Holocene Sedimentary History of Chilliwack Valley, Northern Cascade Mountains
by
Jon Francis Tunnicliffe
B.A. (Hons), University of Western Ontario, 1995 M.Sc, University of Northern British Columbia, 2000
A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF
Doctor of Philosophy
in
The Faculty of Graduate Studies
(Geography)
The University Of British Columbia (Vancouver, Canada) January, 2008 °c Jon Francis Tunnicliffe 2008 Abstract
I seek to reconstruct the balance between sediment storage and yield across multiple drainage basin scales in a large (1 230 km2) watershed in the Northern Cascade range, British Columbia and Washington. Chilliwack Valley and surrounding area has been the site of numerous studies that have detailed much of its Quaternary sedimentary history. In the present study this information is supplemented by reconstruction of the morphodynamic trajectory of the river valley though the Holocene Epoch, and development of a sediment transfer model that describes the relaxation from the Fraser glaciation. The total Holocene sediment yield is estimated from basins across several scales using field and remotely sensed evidence to constrain the historical mass balance of delivery to higher order tributary basins. Rates of hillslope erosion are estimated using a diffusion-based rela- tion for open slopes and delimitating the volume evacuated from major gully sources. Digital terrain models of paleo-surfaces are constructed to calculate total sediment erosion and de- position from tributary valleys and the mainstem. Chilliwack Lake has effectively trapped the entire post-glacial sediment load from the upper catchment (area = 334 km2), allowing to compare this ‘nested’ system with the larger catchment. Rates of lake sediment accumu- lation are estimated using sediment cores and paleomagnetism. These are compared with accumulation rates in the terminal fan inferred from radiocarbon dating of fossil material, obtained by sonic drilling in the apex gravels. A sediment budget framework is then used to summarize the net transfer of weathered material and glacial sediments from the hillslope scale to the mainstem. The long-term average sediment yield from the upper basin is 62 ± 9 t/km2/yr; contemporary yield is approximately 30 t/km2/yr. It is found that only 10-15% of the material eroded from the hillslopes is delivered to mouths of the major tributaries; the remaining material is stored at the base of footslopes and within the fluvial sedimentary system. Since the retreat of Fraser Ice from the mouth of the valley, Chilliwack River delivered over 1.8 ± 0.21 km3 of gravel
ii Abstract and sand to Vedder Fan in the Fraser Valley. In the sediment budget developed here, roughly 85% of that material is attributed to glacial sources, notably the Ryder Uplands and glacial valley fills deposited along the mainstem, upstream of Tamihi Creek. In tributary valleys, local base-level has fallen, leading to the evacuation of deep glacial sedimentary fills. Many of the lower reaches of major tributaries in upper Chilliwack Valley (e.g. Centre and Nesakwatch Creeks) remain primarily sediment sinks for slope-derived inputs, since base-level fall has not been initiated. In distal tributaries (Liumchen, Tamihi and Slesse creeks), paraglacial fans have been incised or completely eroded, entrained by laterally active channels. A transition from transport-limited to supply-limited conditions has been effected in many of these reaches. Slesse Creek has struck an intermediate balance, as it continues to remobilize its considerable sediment stores. It functions today as the sedimentary headwaters of Chilliwack Valley. Using grain size data and fine-sediment geochemical data gathered from Chilliwack River over the course of several field seasons, a simple finite-difference, surface-based sediment transport model is proposed. The aim of the model is to integrate the sediment-balance information, as inferred from estimates of hillslope erosion and valley storage, and physical principles of sediment transport dynamics to reproduce the key characteristics of a system undergoing base-level fall and reworking its considerable valley fill during degradation. Such characteristics include the river long profile, the river grain-size fining gradient, the percentage of substrate sand, and the diminution of headwater granite lithology in the active load. The model is able to reproduce many of the characteristics, but is not able to satisfy all criteria simultaneously. There is inevitably some ambiguity as to the set of parameters that produce the “right” result, however the model provides good insight into long-term interactions among parameters such as dominant discharge, grain size specifications, abrasion rates, initial topography, hiding functions, and hydraulic parameters.
iii Table of Contents
Abstract ...... ii
Table of Contents ...... iv
List of Tables ...... viii
List of Figures ...... x
Acknowledgements ...... xxvi
1 Introduction ...... 1 1.1 Problem Statement ...... 1 1.2 The Study Basin ...... 5 1.2.1 Physiography ...... 8 1.2.2 Regional Studies ...... 9 1.3 Thesis Structure ...... 11
2 Hillslope and Tributary Sediment Stores ...... 13 2.1 Introduction ...... 13 2.2 Data Sources and Associated Errors ...... 15 2.2.1 The Magnitude of Error ...... 15 2.3 Network Structure and Process Domains ...... 17 2.4 Sediment Deposition in Lower-Order Catchments ...... 21 2.5 Sediment Source Areas ...... 26 2.5.1 Large Bedrock Failures ...... 26 2.5.2 Gullies and Diffusive Slope Processes ...... 27 2.5.3 Sediment Source Areas: Surficial Materials and Gullied Terrain . . . 28
iv Table of Contents
2.5.4 Sediment Source Areas: Open Slopes ...... 33 2.6 The Fluvial Domain: The Lower Tributary Valleys ...... 38 2.6.1 Lower Tributary Valley Fills ...... 41 2.7 Discussion ...... 43
3 Chilliwack Lake ...... 46 3.1 Study Area ...... 47 3.2 Seismic Methodology ...... 49 3.3 Interpretation of the seismic record ...... 50 3.4 Fan Deltas ...... 54 3.5 Ground Penetrating Radar Surveys ...... 56 3.6 Lake Cores ...... 61 3.6.1 Lake Core Descriptions ...... 62 3.6.2 Tephra and Other Disturbance Layers ...... 63 3.6.3 Magnetic Parameters ...... 64 3.6.4 Palaeomagnetism ...... 69 3.7 Rates of Sediment Accumulation in the Holocene Epoch ...... 70
4 Evolution of Chilliwack Valley Mainstem ...... 77 4.1 Initial Conditions ...... 78 4.2 Mid Valley Fill ...... 81 4.3 Glacio-Lacustrine Deposition ...... 82 4.4 Lower Valley Fill ...... 85 4.4.1 Ryder Lake Upland Moraine Complex ...... 86 4.5 Vedder Fan ...... 90 4.6 Architecture of the Vedder Fan ...... 92 4.7 Well-log database ...... 95 4.8 Apex Gravels - Core Descriptions ...... 95 4.9 Chronology and Volumetric Estimation ...... 99 4.9.1 Isopach Diagrams ...... 102 4.10 Discussion ...... 105
5 Characterization of Valley Sediments ...... 108 5.1 Introduction ...... 108
v Table of Contents
5.2 Sampling of Tributary and Mainstem Gravels ...... 109 5.2.1 Fining Patterns ...... 114 5.3 Lithology and Geochemistry ...... 119 5.3.1 Coarse Clast Lithology ...... 119 5.4 Silt Geochemistry ...... 123 5.4.1 Methods ...... 123 5.4.2 Factor Analysis ...... 126 5.5 Summary and Conclusions ...... 131
6 A Morphodynamic Model of Postglacial River Evolution ...... 133 6.1 Introduction ...... 133 6.2 One-dimensional representation ...... 136 6.2.1 The Degrading River Valley System ...... 137 6.2.2 Bed Shear Stress Distribution ...... 139 6.2.3 The Active Layer ...... 140 6.3 Model Development ...... 140 6.3.1 Grid Resolution and Hydraulics ...... 141 6.3.2 Abrasion ...... 144 6.3.3 Mass Balance ...... 145 6.4 Boundary Conditions ...... 146 6.4.1 Basin Hydrology ...... 147 6.4.2 Stratigraphy and Bedrock ...... 150 6.4.3 Upstream Feed and Tributary Inputs ...... 152 6.4.4 Time and Intermittency ...... 156 6.5 Model Performance ...... 158 6.5.1 Long-Profile Adjustments ...... 160 6.5.2 Textural Response ...... 164 6.5.3 Clast Fining and Abrasion ...... 165 6.5.4 Subsurface Sand Content ...... 167 6.5.5 Growth of Vedder Fan ...... 169 6.5.6 Suspended Load ...... 172 6.6 Discussion and Conclusions ...... 173
vi Table of Contents
7 Conclusions ...... 178 7.1 Process Domains ...... 179 7.1.1 Terminal Deposits of Chilliwack Valley ...... 181 7.1.2 Mainstem Deposits ...... 182 7.2 Textural Evolution of the Mainstem ...... 183 7.3 Models of the Holocene Fluvial System ...... 184
Bibliography ...... 186
vii List of Tables
2.1 Estimated volumes eroded from gullied morainal and colluvial cover . . . . . 33 2.2 Estimated volumes eroded from planar to convex slopes ...... 37 2.3 Volume of glacigenic fill evacuated from major tributaries...... 43
3.1 Volumetric estimates for catchment erosion and fan deltas bedload delivery since deglaciation, not including outwash stores...... 58 3.2 Radiocarbon ages from Chilliwack Lake...... 62 3.3 Sediment delivery to Chilliwack Lake: Volumetric estimates ...... 74 3.4 Mineral sediment delivery to Chilliwack Lake: Late Holocene (<2 000 years BP) estimates based on core chronology...... 74
4.1 Net bulk volume eroded from the mainstem between Chilliwack Lake and Borden Creek ...... 81 4.2 Estimated bulk erosion volumes for three different assumed topographic con- figurations in the Lower Chilliwack Valley in post-glacial time. The true value is assumed to be intermediate between I & II. The bounds of minimum net erosion are shown in Figure 4-8 ...... 88 4.3 Radiocarbon ages from drilling at Vedder Fan. All samples were dated by conventional radiometric technique. Intercept ages and age range in calendar years before AD 1950. The age ranges in parentheses represent 1σ error limits. The ages were determined using the INTCAL98 database...... 99 4.4 Volumetric estimates of Vedder Fan composition (m3 × 106) based on well log records. Values are cumulative over the time intervals indicated. The overall expected accuracy of the estimates is ±16%, based on the range of possible spatial extents and depths of the fan. Values in brackets indicate additional volumes attributable to the Sumas Lake Basin...... 105
viii List of Tables
4.5 Bed material yield below Vedder Crossing - lowest, mean and highest annual averages over the course of study periods for three investigations using different methodologies ...... 106
5.1 Correlation scores among 19 elements in fine grained (< 63µ) sediments (both active channel and hillslope sources) Chilliwack Valley. Values greater than 0.65 or less than -0.65 are highlighted...... 125 5.2 Factor Scores from elements in the factor analysis using Varimax rotation. Principal loadings for each element are highlighted...... 129 5.3 Mixing estimates based on Fe-Factor and associated raw element variables. Values indicate the approximate proportional addition to the mainstem sedi- ments at each tributary junction. +∞ scores indicate scores greater than one; −∞, less than zero...... 130
6.1 Representative volumes of the eroded stratigraphic units in the lower main- stem. Volume is tabulated within 360 m cells, each having a specified width and vertical walls, and thus some spatial (volumetric) resolution is lost. Vol- umes for an alternate model configuration with larger accumulations within the lower valley are indicated in parentheses...... 152 6.2 Summary of static and varied model parameters ...... 159
ix List of Figures
1-1 A spatio-temporal view of sediment transfer across the landscape, adapted from Church [1996]. The box indicates the range of scales that are addressed in this thesis - river distances of up to 100 km, and timescales ranging from 10 years of river measurements out to 14 ka cal. B.P.). The annotations in the diagram indicate appropriate modes of explanation across various spatio- temporal scales...... 3 1-2 Conceptual picture of the transit of a large-scale sedimentary disturbance through space and time (after Church [2002]). The disturbance decays quickly away from proximal watersheds. Dispersion is due to deposition of material at tributary junctions and in sedimentary reservoirs along the system, vegetation of formerly active sources, and differential mobility of grains in the mixture. The figure illustrates a “primary” disturbance of post-glacial time, upon which smaller-scale perturbations are super-imposed...... 5 1-3 Chilliwack Valley and surrounding area...... 7 1-4 Lithologic assemblages of the study area...... 10
2-1 Definition of the reference post-glacial surface. Material deposited on top of the post-glacial surface constitutes an input to the post-glacial budget, material eroded from the valley wall and glacial fill are considered to be outputs. The
reference postglacial topography is V0...... 14 2-2 Chilliwack River Valley...... 18 2-3 Three different basin scales. The left side of (a) shows linear, first- and second- order debris flow channels, next to a 3rd order alpine colluvial basin, in the headwaters of the Slesse basin. Grid spacing is 500 m. (b) 5th-order Bear Creek, with a developed fluvial network (grid spacing is 1 km). Note the relatively small outlet fan in the latter...... 19
x List of Figures
2-4 Histograms showing the distribution of upstream area within valley network links, from headmost channels to the Chilliwack River at Vedder Crossing. The mean area for each order class is shown with a cross (+). The process domains identified by Brardinoni and Hassan [2006] are overlaid in grey. The total number of links for each order is indicated...... 20 2-5 Diagrams illustrating estimation of fan deposits using the technique of Camp- bell and Church [2003] and using CAD shapes...... 23 2-6 Estimated Holocene bulk sediment deposition (sand and coarser) plotted against contributing catchment area. Catchments are coloured according to their Strahler order. Two reference lines are shown, indicating that the envelope of maximum deposition scales approximately to the power of 1.5. A similar exponent is used for scaling the size of fans and deltas from larger fluvial basins. 24 2-7 Pierce Creek, July 2003. Source area of the 1996 event is shown with an arrow 25 2-8 Data from Kirchner et al. [2001] (black diamonds) with data from the Chill- iwack Valley overlain (open circles). Sediment accumulation estimates from Chilliwack Valley display more scatter, but they are largely consistent with the long-term average rates of specific sediment yield calculated by Kirchner et al. [2001]...... 26 2-9 Histogram of gradient distributions in gullied topography and on planar to convex hillslopes (vegetated and bedrock) in the first to fifth order catchments of Chilliwack Valley. A limiting gradient for the vegetated open slopes is estimated to be 1.4 ...... 28 2-10 Photos of (a) Holocene channel incision and (b) exposed sidewall deposits of till and glacio-lacustrine material. (c) is a stratigraphic section from a left-bank tributary to Lower Slesse Creek. Total incision is over 25 m, but the exposed 10 m section gives some idea of the complex nature of the initial (Pleistocene) hillside stratigraphy. Some similar sections were found in nearby tributaries, but exposure of such a stratigraphic record is uncommon...... 30 2-11 Photo and figure of a landslide headscarp at Foley Creek. This is the initiation point of a debris flow gully, incised into a sandy, compacted till deposit. . . . 31
xi List of Figures
2-12 The distribution of hillslope gradients within the gullied topography of Chill- iwack Valley. Contours indicate the cumulative density of points, in incre- ments of 0.2. The slopes at the initiation points for 492 landslides in the EBA database are overlaid...... 32 2-13 Hillslope relaxation response. Histograms indicate the distribution of DEM slopes that are found within the gullied zones. The modern (TRIM) DEM is shown in darker gray; the inferred post-glacial distribution is outlined in black. The shift in the curve describing the distribution of slopes indicates that the modern (TRIM) surface is left steeper after the morainal and colluvial material has been evacuated, due to establishment of steep gully sidewalls...... 34 2-14 Results from the subtraction of DEM surfaces in gullied zones, over the entire Chilliwack Valley. Graph shows the distribution of maximum depths of eroded volume. Linear scale for bar chart is shown on the left, log scale for the curve is on the right...... 34 2-15 Cumulative erosion from gullied hillslope sources in Chilliwack Valley by slope class. Maximum rates of erosion occur in the range of 35-40◦. Mean depth of vertical erosion increases with slope. Depths of erosion are indicated with bounds of one standard deviation...... 35 2-16 Volumetric estimates of volumetric erosion vs. fan volume (bedload) are com- pared in a number of basins within the Chilliwack Valley, to assess the agree- ment between estimates. Symbols are the same as in previous figure...... 37 2-17 (a) Combined specific erosion potential for bedrock, gullied and forested slopes,
calculated for a proportion of all links in Chilliwack Valley (K1soil = 0.006, Sc
= 1.4 for forested terrain, K1rock = 0.003, Sc = 2 for bedrock, assumed deposit specific density of 1.6 kg/m3). A maximum rate of erosion is attained in some catchments smaller than 1 km2. (b) Inferred rates of tributary coarse sediment yield are plotted with the envelope of hillslope sediment mobilization shown in (a), above (grey region). Dashed line indicates the upper bound of the region with a doubling of the diffusion coefficients...... 39
xii List of Figures
2-18 Longitudinal profiles of 6 major tributaries in the Chilliwack Valley. Vertical exaggeration is 20x. Black triangles indicate the transition from the collu- vial process domain to the fluvial domain. White triangles indicate secondary knickpoints, conditioned either by glacial erosion in the master valley (Lium- chen and Tamihi), or, in the case of Foley Creek, by a large landslide. . . . . 40 2-19 Longitudinal section of evacuated glaciofluvial valley fill, superimposed on the modern river profile in Liumchen, Tamihi and Slesse Creeks. Bedrock features, such as hanging glacial sills (described in the text) are shown in light gray. The Slesse Creek fill overlies a lacustrine layer near the junction with the mainstem. Vertical grid spacing = 100 m ...... 42 2-20 (a) Upper strata of the valley fill in Foley Creek ( 350 m upstream of Foley mouth). Exposed is a mix of glaciofluvial and debris flow deposits. (b) Upper strata of the glaciofluvial valley fill in Foley Creek (∼1.2 km upstream of Foley mouth). The coarse fluvial beds are approximately 45 m above the modern channel. The majority of the boulders are most likely Mt. Barr granite, indicating the headwater provenance of the bulk of the sediments (c) Truncated remains of a fan deposit that once interfingered with glaciofluvial fill in lower Tamihi Creek...... 44
3-1 Chilliwack Lake, with TRIM digital terrain model. Lines A, B, C and D indicate seismic sections discussed below...... 48 3-2 Figure showing mid-lake CHIRP data on the North end of ‘C’ transect, Figure 3-1, and the continuous drape of lacustrine beds leading up onto the edge of the Paleface fan delta. See site (b) in the next figure for the larger setting within the lake sediments...... 49 3-3 Composite seismic image of Chilliwack Lake (centreline transect, 9 km) show- ing major depositional units along the axial section. Five labelled features are discussed in the text: (a) continuous, laminated Holocene lacustrine sedi- ments, (b) sandy outwash material (c) distorted reflectors within Pleistocene strata, (d) Paleface Creek delta, and (e) Depot Creek delta, showing portions of the pre-Fraser glaciation topography (see lateral cross-section, Figure 3-4). Vertical exaggeration is roughly 45x, though seismic velocity through the lower strata is likely somewhat higher than in the lacustrine zone...... 52
xiii List of Figures
3-4 Cross-section B (see Figure 3-1), the closest transect to the down-valley extent of Chilliwack Lake, and the deepest section (120 m water depth). Units a, b and c correspond to units labeled in the previous figure. Top figure shows the un-migrated data, with the characteristic ‘bow-tie’ structures. The migrated section, below, shows the synform structure at depth, though distortion is introduced to other parts of the trace. The valley walls are planar; curvature at the upper edges of the trace is due to turning of the boat and the array. . 53 3-5 Location of subsurface surveys at Depot and Paleface Creek fan deltas. Un- derwater topography was surveyed using sonar on a 100 m grid. Lake core locations and seismic trace paths are indicated. Section ’D’ refers to the seis- mic trace in Figure 3-6...... 55 3-6 Section D (see Figures 3-1 and 3-5). Two seismic techniques used on the same profile at the toe of Depot Creek Fan: CHIRP (above) and acoustic air gun (below). The CHIRP record shows a detailed picture of the uppermost unit visible in the air gun profile. The airgun record shows the deeper strata . . . 56 3-7 Profile C (see Figure 3-1) with superimposed seismic record. Perspective view, facing south east, shows the relationship among the upper beds of lacustrine material, outwash beds, and several antiform reflectors at depth that likely indicate the structure of buried Pleistocence fans emerging from Paleface and Depot Creeks. The sand limit on the fans is inferred from the CHIRP record. Transverse seismic transects are indicated with black lines...... 57 3-8 GPR transects at Paleface fan delta. (a)Location map for the GPR transects. (b) Down-dip section. 0 m indicates the lake shore. (c) Strike section, moving NNE to SSW. A radiowave velocity of 0.07 m ns−1 is assumed. This makes the limit of penetration 35 m (500 ns). The lower bounds of the sediment package were not detected...... 59 3-9 Isopach diagrams, (a) Paleface and (b) Depot Creeks. A hypothetical post- glacial surface (top of outwash unit ‘c’, Figure 3-3), built with a 4th order polynomial grid, is subtracted from the modern topography of the lake fan deltas to yield an estimate of total coarse (sand and gravel) sediment accumu- lation from these catchments over the Holocene Epoch. The fines component of the fan deltas extends much further out into the lake and are estimated in a separate analysis...... 60
xiv List of Figures
3-10 Site map: Chilliwack Lake with the location and description of seven cores recovered from the lake...... 61 3-11 CHWK-06 - Vibracore from the deepest section of Chilliwack Lake. Dark bands in the column at left indicate discrete beds with concentrations of coarser clastic material and/or fine organics. The upper portion of the core may have undergone some compaction during the coring operation, interrupting the otherwise coherent gradient of increasing density with depth...... 64 3-12 Cross-core comparisons of two parameters, water (%) and loss on ignition (LOI). 65 3-13 Ternary diagram showing compositional fields for a number of Holocene vol- canic sources within the Cascades and Coast Mountains. Points (Xs) show microprobe readings taken on shards from the Chilliwack Lake tephra layer. . 65 3-14 Three cores from Chilliwack Lake show a number of fire episodes over time. From left to right: CHWK-06 (Distal 2), CHWK-05 (Distal 1), and CHWK-04 (Paleface). Positive X-Rays (4 mAs at 75 kV) reveal very fine low-density ash layers. Based on paleomagnetic dating and stratigraphic sequence, two event beds are correlated across two distal cores and a third from as far away as Palefacefan delta. The events span an approximate range of 1 350 to 1 750 cal. years B.P...... 66 3-15 Percentage of coarse (>16 mm) clastic lithologies found in active bars, Upper Chilliwack River (open squares) and major tributary sources (Bear and In- dian Creeks, black diamonds). There is a disproportionate representation of volcanic lithologies in the channel, despite the mostly granitic source material provided by the major lower tributaries, which represent 41% of the upper catchment drainage area...... 67 3-16 Graph showing the relative increase in iron and elements associated with heavy minerals and magnetic oxides within the silt fraction of channel sediments (Up- per Chilliwack River) and lake sediments. Fe values range from 3 to 5%. Here they have been log-ratio transformed, centered on zero. Individual elements Cr, Mg, Ti and V have been similarly transformed, then summed, to create a composite index. Individually, V has the strongest down-valley gradient, and Ti the weakest...... 68
xv List of Figures
3-17 Susceptibility, measured at 5 cm intervals along the length of each core. There is an evident longitudinal gradient, with magnetic mineral concentration in- creasing down-valley...... 68 3-18 Sedigraph grain size curves from lake core sediment samples, Chilliwack Lake 69 3-19 Traces of magnetic inclination and declination from four cores are shown in the first panel, as well as their average. These are the raw (unsmoothed) data, interpolated onto a standardized 200-element array to facilitate cross- core statistics. Standard deviation among the magnetic readings are shown in the middle panel. The shorter core extends only to 2 500 B.P., and thus sample size diminishes prior to this date. The final panel shows Fish Lake and Mara Lake datasets compared with the Chilliwack series...... 71 3-20 Paleomagnetic chronology (calendar years) mapped to sediment core depth (cm). Red dots indicate fire disturbance events recorded in the deepest core. Elevated rates of sedimentation are inferred at the Upper Delta (CHWK-03) based on radiocarbon dating of an organic layer at 85 cm (180 ±40 years BP - roughly A.D. 1670-1800; radiocarbon calibration for this era is only loosely constrained). Before approximately 3 200 BP there appears to be a slightly elevated rate of accumulation. Reduced major axis regression within the distal cores indicates a significant change in slope (α=0.01), though there are fewer core samples deeper than 2 m to confirm this shift in trend...... 72 3-21 Isopach map showing the inferred depth of lacustrine sediment since the end of outwash deposition and the onset of lacustrine conditions at Chilliwack Lake, based on seismic and sonar surveys. The seismic transects are overlaid in white. 73 3-22 (a) Volume of lacustrine sediment with depth. Uncertainty bounds are in- dicated by multiple profile lines. Model is based on seismic imaging of the sediments and follows assumptions outlined in the text. (b) Variation of spe- cific dry weight density of sediments with depth in each of the lake cores. The logarithmic trend lines indicate the expected pattern of density with depth. . 73 3-23 Fine sediment yield for Chilliwack Lake shown in comparison to other lakes in British Columbia. Dots size indicates the proportion of basin glacial cover. Data collated from Gilbert et al. [1997]; Desloges and Gilbert [1998]; Schieffer et al. [1999]; Hodder et al. [2006]...... 75
xvi List of Figures
4-1 Six-part deglacial history of Chilliwack Valley: (a)-(c) depicts the retreat phase of the final stage of the Sumas ice lobe, active floodplain outlined in green, 11 200 to 10 500 14C years B.P. (d) A remnant lobe of ice in Sumas Valley, following ice retreat, 10 500 - 10 000 14C years B.P. (e) Sumas Lake fills the depression left by the ice, basin fills throughout Holocene (f) The modern landscape: City of Chilliwack and Sumas Valley. Chilliwack River has been channelized into the Vedder Canal. Adapted from Saunders et al. [1987], Cameron [1989] and Clague et al. [1997]...... 80 4-2 Isopachs of the estimated net erosion from the mid-valley, at Slesse confluence. The maximum erosion depth is 106 m. Shallow erosion is shown across the intact remnants of Larson’s Bench, after a pre-erosional surface was fit to it. . 82 4-3 Longitudinal survey of terraces between Foley and Tamihi Creeks. Degradation appears to be hinged on a knick point close to the outlet of Foley Creek. Terrace flights have been grouped as high (red), middle (light blue) and low (green). Points surveyed on the terraces are coloured accordingly; points on the modern river are coloured dark blue. River topography is taken from the BC TRIM DEM breaklines ...... 83 4-4 Fine sediment source material is delivered episodically from deep glacio-lacustrine deposits in the mid-valley reaches...... 84 4-5 Larson’s Bench is incised by a late Holocene channel. The formerly active layer of the incising channel overlies sandy deltaic fill on the left; lacustrine clay on the right...... 85 4-6 Cross section showing the available evidence (exposures and well logs) to de- scribe the Pleistocene and post-glacial valley fill. The modern channel is de- marcated with a dashed line. Upper planform map shows detailed terrain mapping [Armstrong, 1980; Ryder, J.M. and Associates, 1995] that outlines major lacustrine and morainal strata. Ryder Lake Upland is depicted with a veneer of aeolian material and thick till accumulations beneath. Points on the plan map indicate exposures and well logs that are graphed in the section detail. Vertical exaggeration is 20x. There is assumed to be considerably more lateral complexity within the valley’s deposits (and eroded volume) than can be reconstructed with this evidence...... 86
xvii List of Figures
4-7 Reconstructed geometry of the major valley landforms in Lower Chilliwack Valley, downstream of Tamihi Creek. (a) shows a composite set of cross sec- tions of the valley near Ryder Creek, 3 km downstream from the crest of Tamihi Moraine. The shaded area represents the zone of minimum net erosion of glacial material (scenario III, see Figure 4-8 and text). (b) is a longitudinal valley cross-section looking North, encompassing the former locality of Tamihi moraine. A mid-valley lake elevation is shown, based on the assumed elevation of the delta front at the distal end of the Larson’s Bench sandur. Cross-section lines from Ryder Upland showing the maximum height of the remnant moraine are in the background...... 89 4-8 Scenario III, with minimum bulk volume erosion from valley sidewall sediment sources, including the Ryder Lake Upland. Contours of erosion and deposition are in increments of 15 m. Total erosion is 344 ±45 × 106 m3. Deposition evident at the base of Ryder Upland is approximately 35 × 106 m3...... 90 4-9 Figure showing a band of elevations between 1 and 35 m a.s.l. in Sumas Valley, Vedder Fan and the Fraser River. The bounds of Figure 4-14, showing City of Chilliwack Drilling work (2006), are highlighted. Note the semi-circular geom- etry of the Vedder Fan, and the relatively low-lying surrounding topography. TRIM BC digital elevation data...... 91 4-10 Historical planform of Chilliwack River north of Vedder Crossing, ca. 1891 (top) and 1991 (bottom). An interlinked network of channels (four of the larger threads are labelled) alternately occupied and abandonned various section of the fan as it evolved. The historical figure was derived from an ordnance survey of the Chilliwack area. The modern map was generated from BC TRIM mapping. 93 4-11 Model of alluvial fan growth, after Blair and McPherson [1994]. The lower bounding surface slopes slightly upward, as deposition keeps pace with a slowly rising base level. In the case of Vedder Fan this corresponds to active deposition on the Fraser Valley floor over the course of the Holocene...... 94 4-12 Gravel quarry near Vedder Road at Watson. Looking westward: flow was from left to right. Photo credit Vic Galay, Northwest Hydraulic Consultants. . . . 96 4-13 Gravel quarry near Vedder Road at Watson. Photo is looking southward, and the direction of flow was out of the page. Photo credit Vic Galay, Northwest Hydraulic Consultants...... 96
xviii List of Figures
4-14 Locations of City of Chilliwack drilling operations, 2003-2006 (1995 aerial pho- tography). See Figure 4-9 for map location with respect to the larger fan area. 97 4-15 Grain size distribution from a sampling of units recovered from sonic drilling. Representative fractions > 32 mm could not be effectively recovered from the core samples, but the fractions that were examined effectively show the bi- modal nature of the fan deposits...... 98 4-16 Cross-section of Vedder Fan, based on examination of sonic core cuttings and assembled well logs. Letters refer to locations of photos in Figure 4-17. Nar- rower logs with solid colours are database wells with minimal detail and have not been examined. Dashed lines indicate interpolated surfaces with an asso- ciated date based on radiocarbon samples. Most elevations are approximate, unless significant figures are indicated...... 100 4-17 Photos of sonic core material, illustrating facies assemblages that are indicated in Figure 4-16. Wood dated at 10 125 ±245 cal. years B.P. was recovered from the silt unit shown in (i) ...... 101 4-18 Regression curve through multiple radial sections of Vedder Fan ...... 103 4-19 Isopach diagrams of gravel, sand and lacustrine material within the strati- graphic bounds of Vedder Fan and surrounding area. Bounds of former Sumas Lake are shown in black. See text for description of numbered localities in 4-19c.104 4-20 Cumulative volumetric growth of Vedder Fan from 11 035 BP to present, based on the assumed stratigraphic relationships indicated in Figure 4-16. Error bounds indicate 16% error assumed in the calculations...... 106
5-1 Sampling bar material in the lower Chilliwack Valley (site # 111-04) . . . . . 111 5-2 Sampling sites within Chilliwack Valley. (a) Lower Valley and (b) Upstream of the Slesse Creek confluence. Open circles indicate surface sampling only, and filled circles indicated both surface and bulk sampling...... 112 5-3 Duplicate samples taken at (a) just upstream of the Tamihi Bridge (113-04), and (b) a large bar complex upstream of Borden Creek (135-04). Lines indicate the relative difference among samples for each size fraction, relative to the first of the two duplicates (‘0’ datum). Samples are not significantly different within each sedimentary link, but show a closer affinity than between links...... 113 5-4 Grain size distributions for bulk and Wolman samples within the Chilliwack mainstem (a,b), Tributaries (c,d) and Glacial Tills (e)...... 115
xix List of Figures
5-5 Mean, standard deviation, skewness and kurtosis of gravels (Ψ scale) sampled in Chilliwack Valley. (a) shows the statistical moments of the full distribution, including fines, (b) shows the same statistics with the distribution truncated at 2 mm (no fines)...... 116 5-6 Downstream fining along Chilliwack Valley Mainstem. (a) shows the cumula- tive fractions of the subsurface sediments from Chilliwack Lake to the end of the gravels in Vedder Canal. Samples from Vedder Crossing to the canal were taken by Y. Martin and BC Environment (1989-1991). (b) shows the fining pattern for surface samples. No samples were taken beyond Vedder Crossing. (c) is similar to (b), except the fining gradient is shown from the mid-reaches of Slesse Creek (grey shading) to Vedder Crossing. The comparison shows clearly the geographic source terrain of the coarse material in the mid-reaches of Chill- iwack River. Dashed lines indicate regression fits to the data downstream of Slesse Creek...... 117 5-7 Percentage sand (<2 mm) in surface and subsurface deposits, plotted along the length of Chilliwack River...... 120 5-8 (a) Downstream variation in percentage granite composition of streambed sed- iments. The size composition of the total granite percentage is broken into 3 classes: 8-22 mm, 32-90 mm and 128 mm and larger. Relatively few granite pebbles were recorded, while a large number of boulders are evident. The dark line indicates the relative percentage of granitic terrain upstream (not including the drainage above Chilliwack Lake). (b) Shows the percentage of granitic clasts in the 32-90 mm category relative to both upstream granitic terrain (black) and percentage of granitic boulders (light gray). This is an index of how much mobile granitic material we might expect to see in the channel versus the observed. See text for discussion. The Sternberg relation for diminution of grain size from 90 mm to 32 mm is shown (dotted line, axis to the right)...... 121
xx List of Figures
5-9 Bivariate plot showing the major geochemical domains among channel sedi- ments and glacial deposits in the Chilliwack Valley. Among the raw element data, K and Fe provide the best discriminating potential. Fine channel sed- iments from batholith sources (upper left) and Chilliwack Group or Cultus Group sources (lower right) are reasonably distinct. Mainstem alluvium plots as an intermediate field, and glacial deposits plot to the lower left. A few seemingly anomalous points are evident: (1) a sample taken from glaciolacus- trine bluffs at the Slesse/Chilliwack confluence. There is a strong affinity here for headwater lithologies (2) two samples taken from the Slesse Park landslide, which show a concentrated (proximal) mix of the two source terranes. (3) is a distinct sandy bed from Slesse Park landslide that clearly has a strong in- dication of Slesse source material. There is also a ‘mainstem alluvium’ point here that was sampled downstream of the Nesakwatch Creek confluence, il- lustrating its strong influence on mainstem sediment composition. (4) is from Borden Creek, whose source terrain appears relatively high in silica and low in indicator elements...... 127 5-10 (a) shows the fine sediment samples and geochemical data from source rocks cast in Fe-K factor space. There is clearly an influence from both sources on the elemental composition of the sediments. Source rock information comes from compilations in Richards [1971], Sevigny and Brown [1989] and Tepper and Kuehner [2004]. (b) is a vector diagram illustrating the magnitude and direction of each element in Fe-K factor space. (c) shows the relation between mainstem samples (open circles below Slesse Creek, closed circles joined by lines above, distance to Vedder Crossing indicated) and tributary sediments (crosses represent the mean of all samples within each tributary). There is a clear influence of granitic lithologies on the upper mainstem sediments that diminishes for samples lower in the valley. See text for further discussion. . . 128 5-11 Longitudinal pattern of ‘K’-Factor variation, in comparison to the relative proportion of granitic source terrain upstream. Peaks in the gradient are evident at Slesse and Tamihi Creeks, and one near 30 km, a short distance above Alison Pool...... 130
6-1 Figure illustrating 1-D representation of channel floodplain ...... 139
xxi List of Figures
6-2 Flow chart for the ACRONYM-based Chilliwack sediment routing model. Ini- tial and boundary conditions are fed by the user (upper left), then hydraulics and sediment transport are calculated, generating a new bed configuration. The model is updated, and continues in an iterative process. Some of the key variables and adjustable parameters are indicated. GSD refers to grain size distribution ...... 142 6-3 Mean of average daily flows, 1977-2005, at Chilliwack Lake, Chilliwack Canyon, Slesse Creek and Vedder Crossing...... 148 6-4 Partial series for average daily flows at Vedder Crossing, based on the 1952- 2005 gauging period. There are two distinct flood regimes for spring/summer and fall/winter. The mean annual flood for the fall/winter is estimated to be a daily average of 320 m3/s ...... 149 6-5 Figure illustrating the relative contribution of various daily mean flows to total suspended sediment transport...... 150 6-6 (a)Long profile with stratigraphy. The posited initial longitudinal profile is at the top of the stratigraphic reconstruction. Tamihi moraine has either filled the lower valley (‘High Moraine Scenario’), or only partially filled it. The river flows across the remains of the drained lake and upper delta. While the boundaries are more gradational in reality, they are represented as discrete sections in the model. The modern profile is shown below in gray. Bedrock and bouldery knickpoints are indicated. (b) Graph of the volume of glacial material that was stored above the modern profile within each stratigraphic grid cell, taking account of the valley floodplain width...... 151 6-7 Isolation of sand, gravel and boulder modes within the grain-size distribution, using curve-fitting techniques. The boulder lag mode potentially masks the shape of the upper limit on the gravel mode...... 153 6-8 a) the average among all fluvial sand modes isolated from surface gravel sam- ples taken along the length of Chilliwack River. b) illustrates how varying the sand mode for a given sample alters the shape of the grain size distribution. The mixing proportion ranges from -16 to +16% of the initial distribution. . 153
xxii List of Figures
6-9 Schematic of the model grid in the lower valley. Each computational cell represents 360 m of river length, and the total active width of the floodplain. Sediment source points are indicated with arrows. The large valley wall slump upstream of the Tamihi moraine (mid-lower right) is modelled as an example of a large discrete input to the channel...... 155 6-10 Gravel and sand sediment budget for the Chilliwack model domain below Ne- sakwatch Creek confluence. The net bulk volume of outwash material eroded from the major valleys has been derived in previous chapters. Three alterna- tive scenarios are presented for the total volume of material eroded from the lower valley at Tamihi Moraine. Bedload quantities are based on the relations developed between basin size and volumetric bedload yield developed in Chap- ters 2 & 3, partitioning the total yield to discount tributary valley fills and account for only gravel and sand from hillslope sources...... 157 6-11 Polynomial curves describing the sediment volume / outlet elevation relation- ship for Slesse and Tamihi Creeks. The outlet elevation is shown as a relative scale...... 158 6-12 Space-time diagrams illustrating long-term transport trends in Chilliwack Val- ley. (a) shows the balance of degradation and aggradation across space and time. Rates of change are in units of vertical metres per Model Year, with contours showing 1 m increments, up to a maximum range of 6 m. The ele- vation at the downstream boundary is the Fraser River floodplain, rising at a rate of 40 cm per model-year. (b) shows the corresponding unit bedload flux rates in the model. On the left axis, time in Model Years, Flood Years and an hypothetical scale for calendar years is shown...... 161 6-13 Longitudinal profile, showing successive bed elevations over the course of a model run. Bed elevations at node 44 (downstream of Borden Creek) are shown in geometric series through time (inset)...... 162
xxiii List of Figures
6-14 (a) Illustration of the range of concavity that may be achieved during degrada- tion. The worst-case (RMS = 4) is a planar profile between the knickpoint and the lower valley. (b) The RMS difference between river profile and model re- sults, for a range of specified discharges. Parameters were freely varied within the ranges specified in Table 6.2 to generate a suite of model runs. Bubble size indicates the RMS difference for the longitudinal subsurface sand profile. Results generally indicate that the profile fit improves with lower discharge, and often improves at the expense of the sand profile RMS. Circle indicates reference runs, which had a relatively coarse composition and a low abrasion value. All of the lowest points have high abrasion rates...... 163 6-15 Tributaries that are likely to disrupt the downstream fining pattern will plot above Rice’s discriminant function Rice [1998] ...... 165 6-16 Modelled fractional reduction in grain size for the final equilibrium channel, compared with field results. The subsurface samples from the field are com- pared with the modelled bedload for a range of abrasion values...... 166 6-17 The effect of abrasion on the long-profile fit: with a higher abrasion param- eter specified, the long-profile fit improves accordingly. The RMS difference between modelled and field measurements of the subsurface sand content (bub- ble size) profile worsens, as a surplus of abraded material is added to the mix. Reference runs are circled; a coarser gravel mix achieves a better fit to the long profile, without having to specify a higher abrasion coefficient...... 168 6-18 Subsurface sand percentage (<2 mm) from field samples compared with the modelled bedload...... 168 6-19 (a) Cumulative deposition on Vedder Fan over the course of model runs with varying parameter values. (b) The effect of variations in abrasion (0 to 0.040), sand/boulder content (±10% and 5% variation, respectively), D50 (range through 1 phi mode), profile form (3 scenarios) and tributary flux (1 to 3x) are shown relative to the reference run (‘100%’) to illustrate the range of variability in the time taken to deliver 1.5 × 109 m3, based on the parameter specification. 170
xxiv List of Figures
6-20 Cumulative deposition on an idealized Vedder Fan over the course of the ref-
erence model run. Model stratigraphy of Vedder Fan, showing D50 at left and the proportion of the total sediment yield contributed by mainstem gravels represented in the fan at right. Blue contours indicate fan topography at even 2.5 ka (approx.) intervals. The modern topography is overlaid in red...... 171 6-21 Bedload transport gradient in space and time, illustrating changing specific transport rate along the length of the channel during the process of downcut- ting through the valley sediment stores and re-equilibration of the channel. . 174 6-22 Bedload transport rates at Vedder Crossing throughout a reference model run. The lower three lines indicate flux rates for material sourced from upstream of the model grid, from tributaries (dashed), and from the mainstem channel. The sum of these fluxes is represented by the upper black line...... 175
7-1 Contemporary fluvial suspended sediment yield as a function of drainage area [Church and Slaymaker, 1989], overlaid with long-term estimates of coarse sediment delivery in Chilliwack Valley. Black points and open circles are from Church and Slaymaker [1989]; see their text for description of the different sediment yield regimes. Coloured points are from the present study (see Chap. 2)...... 181
xxv Acknowledgements
I would like to thank professor Michael Church for his insight, advice, and his perseverance in attempts at uncovering the articulate and meticulous scientist that is buried, somewhere, deep within me. The pages ahead are much clearer for his efforts, though I take full responsibility for any lingering lapses in accuracy and logic. I also would thank my committee members Olav Slaymaker and John Clague for their enthusiasm and support in this endeavour. Brett Eaton, Marwan Hassan, Andre Zimmerman, Dave Campbell, Josh Caulkins and Dave Lutzi have been the best of supportive colleagues during my time at UBC, and I owe them a debt of gratitude for their help on many fronts, both in the field and in our many discussions and cogitations. During my time at NIWA, Murray Hicks and Jeremy Walsh provided much added insight into sediment budgets and hydraulic models, and I thank them both for sharing their experience and ideas. My deepest acknowledgements go to my wife Kristiann and son Max, who kept everything else on course while Dad was distracted with writing and research ventures. It has been a great adventure, and continues to be so, thanks to Kristiann. The are many people to thank for their assistance along the way. The chapters that follow could not have come to fruition without their help. Bruce Thomson provided much of the background: the maps, the data, and in particular, his unparalleled knowledge of the area was a very important asset in the development of this work. Rob Huggins (Geometrics, Sunnyvale, CA) provided much help and expertise in conduct- ing the seismic transects on Chilliwack Lake and seismic refraction surveys on Larson’s bench. Geometrics generously provided a GEODE data acquisition system, a geophone streamer ’eel’ and other essential equipment. Bob Gilbert (Queens) kindly lent his CHIRP system for sub- surface profiling of the lake. Brian Menounos and Melanie Grubb (UNBC) provided much assistance and helpful advice. Brian spent a great deal of his time on the geophysical work
xxvi Acknowledgements and supplied the vibracoring equipment for the cores recovered from Chilliwack Lake. I would also like to thank staff at BC Parks, for facilitating several survey campaigns on Chilliwack Lake. Randy Enkin and Judith Baker (GSC Pacific) introduced me to the world of paleomag- netism, and carried out all of the susceptibility measurements using their OLGA susceptibility meter. The results helped significantly with respect to establishing the chronology of sediment deposition in Chilliwack Lake. All of the seismic data processing was done using the computers and Globe Claritas software in Ron Clowes’ seismology research group (EOS). I would like to thank Phil Hammer and Jounada Oueity for their help in sorting out the many intricacies of data processing. Ted Hickin and Michael Roberts (SFU) generously lent their equipment and software for the GPR studies of Paleface delta. The field work was carried out with Natalie Helmstetter and BJ Kelly, and I thank them both for their time in the field and discussions of GPR technique. Thanks to Vic Levson at the British Columbia Geological Survey for making available his exceptional database of geophysical logs and water wells in the Chilliwack area. Janet DeMarcke (City of Chilliwack) and Dan Emerson (Emerson Consulting) kindly let me take part in their research on the groundwater conditions and stratigraphy of Vedder Fan. They both supplied much data and provided access to vibracore drill cores during the City’s drilling campaign. I would also like to thank Kathryn Black (UBC) who carried out much of the analytical work on the drill cores, and made significant contributions to the development of the subsurface model. The geochemical tracing component in the thesis work was inspired through discussions with W.K. Fletcher. Thanks go to Richard Friedman and Jim Mortensen for discussions and making their lab space and resources available. Field work was a large undertaking that couldn’t have been done without much assistance. In a addition to the colleagues mentioned above who were recruited at regular intervals to heft buckets of gravel around the bars of Chilliwack River, I would like to thank Dason Commodore, Sydney Kjellander, Jason Rempel, and Dev Khurana. Staff at Area Services Unit, Chilliwack CFB were particularly helpful in providing access to their properties in Chilliwack Valley. A Natural Sciences and Engineering Research Council of Canada grant to M. Church supported this research.
xxvii Chapter 1
Introduction
1.1 Problem Statement
The last glacial cycle has left a distinct imprint of erosion and deposition in the Canadian Cordillera. In British Columbia and other formerly glaciated regions, the phenomenon of paraglacial sedimentation [Church and Ryder, 1972; Ballantyne, 2002b] has been well docu- mented. In the large mountain valleys, sediment that was originally eroded by the advancing ice continues to work its way through the sedimentary network, many thousands of years after deglaciation [Church and Slaymaker, 1989]. A large volume of sediment derived from relict glacial landforms has a continuing influence on rates of sedimentation in large valleys. Most dating evidence that has been recovered from paraglacial landforms such as outwash terraces, fans and lake deltas [Ryder, 1971; Jackson et al., 1982; Clague, 1986; Brooks, 1994; Ballantyne and Benn, 1994; Ballantyne, 2002a] indicates that the sedimentary relaxation following deglaciation follows an approximately exponential pattern. Initially high rates of sediment transfer from proglacial zones gradually taper off as glaciers retreat and meltwater discharge and sediment supply diminish. The sediment mass is then gradually reworked within the drainage network. As upstream sediment supply is gradually exhausted and meltwater discharge decreases, aggraded landforms such as glacial outwash and alluvial fans become incised and remobilized material is carried downstream. Their initially unvegetated surface renders them particularly vulnerable to erosion. The effects of base-level fall generated by retreating ice gradually propagate upstream. Incision is eventually checked by vegetation and stream bed armouring in the postglacial period. Episodes of renewed downcutting occur intermittently, inducing smaller-scale cycles of aggradation and degradation [Schumm, 1973; Church, 2002; Dadson and Church, 2005].
1 Chapter 1. Introduction
The effects of glacial disturbance on a large valley are complex and time transgressive - it requires several thousand years for the postglacial sedimentary regime to become fully established in large (>1 000 km2) valleys. The fluvial system experiences a downward shift in transporting capacity as sediment transport processes move toward a meta-equilibrium with postglacial hydroclimatic conditions [Baker, 1983; Knox, 1983; Chatters and Hoover, 1992; Kesel et al., 1992]. As the river responds to the changes imposed by falling base-level, it may do so by adjusting one or many channel characteristics [Schumm, 1973; Schumm and Rea, 1995]. Equilibration of the fluvial system is governed by sediment supply. A fundamental con- trol over river regime is the rate and calibre of material supplied to the mainstem from its wider catchment. Many studies of long-term river metamorphosis have focused on a land- scape element at a particular scale (e.g. valley floodplain), without due consideration of the sedimentary conditions in the larger fluvial network. Measurements of flux, or reconstruction by means of proxies, from a single point or reach in a watershed do not provide the necessary quantitative insights into linkages among storage elements that govern fluvial response on bars, floodplains, hillslope and fan deposits. At the larger scale, the fluvial system seems to be subject to complex (linked, causal) changes in configuration over long periods that are difficult to deal with using equilibrium transport models. Figure 1-1 shows a plot of time-space relations and some of the different virtual velocities at which water and sediment traverse the landscape. The virtual velocity of active channel sediments is of order 100 m/yr [Church, 1996]. However, at greater spatial and temporal scales, there is a tendency for this velocity to diminish considerably due to increasing opportunities for deposition at junctions and breaks in slope, resulting in extended periods spent in storage. Thus, one may speculate that the long term rates of transit for river gravels are closer to 1 to 10 m/yr. As the spatiotemporal domain under consideration extends to larger and larger scales, contingencies arise whereby established equilibrium river regimes may be modified. Compli- cations include climate fluctuations, sediment storage, and any adjustment or accident that results in a change of the geometric configuration of the channel or drainage network, limiting or enhancing the rate of sediment flux. The problem of contingency emphasizes the need for renewed investment in historical approaches to landscape studies. Sugden et al. [1997] and Summerfield [2005] point out that the lack of studies in this vein has hindered progress in developing our understanding of long-term landscape evolution. This is presently an impor-
2 Chapter 1. Introduction
4 global 10 t) 3 10 chaotic regional 2 no detection 10
1 e fluvial sedimen 10 -1 0 yr (activ 10 0 m -1 -1 10 10 stochastic 0 m yr -2 1 -1 10 contingent -3 1 m yr 10
-4 10 deterministic
Distance (km)
-5 10
-6 10 -8 -6 -4 -2 0 2 4 6 8 10 10 10 10 10 10 10 10 10
second minute hour day year Time (years)
Figure 1-1: A spatio-temporal view of sediment transfer across the landscape, adapted from Church [1996]. The box indicates the range of scales that are addressed in this thesis - river distances of up to 100 km, and timescales ranging from 10 years of river measurements out to 14 ka cal. B.P.). The annotations in the diagram indicate appropriate modes of explanation across various spatio-temporal scales. tant frontier in geomorphological research, as it challenges the discipline to model and predict change. Sediment budgets have commonly been used to evaluate the relative magnitude of sedi- mentary contributions from different sources within a watershed in order to better understand and manage transfer of material through the drainage network [Dietrich and Dunne, 1978; Reid and Dunne, 1991; Benda and Dunne, 1997; Malmon et al., 2003]. In a long-term ap- proach to modelling an evolving landscape, it is possible to apply a similar mass-balance framework to constrain the evolution of storage elements and the availability of sediment to the fluvial system over the course of time. In some mountainous regions, tectonic uplift is considered an important factor in the Holocene time frame, but in the Chilliwack Valley this effect is considered to be relatively marginal. By reconstructing the history of a glaciated valley and investigating the geometry, sedi- mentology and chronology of landforms, information on the long-term rates of transfer may be
3 Chapter 1. Introduction inferred. Furthermore, we may understand better the mechanisms of the relaxation response as the mass of coarse materials is reworked and transported onwards and out of the basin. This will generate a dataset that may be used to test hypotheses of landscape evolution in formerly glaciated terrain. The analogy of a cascade of reservoirs has been used in a number of studies [Church and Slaymaker, 1989; Warburton, 1993; Ballantyne, 2002a; Church, 2002]. Conceptually, disturbance translates from upper catchments to lower trunk valleys, very much as a flood hydrograph, though the time scale is considerably longer and transit is complicated by a number of factors. Smaller disturbances distributed through Holocene time will be superim- posed on the primary paraglacial signal. Basins at different spatial scales may respond to disturbance on different temporal scales (Figure 1-2). A perturbation visited upon a single low-order catchment will not have a large impact downstream. However, if disturbance from several catchments simultaneously impacts the sediment balance of a higher-order system, there may be a detectable surge in the trunk channel. The hypothesis pursued in this thesis is that the pattern of postglacial sediment yield is strongly conditioned by (1) the volume and geometry of aggraded glacial stratigraphy in the valley, (2) the rate and timing of sediment delivered from tributaries, (3) the rate of base-level fall in the mainstem, and (4) the size grading of material delivered to the mainstem channel. While the first three items have often been dealt with in other studies, this work seeks to analyze all four boundary conditions as an integrated whole, in light of recent advancements that have been made in modelling the transport of gravel mixtures. The following chapters develop the boundary conditions as fully as possible in order to test the sensitivity of these controls on the Holocene river morphodynamics in Chilliwack Valley. I propose that the sedimentology of the glacial valley fill in Chilliwack Valley has had a central and persistent influence on the development of the post-glacial river system, most notably on the form of the final long profile, the asymptotic transport equilibrium that is achieved, and its characteristic response to disturbance. If properly parameterized, a physically-based numerical 1-dimensional sediment transport model with appropriate conservation of mass and treatment of multiple grain size fractions provides a good laboratory for testing some of the theorized notions of paraglacial system response. The transit of the reworked glacial material through the watershed is moderated by depo- sition at tributary junctions and in sedimentary reservoirs along the system, the vegetation of formerly active sources, and the differential mobility of grains in the mixture. The textural
4 Chapter 1. Introduction
Rate of Sediment Flux in Relation to the Subaerial Norm
3
2
1 km2
1 10 km2
12 ka 100 km2 8 ka Catchment Area
Time (post-glacial) 4 ka 1000 km2 Present
Figure 1-2: Conceptual picture of the transit of a large-scale sedimentary disturbance through space and time (after Church [2002]). The disturbance decays quickly away from proximal watersheds. Dis- persion is due to deposition of material at tributary junctions and in sedimentary reservoirs along the system, vegetation of formerly active sources, and differential mobility of grains in the mixture. The figure illustrates a “primary” disturbance of post-glacial time, upon which smaller-scale perturbations are super-imposed. character of the network is an important moderating factor. Residual lag (boulder) modes condition the river bed in a number of important ways, most notably in the stability of the bed structure, hydraulic roughness, and in the evolution of the long profile. The relative quantities of sand in the system will also condition transport and dictate rates of response to disturbance. The modern river is the unique product of the source material yielded by glaciation.
1.2 The Study Basin
Chilliwack River has a drainage area of 1230 km2 (not including the Cultus Lake / Columbia Valley drainage), with most of the major headwaters situated in the North Cascades National Park in Washington State (Figure 1-3). It is the only major left-bank tributary of the Fraser River in the Lower Mainland, descending from the North Cascade Mountains through a broad
5 Chapter 1. Introduction glacial valley, emerging to form an expansive alluvial fan on the floor of Fraser Valley. The river is still actively eroding many remnant glacial and paraglacial deposits. Coarser deposits of reworked glacial material remain in place throughout the drainage network. A great deal of Holocene alluvium remains in storage in the lower reaches [McLean, 1980], and the river adopts a wandering habit through these sedimentation zones. The river has expe- rienced a primarily degradational trajectory throughout the course of the Holocene Epoch. The length of Chilliwack River is roughly 80 km, with a lake in the upper valley truncating the lower mainstem length to 50 km. The pattern of sediment yield in British Columbia established by Church and Slaymaker [1989] indicates that all basins larger than 1 km2 are still to some degree under the influence of British Columbia’s glacial legacy. Based on the observed behaviour of several large-scale sedimentary disturbances, Church [2002] estimates that the rate of propagation (in years) of a large disturbance traveling as a dispersive wave 2 scales approximately with area (in km , t ∝ Ad), much more slowly than the virtual velocity of active channel sediments. The scaling relation for distance along the channel, L(in km), is approximately L ∝ t1/2. Considering the timescale of interest is 13 000 years, catchments that are larger than 10 000 km2 or have a mainstem length greater than 100 km should still be experiencing transit of the end-glacial disturbance. Catchments that are somewhat smaller are likely to be still experiencing the asymptotic tail of the late-glacial disturbance. In his study of the smaller Squamish Valley (3 850 km2), Brooks [1994] concludes that the Squamish Valley has eroded the majority of its reworked glacial sediment. Evidence of the evolution of the drainage network following Fraser Glaciation is remark- ably well preserved. Traces left from glaciers moving through the valleys indicate interlinkages in time and space among tributaries, the trunk valley, and neighbouring drainages. Many relict features such as sandar, kame terraces, glaciolacustrine deposits and lateral moraines remain clearly discernible and have been documented extensively [Chubb, 1966; Easterbrook, 1971; Munshaw, 1978; Clague and Luternauer, 1982; Saunders, 1985; Saunders et al., 1987; Easterbrook, 1992; Watson, 1999]. An advantage of a system with a primarily degradational history is that the additional complexity introduced by episodes of significant aggradation does not hinder the historical interpretation of landforms. Dating has been successfully carried out on a number of the major glacial landforms. Year zero for the sediment budget is approximately 13 300 calendar years ago (∼11 400 14C years B.P.), when the final layers of deltaic sands were deposited on the shores of an unnamed mid valley lake, and the glacier at the head of the valley retreated from its terminal moraine and
6 Chapter 1. Introduction
Figure 1-3: Chilliwack Valley and surrounding area. became uncoupled from the outwash sandur [Clague and Luternauer, 1982; Saunders, 1985; Saunders et al., 1987]. In the lower valley, wood from an ice-contact face at Cultus Lake has been dated at 11 300 ± 100 14C years B.P. [Lowdon and Blake, 1978]. The Chilliwack Valley has been subject to several episodes of glaciolacustrine sedimen- tation, when blockages at the mouth of the valley caused the formation of a lake [Clague and Luternauer, 1982; Saunders, 1985; Saunders et al., 1987]. These deposits are relatively common in the watersheds that drain into the Fraser Lowland. Ward and Thomson [2004] point out that, in valleys such as the Chehalis and the Chilliwack, the ice accumulation area is quite small in comparison to that of the Fraser Lobe and thus the valley glaciers responded differently to climatic changes. During advance, the valleys lagged behind the Fraser lobe whereas, during retreat, the valleys responded more quickly to climatic amelioration. This resulted in glacial lakes forming during both advance and retreat phases [see also Johnsen and Brennand, 2004].
7 Chapter 1. Introduction
1.2.1 Physiography
Climatic conditions in the valley span a gradient from a warm maritime climate in the west to a colder continental climate in the east. Climate also varies significantly by elevation. The lower elevation valley bottom areas in the east are characterized by warm, dry summers and moist, cool winters with moderate snowfall; valley bottoms in the west are similar in summer but have moist, mild winters with little snowfall. At higher elevations, summers are short, cool and moist with long, moist and cold winters [George et al., 2005]. Mean annual precipitation in the period 1961-2005 is 137 cm at the fish hatchery near Slesse Creek, with roughly 50% of the precipitation falling in four months from October to January. Up to 10% of the total annual precipitation may fall as snow in the valley bottoms. At higher elevations, there may be deep snow cover for up to 9 months. The valley floor and lower tributaries are generally designated as Coast Western Hemlock (CWH) in the BC Biogeoclimatic Scheme [Meidinger and Pojar, 1991]. This zone primarily contains coniferous forests, or ‘temperate rainforests’ composed of western hemlock (Tsuga heteropylla) and western red cedar (Thuja plicata). Other species include Pacific silver-fir (Abies amabilis) and Alaska yellow cedar (Chamaecyparis nootkatensis) in wetter areas, and Douglas-fir (Pseudotsuga menziesii), western white pine (Pinus monticola) and bigleaf maple (Acer macrophyllum) in drier areas. Between approximately 900 and 1800 m above sea level, the Mountain Hemlock (MH) ecological zone has mountain hemlock (Tsuga mertensiana) and Pacific silver-fir, with some yellow cedar and sub-alpine fir (Abies lasiocarpa). Active forestry has been ongoing since the 1910s, and by the 1930s, much of the easily accessible flat had been harvested, and operations have extended onto progressively steeper terrain since then [Hay & Co. Consultants, 1992]. Roughly 15% of the forested land base on the Canadian side had been logged by the late 1980s [Jordan, 1990], and George et al. [2005] estimate that 36% of forested land in the valley presently has growth that is younger than 60 years. The headwaters of Chilliwack Valley, including Chilliwack Lake and its tributaries, are within a granitic batholith (Figure 1-4). Radium and Centre Creeks and the headwaters of Nesakwatch, Slesse and Foley Creeks are similarly granitic. Chilliwack Batholith was em- placed between terranes of metamorphosed phyllites and amphibolites in Eocene to Pliocene time. The batholith is the largest pluton in the Cascade arc (960 km2), and consists of at least 40 plutons of granite, granodiorite, tonalite and quartz diorite composition (57% to 78%
SiO2, Tepper et al. [1993]). Chilliwack Batholith also contains stocks of gabbro and diorite, as well as rhyolite and dacite of the Hannegan Formation, located in the headwaters of the
8 Chapter 1. Introduction
Chilliwack Valley. These are likely Pliocene in age [Richards, 1971; Tabor et al., 2003; Tepper and Kuehner, 2004]. The mid section of the valley consists primarily of Chilliwack Group lithologies [Cairnes, 1944; Monger, 1966; Brown, 1987]. This unit is an intra-oceanic island arc assemblage charac- terized by an upper volcanic sequence of pyroclastic rocks and lesser basalt, basaltic andesite, and dacite flows built on a clastic package of volcanic litharenite, slate, limestone and chert [Sevigny and Brown, 1989; Tabor et al., 2003]. Rocks have experienced sub-greenschist to greenschist facies metamorphism. Limestone units within the Chilliwack Group are evident in the mid- to lower reaches of the valley, with some notable outcrops across from the outlet of Chipmunk Creek, and near the mouth of Slesse Creek. The Cultus Group underlies much of the lower portion of the valley, as well as portions of Chipmunk and Foley creeks. Cultus rocks consist of pelites, siltstones, fine-grained sandstones and thin limestone beds. A number of other bedrock units crop out in the study area. These are summarized in Figure 1-4.
1.2.2 Regional Studies
The present study follows other Cordilleran studies of large-scale sediment budgets that are inherently represented in valley landforms. Some important recent contributions are those of Brooks [1994] in the Squamish Valley (3850 km2), and Jordan and Slaymaker [1991] and Friele et al. [2005] in the Lillooet Valley (3150 km2) in the Coast Mountains. Work by Kovanen and Easterbrook [2001] in the Nooksack Valley and Collins et al. [2003] in the Nooksack, Skagit, and nine other major rivers draining the western slopes of the Washington Cascades have also shed light on the large-scale reworking of the landscape following deglaciation. In his study, Brooks concludes that reworked glacial sediments are a considerable component of the historical sediment budget, but not a dominant component in the late Holocene river load of Squamish River. The contrast among these valleys is instructive. Both basins in the Coast Mountains and numerous major valleys in the Cascades are affected by volcanic activity, and thus have experienced major disturbance and sediment inputs from both active and reworked sources [Friele and Clague, 2002]. In particular, studies in Lillooet Valley suggest that repeated volcanic episodes have come to dominate long-term sedimentary processes in the valley. Four landslides in excess of 1 x 106 m3 have occurred in the last century alone [Friele et al., 2005].
9 Chapter 1. Introduction
00’ N
°
Depot Creek
49
Bear Creek
2
Indian Creek
Brush Creek
Creek
Eocene Conglomerate
Chilliwack Batholith Oligocene - Pliocene Intermediate granodiorite, tonalite to quartz diorite, some gabbro.
Skagit Gneiss Complex Cretaceous - Tertiary Banded gneiss, some mafic orthogneiss,mafic migmatite, ultramafic rock, tonalite
Paleface
Easy Creek
Little
Post Creek
Chilliwack
Radium
Batholith Centre Chilliwack Creek tch
Hannegan Formation
Foley Creek Nesakwaek Shuksan Cre
(Silessia)
Amphibolites
Airplane Creek
Slesse Creek
Volcanic Rocks Miocene-Pliocene Andesitic to dacitic breccia, tuff
Aster
Yellow
Schist
Gneiss
Slolicum
Creek
Chipmunk
Group
Chilliwack
Borden
(Tomyhoi)
Creek
amihi
T
a-igneous rocks.
Cultus
Formation
ellow Aster Gneiss
Y
pre-Devonian (?) Well-layered pyroxene gneiss, calc-silicate gneiss and associated marble and met
Some quartz-rich tonalites.
Liumchen Creek
Shuksan Amphibolites Slollicum Schist Cretaceous - Tertiary Cretaceous - Tertiary Mid-amphibolite to andalusite- Greenschist-grade mafic to grade metamorphic rocks intermediate volcanics, phyllite, minor conglomerate.
Kilometers
20
15
10
0
5
faceous siltstone
Chilliwack Group Silurian - Permian
Volcanics: Calc-alkalic island arc, predominantly mafic volcanic flows and breccias.
Clastics: Phyllite, sandstone, semischist, greenstone or greenschist, minor chert, conglomerate
Calcareous Rocks
Late Triassic to Late Jurassic
Cultus Formation
Fine-grained sandstone Tuf
Limestone beds and lenses Dacitic tuff and flows
Nooksack Formation Jurassic-Cretaceous Argillite, sandstone, conglomerate, dacitic volcanic rocks
2.5
0
10 Figure 1-4: Lithologic assemblages of the study area. Chapter 1. Introduction
The collapse of volcano flanks in the valley at 8700 and 4400 years BP is estimated to have delivered 600 x 106 m3 of material to the valley. In the Squamish Valley, a debris avalanche from Mount Cayley (4.8 ka BP) instantaneously delivered 200 x 106 m3of material. Brooks points out that this is more material than all of the reworked glacial sediment from any given Squamish River tributary. Collins et al. [2003] state:
Voluminous lahars from eruptions of Glacier Peak volcano inundated the Cascade drainages of the Sauk, Skagit, and Stillaguamish Rivers (Beget, 1982); remnants of lahar deposits since incised by fluvial erosion can be found in each of these three valleys. At least 60 Holocene lahars moved down valleys heading on Mt. Rainier (Hoblitt et al. 1988). Mt. Rainier’s National Lahar travelled from the Nisqually River to Puget Sound less than 2200 years BP.
In comparison, the Chilliwack drainage appears to have been relatively quiescent through- out the Holocene. Although very large bedrock failures have been documented (some as large as 1.8 x 106 m3; Thomson [1999]), these rare deliveries are not as mobile within the drainage network as the lahars and landslides described above. Given, then, the relatively subdued pace of geomorphic processes throughout the Holocene, the valley provides an important opportunity to observe the course of relaxation from glacial disturbance with minimal large- scale overprinting by other perturbations. Accordingly, this thesis will explore some of the likely scenarios for the subsequent evacuation of glacial sediments, given the evidence that remains.
1.3 Thesis Structure
In order to address the question of sediment transfer on the Holocene time scale, it is essential to develop a quantitative framework that bounds the system through time and space. This is achieved by analyzing the geometry, mass, provenance and chronology of the deposits along the length of the system. Chronology is established using radiocarbon dating, providing an important time-frame for the deposition of valley landforms. While dating evidence is relatively sparse, further in- sights have been gained by applying numerical models to evaluate potential rates of sediment mobilization throughout the Holocene Epoch using data from the various study components. These different avenues of investigation can provide important checks on estimates of sedi- ment yield and evacuation of stored glacial material within the system.
11 Chapter 1. Introduction
Chapter Two examines the network structure of the valley, and estimates volumetric erosion and deposition of glacial and hillslope material within the postglacial time frame. Approximate volumes of erosion and deposition are calculated from air photos, digital eleva- tion models and land surveys. Chapter Three provides a detailed sediment budget at Chilliwack Lake for the postglacial period. The bounding volume of the lake bed layers and deltas provide an integrated picture of sediment yield at the scale of 5th to 7th order catchments within the valley network. The history of deposition in the lake is constrained using evidence from paleomagnetism and radiocarbon dating. Chapter Four presents architectural and volumetric analysis of major landforms in the lower valley. Isopachs of erosion are generated for the major landforms, allowing estimation of the long term yield to the lower valley and Vedder Fan. Large glacial deposits such as Ryder Lake Upland and Larson’s Bench are the dominant components of the postglacial sediment budget. The geometry of Vedder Fan provides some constraints on summary yield from the catchment. Chapter Five looks at the geochemical, lithological and sedimentological character of sediments in source areas and along the length of the mainstem. The grain size distributions of major landforms and active channel sediments along the drainage network are characterized to further establish the pathways of sediment transport. Important landform elements have been sampled in order to determine the relative sequestration or transfer of various grain-size fractions. Source ascription and rates of mixing for fine sediments are presented, for both fine and coarse sediments. Evidence from geochemical analyses and patterns of grain lithology along the mainstem are used to supplement the mass balance calculations. Chapter Six integrates the above information in order to develop a temporal and spatial picture of sediment transport within the valley mainstem. Using conventional hydraulics and sediment transport equations, it is possible to model the passage of the paraglacial sediment wave using a simple 1D finite-difference framework. Certain boundary conditions are assumed, and the sensitivity of those assumptions and other parameters is examined.
12 Chapter 2
Hillslope and Tributary Sediment Stores
2.1 Introduction
This chapter examines the volumetric balance of erosion and deposition of material from Holocene weathering processes, as well as from primary (outwash and morainal material) and secondary (colluvium, paraglacial fans and valley fill) remobilization of glacial sediment stores. The thesis seeks to develop the post-glacial sediment budget for Chilliwack Valley moving from the hillslope scale to the broad river reaches of the lower valley. The aim of this chapter is to estimate the long term yield from the major tributaries (5th and 6th order) to the mainstem. Following a summary of methods and potential errors in the analysis, the chapter is broken into three major topics: (1) the structure of the valley network, (2) sediment mobilization and deposition at the hillslope scale, out to the scale of the major tributaries, and (3) estimation of the volume of glacial fills in the lower reaches of the major tributaries. Many of the landforms within the upper drainage network of Chilliwack Valley retain aspects of their early Holocene, post-glacial configuration allowing some assessment of the total volume of sediment evacuated over time. Steep drift-mantled slopes have delivered abundant material to the lower slopes and valleys immediately following deglaciation. Till has been incised and mobilized as debris flows and debris slides over the postglacial period. Various authors have proposed that valley-side till may have been eroded, redeposited and stabilized within a timescale of decades or centuries rather than millennia (Eyles et al. [1988]; Evans and Clague [1988]; Jackson et al. [1989]; Ballantyne and Benn [1996]; Harrison [1996]; Curry [2000]). Diffusive erosional processes have subsequently smoothed topographic breaks and lowered landform surfaces [Putkonen and O’Neal, 2005]. Much of the mass eroded from hillslopes and glacial landforms, particularly the coarser
13 Chapter 2. Hillslope and Tributary Sediment Stores
Erosion (Output)
Deposition (Input)
Post-Glacial Surface - V 0 Valley Fill Bedrock
Figure 2-1: Definition of the reference post-glacial surface. Material deposited on top of the post- glacial surface constitutes an input to the post-glacial budget, material eroded from the valley wall and glacial fill are considered to be outputs. The reference postglacial topography is V0. sediment fractions, remains within the valley network, modified and remobilized occasionally by mass wasting and by the fluvial system. Rates of sediment sorting and transfer are dictated by the relative position of the deposit within the network hierarchy: erosive capacity is controlled by the total upstream area and local slope, which may be tied to the history of base-level fall in the downstream catchment. Headward migration of degradational trends appears to be a strong controlling factor in the erosion of glacial and paraglacial landforms in many Chilliwack tributaries. The proposed methodology for analyzing the valley sediment budget is to construct a postulated pre-erosional (i.e. end-Pleistocene) surface and to calculate erosion from, and de- position upon, such a surface (Figure 2-1). A similar approach has been used in other studies in which a simple and well-dated initial surface is available (notably Ollier and Brown [1971]; Gorler and Giese [1978]; Ibbeken and Schleyer [1991]). Given the reasonably good under- standing of the end-glacial topography in Chilliwack Valley, this technique holds some promise for unravelling the magnitude of erosional and depositional volumes within the Holocene Epoch. Where possible, maximum and minimum estimates are generated in order to assess the magnitude of potential errors.
14 Chapter 2. Hillslope and Tributary Sediment Stores
2.2 Data Sources and Associated Errors
Most of the analysis of the sediment drainage network and landforms was done using the TRIM (British Columbia Terrain Resource Information Management [1996] digital basemap) provincial dataset (sheets 092H001-004 and 092H011-013) and United States Geological Sur- vey 10 m DEMs. Calculations of erosion and deposition of landforms was mainly carried out using the TRIM and USGS DEMs of the valley topography, supplemented with some topographic field surveys and air photo measurements. The drainage network was extracted from the DEM dataset using ESRI’s ArcGIS hydrological toolbox. Calculation of terrain curvature was carried out using the Landserf package developed by Wood [2004]. The TRIM data are intended for work at 1:20 000 scale. Elevation points are 50-75 m apart, though 3D breakline vectors are more detailed in areas of high relief and complex topography. The suggested grid resolution of a digital elevation model from the point and breakline data is 25 m. Terrain mapping by J.M. Ryder and Associates [1995] was used to assist in the delineation of glacial deposits on the Canadian side of the border. This dataset is also intended for use at the 1:20 000 scale, and was not intended for high-resolution interpretation of landforms. Higher resolution (ca. 1:10 000) mapping by Rode [unpublished] and Thomson [unpublished] was consulted for the Ryder Lake Upland and Foley Creek, respectively. An inventory of over 1 100 landslides and other features such as bedrock failures was compiled using a single-year (1996) set of air photos [EBA Engineering 2002]. This provided a GIS-ready dataset that furnished information on the types and spatial distribution of mass failures. Measurement errors are a critical part of the analysis, yet the true nature of the variance in measurements is difficult to ascertain. Quantities measured are areas and volumes, as well as slopes of hillsides and fan deposits. Depths observed in the field at exposures can be easily measured to within ± 2-3%, as can surface area, subject to correct interpretation of the deposit stratigraphy. However, in the absence of natural exposures or geophysical soundings, estimates of deposit depths often require judgement and guesswork.
2.2.1 The Magnitude of Error
Errors involved in DEM analysis can be divided into at least three major categories: (1) DEM spot elevation and breakline errors, (2) surface interpolation and processing errors and (3) errors of geomorphic interpretation.
15 Chapter 2. Hillslope and Tributary Sediment Stores
Spot elevations collected by semi-automated photogrammetric methods are subject to a number of errors, which may be exacerbated by factors such as micro-relief, dense vegetation, shading or sloping ground. The accuracy specification of the TRIM product for spot elevation points is a Linear Mapping Accuracy Standard (LMAS) for the elevation of less than 5 metres, corresponding to a maximum RMSE of 3 metres [GDBC , 1992]. This likely varies over the landscape, depending on the overall roughness of the terrain. Given the high spatial autocorrelation of points in elevation datasets [Liu and Jezek, 1999] the error term may be somewhat less than 3 m. Some systematic error is evident in the study area DEM, most notably ‘stripes’ associated with photogrammetric lines [Albani and Klinkenberg, 2003] that impart a characteristic herringbone pattern to the topography. Spot elevations (mass points) and breaklines from the TRIM dataset were used to con- struct digital elevation models. A great variety of methods is available for surface interpo- lation, and there is no clear consensus on the best methods for mountainous terrain [Fisher and Tate, 2006]. Estimates of valley landform volumes consist of subtracting a DEM of the modern landscape from an approximation of the post-glacial landscape. Some error resides within the modern DEM, however, in most cases the larger error - which is, moreover, not exactly specifiable - will be in the coordinates of the posited post-glacial surface. Taylor [1997] shows the propagation of error in the general case of a parameter q derived from variables x, ..., z measured with uncertainties ∂x, ..., ∂z. Provided the uncertainties in x, ..., z are random and independent, the uncertainty in q may be calculated as:
sµ ¶ µ ¶ ∂q 2 ∂q 2 dq = dx + ... + dz (2.1) ∂x ∂z Many of the errors are random, and are likely compensating. The composite uncertainty terms for deposit area and depth are calculated for each landform and propagated in quadra- ture along the drainage network. The total error associated with volumetric estimates of each landform is determined by: q 2 2 2 Et = Ez + Eplan + Eintrp (2.2)
2 2 where Ez is the error incurred in deriving the vertical bounds of the deposit, Eplan is 2 the error related to delimiting the planform boundaries, and Ez is a composite error term related to errors of geomorphic interpretation. Volumetric estimates of landforms are most sensitive to the estimation of depth z of
16 Chapter 2. Hillslope and Tributary Sediment Stores aggraded deposits such as alluvial fans and valley fills. The error term is treated as random, however there does tend to be some systematic bias toward large estimates of deposit depth and thus higher volumes. The bedrock boundaries are assumed to follow the curvature of the hillslope above. In the case of fans and debris cones, the base of the deposit at the distal toe is interpreted to be the lowest point in the adjacent channel. Eroded valley fills have been reconstructed as longitudinally curved, laterally planar sur- faces. There is relatively less error in this case, since the reconstructed surface represents the average elevation of the deposit. In some cases, particularly where valley walls show a record of mass flow deposits and the elevation of the former alluvial surface is uncertain, a larger uncertainty term is assigned. Generally, given clear evidence of the former surface, the volumetric error for valley fills is approximately ±5%. Errors of interpretation are consequences of the many assumptions regarding the post- glacial landscape that must be made. These may include assessments of the geometry of the initial deposit, interpretation of a stratigraphic sequence, and/or inferences of post- depositional reworking. A further problem is the attribution of older tills and drift sequences in the valley to a younger event. Obviously many of these errors are difficult to assign outright, and must be applied with judgement. Planimetric area (x, y) of landforms can be established with an estimated precision of ±2%. The depth of the deposit may be determined with a precision of roughly ±10%. Smaller cones or fans are delineated by as few as 12-15 topographic points. The relative volumetric error in this case is potentially greater than for larger features. Given additional potential errors in defining the lower bounding geometry and interpretation of the extents, estimation of fan volumes typically carry an error of about 20%.
2.3 Network Structure and Process Domains
The spectrum of channel types within the tributary valleys ranges from steep first-order gullies with no channel bifurcation to larger fluvial basins with an evolved, branched network (Figure 2-2). Figure 2-3a shows examples of steep, linear hillslope gullies that deliver material directly to the alluvial floor of a 6th order channel. Sediment is stored at breaks in slope at tributary junctions, leading to higher apparent rates of yield from the lower-order catchments, whereas more storage occurs within larger basins having a developed fluvial system. Many of the larger, 6th and 7th order fluvial basins have an elongate, trellis form (Centre,
17 Chapter 2. Hillslope and Tributary Sediment Stores ° Depot Creek 49 00’ N Bear Creek Creek Indian Creek Paleface Brush Creek Lake Chilliwack Easy Creek Little Post Creek Chilliwack
121°30’ W Radium ntre Ce Creek h
Foley Creek Nesakwatc Creek Chilliwack River Valley. (Silessia) Pierce Creek Airplane Creek 20 km Slesse Creek Figure 2-2: Creek Chipmunk Borden (Tomyhoi) 10 Creek
121 50’° W Tamihi 5 15 0 Liumchen Creek (Damfino) Vedder Crossing
18 Chapter 2. Hillslope and Tributary Sediment Stores
3rd - Order ‘Convergent’ Basin
1st - Order Hillslope Gullies 5th - Order Fluvial Basin
Figure 2-3: Three different basin scales. The left side of (a) shows linear, first- and second-order debris flow channels, next to a 3rd order alpine colluvial basin, in the headwaters of the Slesse basin. Grid spacing is 500 m. (b) 5th-order Bear Creek, with a developed fluvial network (grid spacing is 1 km). Note the relatively small outlet fan in the latter.
Nesakwatch, Slesse, Depot), while others exhibit a more dendritic form (Paleface, Liumchen, and Tamihi). The trellis form lends itself to a particular regime of sediment transport. Stream power grows incrementally along the tributary length, as opposed to a more punctuated regime in a dendritic configuration (cf. Strahler [1964]; Gregory and Walling [1976]; Benda et al. [2004]). Material is discharged periodically from weathered bedrock exposures, gullies and zero- to second-order basins along the master channel. With delivery from the lower- order systems to a channel several order higher, there is an abrupt change in channel slope which leads to deposition on the footslopes. There may also be a greater degree of de-coupling between hillslopes and the main channel where a floodplain has developed in the higher order channel. Network topology for Chilliwack Valley was extracted from the BC TRIM DEM using Tarboton’s [1991] suite of ArcGIS tools, TauDEM. Links are segments of the channel network between junctions. Link position is specified by Horton’s ordering system, as modified by Strahler [Strahler, 1952, 1957]. Optimum resolution of the valley network was achieved with
19 Chapter 2. Hillslope and Tributary Sediment Stores
10 10 Chilliwack Valley, n=404
9 1 026 10 1.5274 y = 5436 e Distal fluvial 2 1 311 R = 0.9993 (Glacial trough) 8 10 2 343
2 3 169 7 10 5 641 Colluvial (Valley step) Area (m ) 11 327 6 10 n = 25 222 Hanging Fluvial
stream
5
Up 10 Colluvial All Chilliwack Network Links (Cirque walls) Mean Link Area by Order 4 10 Hillslope
3 10 0 1 2 3 4 5 6 7 8 9 10 Link Strahler Order Figure 2-4: Histograms showing the distribution of upstream area within valley network links, from headmost channels to the Chilliwack River at Vedder Crossing. The mean area for each order class is shown with a cross (+). The process domains identified by Brardinoni and Hassan [2006] are overlaid in grey. The total number of links for each order is indicated.
Chilliwack River at Vedder Crossing as an 8th-order channel. On 1:50 000 scale NTS maps, Chilliwack River is a 5th order channel, so the network is specified in greater detail. Horton [1945] identified a number of important scaling relationships among quantities such as the number of streams, length of streams, and slope of streams within a drainage network. Schumm [1956] refined a similar relationship for catchment area. He noted that the scaling among link areas for successive orders is approximately constant. This is shown for Chilliwack Valley in Figure 2-4. There is a systematic change in upstream catchment area relative to the position of each network link within the drainage network. Within the lower order links, catchment size is log- normally distributed across roughly two orders of area magnitude, reducing to one order of magnitude at 7th order. The log-normal distribution appears to break down in higher orders, presumably due to the greatly decreasing sample size. A regression through the mean basin area points indicates that scaling is quite consistent across five orders of area magnitude. Given the systematic change in channel slope and upstream contributing area (thus chan-
20 Chapter 2. Hillslope and Tributary Sediment Stores nel flow) with scale, there are distinctive process domains within each hierarchic level of the network. Such process domains were initially proposed by Montgomery and Foufoula- Georgiou [1993], and have subsequently been amended to included formerly glaciated land- scapes by Brardinoni and Hassan [2006]. The characteristic area scales for each of the process domains have been superimposed on the diagram (Figure 2-4). It is difficult to as- sign definitive process specifications or provide precise bounding spatial scales across such a diverse landscape; however, Brardinoni’s generalization of how sedimentary processes are conditioned by the large-scale imprint of glaciation on the landscape does hold up well in Chilliwack Valley.
2.4 Sediment Deposition in Lower-Order Catchments
Colluvial and alluvial deposits have been identified throughout Chilliwack Valley, using air photos, terrain maps and TRIM data. In the following analysis, the depositional volume in fans that are judged to be transport-limited and/or decoupled from their master channel is taken as an estimate of coarse sediment yield over postglacial time. These apparent yields are compared among catchment order and area, in order to estimate a bounding envelope to represent the magnitude of the total bedload/mass wasting yield from upland basins. Many deposits have a multi-phase history. Many fans in Chilliwack Valley are polyge- netic, built by a variety of active processes, ranging through debris flows, snow avalanches, and fluvial transport. In some catchments the sediment and flow regimes have changed dra- matically, leaving a smaller, low-gradient modern fan inset into remnants of a high-gradient paraglacial fan. Ryder [1971] notes that the rate at which fan dissection proceeds is likely to depend upon the erosive capacity of the tributary stream. Fan volumes and geometry are thus indicators of the colluvial/alluvial reservoir state and history, essentially, the equilibrium achieved between hillslope sediment yield and transport efficiency within the fluvial network. Alluvial reaches that are degrading will carry away their sediment stores, leaving smaller, incised fans. In Chilliwack Valley there are several states found:
• Transport-limited basins with coarse-grained alpine fans: exposed bedrock faces in the upper basins have enhanced rates of weathering and sediment delivery. These fans are mostly decoupled from the transport system, though some cutting of the fan toe may occur close to the higher-order receiving channel. Accumulation rates were probably quite high in the late-glacial period, as glacial ablation debuttressed many steep slopes.
21 Chapter 2. Hillslope and Tributary Sediment Stores
Rates have probably been significantly lower over the course of the Holocene Epoch. Fans tend to have a steep, straight to concave slope profile. A number of these features grade into debris or alluvial cones.
• Aggraded paraglacial mixed-transport fans: high-gradient fans built by both fluvial and mass wasting processes that have aggraded throughout the Holocene Epoch. The system is generally transport-limited, emptying into a trunk channel that is still choked with glacigenic material. Nesakwatch and Centre Creeks and some headwater basins have a number of examples. There are also many examples of such fans emptying into lower-gradient hanging valleys - the ‘sink-colluvial’ domain identified by Brardinoni and Hassan [2006].
• Degraded paraglacial mixed-transport fans: Since lowering of base level in tributaries, much of the glacial valley fill in the mid to lower tributaries has been evacuated. Fluvial channels have trenched the fan-head and carved away at the base of the fan, sometimes leaving only a few terraced remnants. A modern fan usually develops in the incised remains. Slesse and Tamihi have some examples of relict fans that have been cut away by the mainstem. Some valleys have had renewed episodes of fan aggradation after recent disturbances, most notably logging [Millar, 2000].
• Fluvial fans: outlets of higher-order basins have well-established fluvial fans, which are likely influenced by debris-flow events, but these are not a major construction agent. In some cases there may be relatively little inheritance from early Holocene paraglacial processes. Fan profiles are typically flat to convex.
There are numerous intermediate states; the interplay between basin size, order, slope and geometry, and characteristic grain size determine the balance achieved. 135 relatively intact fans have been identified throughout the valley, which incorporate both remobilized glacial material and Holocene weathering products, and thus potentially indicate relatively high rates of deposition over time compared to non-glaciated regions. Rates of yield have diminished over post-glacial time, and patterns of trees and shrub vegetation on fans or lichens on talus slopes highlight the large inactive zones on many of the deposits. Finer sediments have moved onward as washload and suspended load. The volume of each fan has been approximated using CAD geometric shapes (Figure 2-5). The methodology is based on the technique used by Campbell and Church [2003] in
22 Chapter 2. Hillslope and Tributary Sediment Stores
Figure 2-5: Diagrams illustrating estimation of fan deposits using the technique of Campbell and Church [2003] and using CAD shapes.
Lynn Valley, Southern Coast Mountains. An improvement afforded by CAD shapes is the ability to refine curvature to match the fan’s frontal slope, to adjust to compound slopes on the bounding valley wall, and to construct complex, coalescing fans. Accumulations in talus slopes were generally assumed to have a triangular prismatic shape. The cross sectional geometry was interpolated over a linear or curved path along the lower slope. The largest source of error remains the lower bounding surface, which is usually approx- imated from the valley wall geometry and the height of the valley floor. It is possible that more volume is obscured below as the valley walls may not be planar, and the deposit may interfinger with floodplain and valley fill. Figure 2-6 displays the relation between fan volume and upstream catchment surface area. Points are distinguished by basin order. Each landform ostensibly represents 13 000 years of coarse material accumulation on the lower slopes of valley walls. It is assumed that finer material moves onwards, and there is variable preservation of fine material in the fans. The relation exhibits considerable scatter which can be attributed to measurement error, as well as a complex combination of geologic and morphometric variables. Grouping the basins by geology or by slope class explains only a relatively small proportion of the fan-size variability. Variables related to hypsometry and vegetative cover presumably must also play a role, due to the accelerated weathering in exposed environments at higher elevations. There is a scale-dependent transition from catchments with steep, linear morphology to drainage basins with greater bifurcation and thus sediment storage; this results in two distinct patterns in the dataset (Figure 2-6). The smaller, steeper low-order basins are able to funnel
23 Chapter 2. Hillslope and Tributary Sediment Stores
1E+9
1.5 1.5 2nd Order y = 0.015x y = 5E-05x 3rd Order )
3 1E+8 4th Order 5 5th Order 3 4 6th Order 1E+7 7th Order 1
1E+6 2
1: Pierce Creek 2: Brush Creek 1E+5 Estimated Deposit Volume (m 3: Paleface Creek 4: Depot Creek 5: Upper Chilliwack River
1E+4 1E+4 1E+5 1E+6 1E+7 1E+8 1E+9 Upstream Catchment Area (m2)
Figure 2-6: Estimated Holocene bulk sediment deposition (sand and coarser) plotted against con- tributing catchment area. Catchments are coloured according to their Strahler order. Two reference lines are shown, indicating that the envelope of maximum deposition scales approximately to the power of 1.5. A similar exponent is used for scaling the size of fans and deltas from larger fluvial basins. and accelerate the transit of water and debris, whereas slope breaks within higher order catchments presumably tend to promote shorter step lengths relative to the drainage basin size, thus more sorting and storage occurs. Points 3,4 and 5 in Figure 2-6 indicate end-point sedimentation in Chilliwack Lake. Pierce Creek is a steep 5th order basin (6.8 km2 in area (Figure 2-2) near the Slesse Creek confluence, that has deposited most of its post-glacial load in a conical fan on a terraced glaciofluvial plain 50 m above the mainstem. The total fan volume is approximately 2.3 x106 m3. In a single debris flow event, the creek delivered roughly 63 000 m3 of material, with a recurrence interval estimated to be in the range of 50-100 years Thomson [1999]. The source area of this debris flow was in the steep lower portion of the basin; the rest of the basin is otherwise relatively stable and there is considerable storage of material. Examining Figure 2-6 (‘1’), Pierce Creek sits closer to the fluvial basin trend, with more subdued rates of sediment delivery over the long term. Information on the total Holocene yield from upland catchments in Cordilleran British Columbia is relatively scarce. There are, however, a number of data sets that emerge from
24 Chapter 2. Hillslope and Tributary Sediment Stores
Figure 2-7: Pierce Creek, July 2003. Source area of the 1996 event is shown with an arrow modern, decadal-scale studies with which to make comparisons. Many of these studies have focussed on the relatively softer, volcanic terrain, which may have yielded rates an order of magnitude greater than the metamorphic and granitic terrain considered here. In the Lillooet Valley, roughly 150 km to the North in the Southern Coast Mountains, Jordan and Slaymaker [1991] estimated an average magnitude and frequency of debris flows from basins as follows. For “small” debris flows, an average yield of 20 000 m3 every 10 years is assumed, 25% of which is delivered onward to the channel. For “large” debris flows, an average yield of 50 000 m3, at the same frequency was used. At several sites in the southwestern British Columbia, including Lillooet, Squamish, and Lower Fraser valleys, Jakob and Bovis [1996] estimated volumetric output from 34 notable debris-flow basins, ranging in size up to 15 km2. The magnitude of sediment delivery was correlated with a number of morphometric characteristics. The authors noted that most debris flows occur with magnitudes in the range of 25 000 to 30 000 m3 for a single event - with large flows ranging up to 200 000 m3. Smaller events may not be detected in the dendrochronological and depositional record. Recurrence intervals were commonly less than 10 years. Another data set with which we can compare is that of Kirchner et al. [2001], Figure 2-8. Their database provides estimates of fan accumulation over the course of up to 30 ka in the unglaciated Idaho Batholith. The long-term estimates of averaged annual accumulation
25 Chapter 2. Hillslope and Tributary Sediment Stores
104
103 )
-1 Vedder Fan /a 2
102 Specific Yield (T/km 101
100 104 105 106 107 108 109 1010 Upstream Area (m2)
Figure 2-8: Data from Kirchner et al. [2001] (black diamonds) with data from the Chilliwack Valley overlain (open circles). Sediment accumulation estimates from Chilliwack Valley display more scatter, but they are largely consistent with the long-term average rates of specific sediment yield calculated by Kirchner et al. [2001]. in Chilliwack Valley seem to be in line with the pattern in Kirchner’s dataset, although the effects of glaciation in the Chilliwack dataset probably result in higher apparent rates.
2.5 Sediment Source Areas
2.5.1 Large Bedrock Failures
Rocks of the Chilliwack Group and Cultus Formation are noted as being somewhat more susceptible to large-scale mass failure than are crystalline lithologies, but this is not a highly significant trend [Thomson, 1999]. The Chilliwack Valley does not have large outcrops of tuffs, breccias and other volcanic lithologies that have often been implicated as indicators of enhanced debris flow activity [Rood, 1984; Jordan, 1994; Jakob and Bovis, 1996; Jakob, 2005]. Some of the larger failures in the basin, such as the Pierce Creek landslide, are related to seams of limestone running through the Chilliwack Group. Others are related to planes of weakness found within phyllitic rocks.
26 Chapter 2. Hillslope and Tributary Sediment Stores
The nature of tectonic building by thrust faulting in the region has led to some degree of asymmetry in valley sectional profiles [Groulx, 1993]. The southwest-facing sides of the Airplane, Lower Chipmunk, Nesakwatch and Centre Creek valleys are part of a thick, tabular body of amphibolite rock that conformably overlies sedimentary strata [Monger, 1966]. The hanging wall of this body is more susceptible to erosion, which appears to have led to over- steepening over the course of glaciation and enhanced gully dissection over time. This may account for higher total rates of erosion in Airplane, Foley, Nesakwatch and Centre Creeks. Twenty two historic bedrock failure zones are described by Thomson [1999] and EBA [2002] in the Canadian portion of the study area, totalling 4.5 km2, for an overall density of roughly 0.6 percent. Rates of delivery from deep-seated bedrock sliding in Chilliwack Valley are lower than in the neighbouring Nooksack drainage, where Easterbrook et al. [2007] documented at least 13 large events that have occurred in postglacial time. Several of the landslides cover 10 km2, and the largest (the Church Mountain Slide) delivered an estimated 2.83 × 108m3 at approximately 2 500 BP [Carpenter, 1993]. In Chilliwack Valley, some of the larger source areas are roughly 0.5 km2, with historical deliveries on the order of several tens of millions of cubic metres. Onward mobilization within the fluvial network in the post-glacial era is generally quite limited, however, there is often a profound impact on patterns of sediment transport and channel configuration. Foley Creek is a notable example, with a mid-valley lake (0.1 km2 in area) created behind the debris of a large bedrock slide. A lake delta and a “forced” alluvial channel [Montgomery and Buffington, 1997] have been created upstream of the lake.
2.5.2 Gullies and Diffusive Slope Processes
In order to match the observed patterns of detrital deposition in the upland basins with likely source areas, gross rates of mobilization were estimated throughout the catchment. The hillslope areas were divided into three major type areas (gullies, soil-mantled, and bedrock terrain) where the active geomorphic processes and regime of sediment delivery are likely to be different. Bedrock areas were distinguished from vegetated zones using Landsat spectral imagery; gullied zones were identified using measures of planform concavity extracted from the TRIM DEM. Gullied terrain is found primarily within the soil mantled hillslopes of the valley, but also includes portions of the bedrock zone. It is assumed that a majority of the volume evacuated from lower post-glacial hillslopes is represented by these planform concavities, often carved
27 Chapter 2. Hillslope and Tributary Sediment Stores
8%
Vegetated Open Slopes Bedrock Open Slopes 6% Convergent Gullies
4% Percent Plan Area 2%
c
S = Critical Slope
0 0 0.5 1 1.5 2 Gradient
Figure 2-9: Histogram of gradient distributions in gullied topography and on planar to convex hillslopes (vegetated and bedrock) in the first to fifth order catchments of Chilliwack Valley. A limiting gradient for the vegetated open slopes is estimated to be 1.4 into till blankets, colluvium on steep slopes and into jointed, weathered bedrock. The second two zones are planar to divergent hillslopes, which are less active than gullied areas, but occupy a considerably greater proportion of the landscape. Gullied terrain accounts for roughly 14% of the valley hillslope area, the bedrock zone accounts for 16%, and vegetated slopes 70%. On soil-mantled hillslopes, both slow, quasi-continuous diffusive processes and more episodic shallow failures deliver sediment to the drainage network. The slow diffusive processes include creep, slope wash, tree-throw and bioturbation. Figure 2-9 shows the distribution of slopes within the three zones: convergent gullies, and convex-to-planar vegetated and bedrock terrain. The vegetated zone has only a very small fraction of its area on slopes greater than 1.4 (54.5◦), while gullied topography and open-slope bedrock zones both extend into a steeper range. This would suggest that there is a threshold slope within the soil-mantled zone beyond which much greater rates of failure tend to occur. This is explored further below; the behaviour is consistent with a number of non-linear models of hillslope evolution [Howard, 1994; Roering et al., 1999; Martin, 2000; Montgomery and Brandon, 2002; Dietrich et al., 2003], and this type of model is used here to approximate rates of flux from the planar slopes within the valley.
2.5.3 Sediment Source Areas: Surficial Materials and Gullied Terrain
Blankets of morainal material and colluvium cover most of the mid- to lower slopes in the major Chilliwack Valley tributaries. During alpine glaciation, ice lobes descended from cirque
28 Chapter 2. Hillslope and Tributary Sediment Stores basins and moved out to fill the 5th to 8th order valleys, building moraines at their lateral extents. Lateral gullies and debris flow basins delivered sediment to the sides of the glacier. Many of the deposits on valley walls have been gullied and incised, remobilizing relict glacial material to the lower footslopes and to the fluvial system. Gullied sections of these deposits reveal quite variable depths; road cuts show depths of up to several tens of metres. Gullies on steep valley walls and interfluves tend to have simple, linear geometry with limited branching in the upper reaches. Hillslope gully cross-sectional geometry is typically a ‘V’ shape with vertical depths typically on the order of 5 to 30 m and widths of roughly 10 to 60 m (Figure 2-10a). These planform concavities are regions of groundwater and overland flow convergence that promote the most effective sediment mobilisation. Exposures of incised materials in gullies reveal more than uniform morainal drift. An example from a second-order tributary is shown in Figure 2-10(b) and (c). Remnants of similar stratigraphy at about the same elevation (lower to mid-slope) are found throughout neighbouring tributaries. Significant deposition and ponding against ice occurred over the course of glaciation, leaving thick deposits of glaciolacustrine material, as well as a chaotic mix of colluvium, slope wash and fluvial deposits. As the ice pulled away, rapid incision likely occurred in the steeper reaches of each gully. Stratigraphy is also visible in the active headscarps of failures throughout the watershed. The headwall sections of some headscarps show a compacted diamicton, with subrounded to angular, glacially abraded material. A number of headscarps examined show an upper, dry weathered horizon and a wet lower section (Figure 2-11). There may be preferential failure of finer grained materials that retain moisture better than coarser grained deposits which are better drained (B. Thomson, pers. comm.). There is significant weathering of these materials as well, gradually reducing stability over time. Materials fail by headward retreat, with episodic transport of the sediment mass over time. Material from slope wash and creep may fill declivities, gradually building up over time. This material may be released as a debris flow following the occurrence of major rainstorm event, once a critical limit of accumulation has been reached. EBA [2002] conducted a study of mass wasting in the Chilliwack Valley, identifying over 1 100 modern and historical slides. This inventory provides a window of landslide activity that is visible on the landscape in 1996 aerial photography. The age of the slides has been roughly estimated, and the inventory includes historically reactivated features such as large, chronic bedrock failure zones.
29 Chapter 2. Hillslope and Tributary Sediment Stores
a) c) C S S G Top of sequence, weakly bedded 448m Clasts up to 30mm. Sandy matrix with marginal soil development. Clay layer with drop stones 25mm+
Colluvial diamict. Poorly sorted with clasts 256mm+. Mostly 447 subrounded/subangular meta-seds. Some fluvial bedding toward lower contact
446 Chaotic, unsorted sequence of highly weathered material overlain unconformably by clay layer. Abundant evidence of Fe leaching through profile from 445 moisture in upper layer. Bedding dipping gently upwards, down-valley
Sand and pebble fluvial units - very well sorted. Intermitted band 444 of clay along the section.
Bedded fluvial gravels and silt. b) Rounded clasts 50mm+ 443
Silt lense (5-10cm) dipping 442 steeply downvalley.
Assorted fluvial bedding - particles up to 150mm.
441 Bed of silt with wavy and broken bands of interlensing sand. Stones up to 15mm
Coarse diamict with 440 boulders up to 300mm+. No apparent bedding, same ? ? pattern of Fe leaching as above - moisture emerging ? ? ? from overlying silt layer. ? Highly weathered.
Figure 2-10: Photos of (a) Holocene channel incision and (b) exposed sidewall deposits of till and glacio-lacustrine material. (c) is a stratigraphic section from a left-bank tributary to Lower Slesse Creek. Total incision is over 25 m, but the exposed 10 m section gives some idea of the complex nature of the initial (Pleistocene) hillside stratigraphy. Some similar sections were found in nearby tributaries, but exposure of such a stratigraphic record is uncommon.
30 Chapter 2. Hillslope and Tributary Sediment Stores
Upper, Oxidized Layer
Saturated Zone
Figure 2-11: Photo and figure of a landslide headscarp at Foley Creek. This is the initiation point of a debris flow gully, incised into a sandy, compacted till deposit.
Most shallow landslides occur within the convergent, gullied topography described above. The bounds of such areas were delineated using indices of planform convergence such as may be generated using tools such as LandSerf [Wood, 2004] and ArcGIS. In Landserf, a 60 x 60 m window was used to identify zones with negative plan-form curvature (i.e. concavity). A threshold value was then found (-0.25) to extract the gully boundaries - spurious depressions were also picked up, but these could be reduced using a low-pass or majority filter. 90% of the debris-flow tracks identified in the EBA database are found within these zones. The channel within these gullied zones is steep, and the gullies extend only for short distances, generally less than a kilometre. Figure 2-12 is a plot of upstream contributing area versus slope for all of the gullied (convergent) topography within Chilliwack Valley. Most of the points cluster near a gradient of 0.7 (35◦), with lower slopes for larger gully basin areas. Most of the material on slopes steeper than 1.2 to 1.5 (50◦- 56◦) has already failed. Headscarp points of historic debris-flow gullies identified in the EBA database are overlain, and would appear to occupy roughly similar space in the landscape. These slopes were interpolated from a 25 m DEM and are thus approximate. Gullies and incised terrain in the original DEM were ‘filled’ by interpolating across the boundaries of the zones of planform convergence. The resultant smoothed topography consti-
31 Chapter 2. Hillslope and Tributary Sediment Stores
0 10
Gradient (m/m)
-1 10 2 3 4 5 6 10 10 10 10 10 2 Upstream Catchment Area (m )
Figure 2-12: The distribution of hillslope gradients within the gullied topography of Chilliwack Valley. Contours indicate the cumulative density of points, in increments of 0.2. The slopes at the initiation points for 492 landslides in the EBA database are overlaid. tutes a putative ‘pre-erosional’ surface. An index of gully erosion volumes was then generated by subtracting the original DEM (i.e. modern) surface from the pre-erosional (i.e. constructed post-glacial) surface. The procedure involves some important assumptions about where erosion has occurred in Holocene time, and is subject to errors inherent in working with a 25 m2 grid representation of the landscape, particularly at hillslope scale. The method nevertheless provides a spatially distributed estimate of the total mass eroded from gullies (Table 2.1). Some tributaries, notably Nesakwatch Creek, have higher rates of specific erosion that may be attributed to a denser network of gullies within the drainage. The volume of gully erosion is consistent with (equal to or greater than) the volume of coarse material estimated to have been deposited at the base of the slope for 66% of the depositional sites discussed in Section 2.4 (summarized in Figure 2-6). Outlet deposition is of course an imperfect measure of the total sediment output, and does not account for finer sediment deliveries that have moved onward. There are additional contributions from planar
32 Chapter 2. Hillslope and Tributary Sediment Stores to divergent slopes are important to the sediment balance as well, and these are calculated below.
Table 2.1: Estimated volumes eroded from gullied morainal and colluvial cover
Tributary Eroded Bulk Specific Bulk Volume (m3 × 106) Volume (m3/km2) Liumchen 34 ±4.7 48 ±6.6 Tamihi 97 ±13 57 ±7.7 Slesse 152 ±20 72 ±9.8 Chipmunk 33 ±4.5 57 ±7.7 Foley 71 ±9.7 70 ±9.5 Nesakwatch 96 ±13 130 ±17 Centre 57 ±7.8 110 ±15 Paleface 38 ±5.2 79 ±10 Depot 66 ±9.0 88 ±12 Upper Chilliwack 177 ±23 72 ±9.8
Slope classes represented within the gullied zones are shown in Figure 2-13. The overall distribution in the modern DEM is steeper than the “post-glacial” surface because material has been eroded, leaving steeper channels and side-walls in the gullies. The EBA database is overlain (lighter gray) to show where active erosion is occurring in the modern landscape. The total planimetric area occupied by each erosion depth class within the gullied zones is shown in Figure 2-14. The inferred depths of erosion over the Holocene are typically in the range of 1-8 vertical metres (mean = 6.3, median = 4.4), with some depths ranging to over 30 m. Cumulative volumetric erosion among 17 slope classes is shown in Figure 2-15.
2.5.4 Sediment Source Areas: Open Slopes
Surficial failures on open to convex slopes do not leave the same prominent signature on the landscape as gullies, but are nevertheless an important mechanism for sediment delivery over the Holocene time scale. Shallow landsliding occurs along the interfluves of some of the major valleys and on many of the steep valley slopes. The gullied zones described above are also pathways for material delivered from these upslope areas. In the alpine areas of the catchment, conditions are generally weathering-limited and slopes may attain relatively steep gradients. Broad swaths of exfoliating bedrock surfaces are evident in the alpine zone, and large accumulations of sediment are found at the base of many bedrock slopes. Products of continuous processes such as ravel and spalling, and of
33 Chapter 2. Hillslope and Tributary Sediment Stores
120000 120
Reconstructed (Postglacial) 100000 DEM Slopes within Hollows 100 and Gullies
80000 Modern (TRIM) DEM 80 Slopes within Hollows and Gullies 60000 60
40000 40
Topographic Points within Gullies 20000 20
EBAEBA Database Database Number of Active Headscarps (EBA) 0 0 0 10 20 30 40 50 60 70 80 90 Slope Angle (q)
Figure 2-13: Hillslope relaxation response. Histograms indicate the distribution of DEM slopes that are found within the gullied zones. The modern (TRIM) DEM is shown in darker gray; the inferred post-glacial distribution is outlined in black. The shift in the curve describing the distribution of slopes indicates that the modern (TRIM) surface is left steeper after the morainal and colluvial material has been evacuated, due to establishment of steep gully sidewalls.
1E+6
2E+5
1E+5
1.5E+5
pixles (Linear) 1E+4
2 2 1E+5
1E+3 5E+4
Number of 10m Number of 10m pixles (Log)
0E+0 1E+2 0 5 10 15 20 25 30 35
Eroded Volume (Vertical Meters)
Figure 2-14: Results from the subtraction of DEM surfaces in gullied zones, over the entire Chilliwack Valley. Graph shows the distribution of maximum depths of eroded volume. Linear scale for bar chart is shown on the left, log scale for the curve is on the right.
34 Chapter 2. Hillslope and Tributary Sediment Stores
1E+9 100
) 8E+8 80 3
6E+8 60
4E+8 40
Cumulative Erosion2E+8 (m 20
MeanDepth of Vertical Gully Incision (meters, w/Std Dev)
0E+0 0 5 5 0 5 0 0 0 0 5 0-5 1 -2 -3 -3 -4 -45 -5 -6 -65 -7 -7 -85 5-10 0 5 5 5 5 0 5 0 10- 15-20 2 2 30 3 40 4 50-55 5 6 6 7 75-80 80 Slope Category
Figure 2-15: Cumulative erosion from gullied hillslope sources in Chilliwack Valley by slope class. Maximum rates of erosion occur in the range of 35-40◦. Mean depth of vertical erosion increases with slope. Depths of erosion are indicated with bounds of one standard deviation. freeze-thaw contribute to the sediment cascade on lower, soil-mantled slopes, or may fall into long-term storage. Overall, alpine zones do not provide as much material to the drainage network as processes operating downslope, in gullies and on soil-mantled surfaces. Forested slopes do not attain the same steep hillslope gradients as alpine zones, presum- ably due to the lower cohesive strength of the surficial material. The histogram of hillslope gradients (Figure 2-9) shows some indications of a stability threshold, suggesting that a non- linear model may be most appropriate for representation of slow, continuous processes such as creep, slope wash, tree-throw and bioturbation and more episodic, shallow failures. In this type of model, erosion rates increase in a nearly linear manner for a shallow range of slopes, then rapidly increase for steeper sections, up to a critical gradient. Beyond this threshold, rates are limited by supply. The equation proposed by Roering et al. [1999] is · ¸ K1∇z qs = 2 (2.3) 1 − (|∇z|/Sc)
in which qs is the volumetric sediment transport rate, ∇z is the local slope, Sc is the 2 critical slope for failure, and K1 is the diffusion coefficient, in units of m /yr. This equation was initially intended for use on soil-mantled terrain, calibrated using a relatively finely spaced (4 m) DEM [Roering et al., 1999]. Roering et al. found that the
35 Chapter 2. Hillslope and Tributary Sediment Stores local erosion rate was dependent on the local curvature of the landscape, and this curvature may not be accurately resolved on coarser DEMs, such as employed here. It has been found, however, that the equation has wider applicability to bedrock terrain at a regional scale, over millennial timescales [Montgomery and Brandon, 2002]. Its use here is intended as an approximate index of erosional potential, with the magnitude of the diffusion term constrained by the estimated volumes of depositional forms along the length of the drainage network. The equation is applied to the TRIM and USGS DEMs of the study area, calculating the potential erosion for the local slope at each grid cell within planar bedrock and vegetated zones, integrated over 13 ka. This summation of flux rates provides an index of the total volumetric sediment load mobilized within each major catchment, neglecting for a moment the effects of intermediate storage (Table 2.2). The fit between this index value and the volumes estimated to have been deposited at each cone/fan deposit (Figure 2-6) is much better than any other single index, particularly for lower-order basins. In their work in the Oregon Coast Range, Roering et al. [1999] found the optimum fit 2 to their hillslope flux data was a diffusivity term (K1) range of 0.0031 to 0.0045 m /yr.
The critical slope value, (Sc), was found to be have an optimum range of 1.2 to 1.35. In their model fit, the critical slope value was found to be less sensitive than the diffusivity term. In order to balance hillslope erosion with the depositional volumes found on hillslopes throughout Chilliwack Valley, a diffusivity range of 0.006 was selected for the soil mantled areas, and 0.003 for the bedrock zones. Long-term flux rates for the North Cascades landscape are expected to be higher due to the tills that blanket many of the hillslopes. The critical slope chosen for the soil mantled zone was 1.4. A limiting gradient of 2 was chosen for the bedrock zone, however, since the slopes do extend to a much steeper range, a constant rate was assumed beyond that limit. Figure 2-16 shows the volumetric estimates of catchment channel erosion versus the fan volume for a number of basins. If points plot above the 1:1 line in the figure (deposition > erosion), this would indicate that either erosion is under-estimated or deposition is over- estimated. As the basin scale increases, storage becomes a more prominent effect, and the quality of the match between erosion and yield is reduced. The ratio between erosion and storage is highly variable, with most 2nd order basin fans holding 10% to 100% of the esti- mated erosion, and higher order catchments (5-6th order) generally capturing less than 10%. Uncertainty for measurements of all the fan features is conservatively estimated (i.e. worst case) as ± 30%.
36 Chapter 2. Hillslope and Tributary Sediment Stores
Table 2.2: Estimated volumes eroded from planar to convex slopes
Tributary Eroded Bulk Specific Bulk Volume (m3 × 106) Volume (m3/km2) Liumchen 197 ±32 279 ±45 Tamihi 391 ±57 228 ±33 Slesse 513 ±73 244 ±34 Chipmunk 125 ±19 214 ±33 Foley 253 ±35 248 ±35 Nesakwatch 182 ±23 247 ±31 Centre 125 ±16 240 ±30 Paleface 110 ±14 225 ±29 Depot 154 ±19 204 ±26 Upper Chilliwack 528 ±71 216 ±29
108 2nd Order 3rd Order 1:1 1:2 4th Order 1:10 107 5-6th Order ) 3
106 Volume Deposited (m 105
104 105 106 107 108 109 Volume Eroded (m3)
Figure 2-16: Volumetric estimates of volumetric erosion vs. fan volume (bedload) are compared in a number of basins within the Chilliwack Valley, to assess the agreement between estimates. Symbols are the same as in previous figure.
Figure 2-17a presents the summation of the volume of material eroded from each grid cell in each basin, according to Equation 2.3, with no intervening storage, routed from 3rd to 8th order links. The process domains across basin scales exert a distinctive influence on the
37 Chapter 2. Hillslope and Tributary Sediment Stores pattern. Catchments with areas of up to 5-6 km2 (3rd - 4th order) show higher variance in yield, but also achieve higher delivery rates due to steeper slopes. The scatter in the pattern of specific erosion decreases at larger basin scales due to the integration of yields across a much larger contributing area. Floodplains begin to emerge in smaller tributaries such as Centre Creek and the Upper Chilliwack River as catchment plan area grows to roughly 35 km2. Figure 2-17b shows how storage varies across the basin scales, shaping the overall pattern of specific yield in the valley. Deposition volumes (Figure 2-6) are plotted under the envelope of erosion potential. Rates of mobilization in the headwaters are relatively high, due to higher slopes, accelerated weathering, and the large load of glacial material that has been evacuated from basins. Moving from 3rd up to 6th order catchments, the specific yield of sediment is at least one order of magnitude lower, highlighting the deposition of material in fans and colluvial or alluvial reservoirs along the drainage network. The long-term yield from 5th, 6th and 7th order basins, such as Paleface, Depot and Upper Chilliwack Creek are plotted on the diagram. These are terminal deposits, emptying into Chilliwack Lake, and are estimated to represent essentially the entire yield from the Holocene Epoch (Chapter 3). The pattern of yield downstream in the 6th to 8th order catchments is one of gradually increasing specific yield over the long term. This is inferred to be the result of past remobi- lization of large glacial deposits that filled the lower valley. The extents of these deposits are explored in the following section.
2.6 The Fluvial Domain: The Lower Tributary Valleys
Examination of the physiography of the 6th and 7th order Chilliwack tributary valleys (see Figure 2-2) reveals that intensive erosion by valley glaciers in the lower-order basins has provided an abundant sediment supply that filled the lower portion of the larger tributary valleys and the mainstem beyond. The slope of each channel reduces gradually as it emerges from the rocky alpine headwaters. Figure 2-18 shows a comparison among 6th and 7th order tributary channel long-profiles, illustrating the results of interplay between bedrock structure, glacial and fluvial erosion. There is an evident knick point in most channels, marking a transition from an undulating or roughly convex-up profile, to the typically smooth concave- up curve of a fluvial channel (Figure 2-18). This also indicates the transition from the dominant influence of episodic transport processes in the headwaters to fluvial transport in the mid to lower valley. The transition occurs at a point in the basins with upstream areas
38 Chapter 2. Hillslope and Tributary Sediment Stores
3x103
/yr) 3 2 10 Floodplains Develop Specific Erosion (T/km
102 104 105 106 107 108 109 Upstream Catchment Area (m2)
4 10 Hanging Fluvial Colluvial Valley Step Fluvial Remobilization (Cirque Walls)
K1soil = 0.006, K1rock = 0.003 103
)
-1
/a
2
/km Chilliwack Lake
102
2nd Order 3rd Order
Specific Yield (T 4th Order 1 10 5th Order 6th Order 7th Order 8th Order 100 104 105 106 107 108 109 Upstream Catchment Area (m2)
Figure 2-17: (a) Combined specific erosion potential for bedrock, gullied and forested slopes, calcu- lated for a proportion of all links in Chilliwack Valley (K1soil = 0.006, Sc = 1.4 for forested terrain, 3 K1rock = 0.003, Sc = 2 for bedrock, assumed deposit specific density of 1.6 kg/m ). A maximum rate of erosion is attained in some catchments smaller than 1 km2. (b) Inferred rates of tributary coarse sediment yield are plotted with the envelope of hillslope sediment mobilization shown in (a), above (grey region). Dashed line indicates the upper bound of the region with a doubling of the diffusion coefficients. 39 Chapter 2. Hillslope and Tributary Sediment Stores
100 m 4 km
Tamihi Slesse Chipmunk Nesakwatch
Centre
0.03 Liumchen Foley
0.02
0.01 0.05 0.001
Figure 2-18: Longitudinal profiles of 6 major tributaries in the Chilliwack Valley. Vertical exagger- ation is 20x. Black triangles indicate the transition from the colluvial process domain to the fluvial domain. White triangles indicate secondary knickpoints, conditioned either by glacial erosion in the master valley (Liumchen and Tamihi), or, in the case of Foley Creek, by a large landslide. of 30-40 km2 in the 6th order channels and 110 km2 in the 7th order ones. Bars and floodplains begin to develop in the distal reaches of 6th and 7th order valleys. There is usually a steep mid-section with a coarse bed and very little channel storage. The larger grain size fractions of poorly sorted debris introduced to the upland reaches are typically sequestered in long-term storage while finer material moves downstream. These reaches are supply-limited, with structured boulder beds and logjams along much of their length (“threshold” channels, [Church, 2006]). The substrate is a coarse lag derived from glacial sources and mass wasting. Much of the coarse detritus delivered from upland basins is delayed from onward transfer, resting at breaks in slope on lower valley walls or at tributary junctions. The transition point from bedrock or boulder substrate to labile alluvium has likely migrated downstream over the course of the Holocene, following the evacuation of glaciofluvial sediment stores. Valleys above the degradational limit on the mainstem such as Nesakwatch and Centre Creeks have not undergone a fall in base-level, with the attendant incision and evacuation of glacial sediments. Though there has been considerable evacuation of hillslope sediment stores
40 Chapter 2. Hillslope and Tributary Sediment Stores through the action of debris-flows, the configuration of these valleys remains a useful analog for reconstructing the ‘initial conditions’ of tributary valleys further downstream. Major valleys such as Liumchen, Tamihi, Slesse and Foley must have remained in a comparable aggraded state for a short time following deglaciation.
2.6.1 Lower Tributary Valley Fills
During deglaciation greatly elevated sediment production from valley glaciers filled the lower reaches of the valley tributaries with sandy, heterogeneous fluvial fill. Considerable evidence remains of the former surface in most of the valleys. In the case of Liumchen and Tamihi Creeks, the tributary channels were graded to the Pleistocene (late-glacial) mainstem channel, which ran alongside the ice lobe that filled Chilliwack Valley, some 150 m above the modern channel. It is not clear how long this configuration lasted, but there are remnant deposits of considerable size left in place from this paleo-Chilliwack River between Tamihi Creek and Cultus Lake [see Saunders, 1985; Saunders et al., 1987]. Figure 2-19 shows the longitudinal section geometry of the fills; the elevation indicated at the valley outlet are presumed to have been continuous with mainstem deposits. Since the disappearance of the glaciers and a fall in base-level, the five large catchments downstream of Nesakwatch Creek have evacuated significant quantities of alluvial fill from their lower reaches. Liumchen and Tamihi evacuated their valley fill relatively soon after the retreat of the Fraser Lobe from its terminus at Tamihi Moraine. Slesse, Chipmunk, Foley and the mid-valley section (present-day Larson’s Bench) must have followed a short time later, though the chronology has not been established. Liumchen presently has relatively little storage in the lower valley: material was likely evacuated quickly and streambed armouring set in soon afterwards. A bedrock sill near the outlet of Tamihi Valley is a remnant of the lip of this once-hanging valley (Figure 2-19). The river has regraded to the mainstem by incising through this feature, though clearly the bedrock incision was not wholly accomplished in the Holocene. The notch has likely developed over the course of several glaciations. The Tamihi Creek Forest Service Road travels along a high bench adjacent to the channel, and numerous truncated fans from lateral basins are evident along the length of the valley. There are substantial remnants of glaciofluvial terraces in the headwaters that have been gradually excavated. The total depth of the glacial fill in the canyon of lower Tamihi Creek appears to have been as great as 150 m, which leads to very large volumetric estimates (Figure 2-20c, Table 2.3).
41 Chapter 2. Hillslope and Tributary Sediment Stores
Liumchen Creek
elev. 210m
Tamihi Creek
elev.
Elevation (8x exaggeration) 250m
Slesse Creek elev. 310m
2000 4000 6000 800010000 12000 Distance from Chilliwack Mainstem (m)
Figure 2-19: Longitudinal section of evacuated glaciofluvial valley fill, superimposed on the modern river profile in Liumchen, Tamihi and Slesse Creeks. Bedrock features, such as hanging glacial sills (described in the text) are shown in light gray. The Slesse Creek fill overlies a lacustrine layer near the junction with the mainstem. Vertical grid spacing = 100 m
In Slesse Creek, the former elevation of the glacial fill is evident in a number of exposures in the lower portion of that valley [Saunders et al., 1987]. Deltaic deposits stand 70 m above the modern channel. Bedded glaciolacustrine sediments lie beneath the deltaic sands at an elevation of 255-270 m. Glacial Slesse Creek evidently prograded into a mid-valley lake [Saunders et al., 1987]. Evidence from Foley Creek is particularly clear (Figure 2-20a). The valley fill is exposed in several sections, and shows a sandy matrix with a preponderance of granitic boulders that emanated from the headwaters. The fluvial facies indicate a high-energy aggrading channel (Figure 2-20b). Given the elevation and gradient of the exposed beds, baselevel for the tributary would be consistent with the elevation on the top of Larson’s Bench. Ryder Creek is not a tributary valley fill, but it represents a significant amount of sediment
42 Chapter 2. Hillslope and Tributary Sediment Stores
Tributary Evacuated Fill (m3x106) Liumchen 16.0 ±2.4 Tamihi 128.8 ±22 Slesse 78.0 ±14 Chipmunk 6.0 ±0.8 Foley - Lower 7.1 ±0.5 Foley - Upper 8.2 ±0.5 Ryder Creek 79.0 ±6.5
Table 2.3: Volume of glacigenic fill evacuated from major tributaries. eroded from the Ryder Lake Upland (see Chapter 4). It is included in this section as a significant, discrete tributary input that is directly coupled to the mainstem. The total volume eroded from each valley fill is presented in Table 2.3. The error term incorporates uncertainty due to alternate possible configurations of the fill. Maximum values assume continuity of the surface across the channel; minimum values assume the fills sloped toward the modern valley floor. For most valleys, the true number is probably closer to the maximum value. In some tributaries, the volume of valley fill is comparable to the total hillslope erosion estimated for the catchment over the postglacial period. This evacuated fill is a major term in the tributary sediment budget, and has presumably had a major influence on the sediment dynamics in the lower tributary valleys, and points downstream, over time.
2.7 Discussion
This chapter has examined the relative balance of storage and erosion throughout the drainage network of Chilliwack Valley, in an attempt to identify the important linkages and process zones in the Holocene sediment cascade. The pattern that emerges from this analysis is a landscape that has deposited large quantities of material at the outlet of 1st to 3rd-order catchments with basin areas generally less than one square kilometre. This is consistent with observations of Brardinoni and Hassan [2006], who showed that the hillslope colluvial domain was a distinct area of deposition within formerly glaciated catchments. Material tends to accumulate along the base of steep valley slopes, with varying proportions of onward transfer, depending on the coupling with the fluvial system. In the soil-mantled hillslopes of the Oregon Coast Range, Reneau and Dietrich [1991]
43 Chapter 2. Hillslope and Tributary Sediment Stores
a)
b) c)
Figure 2-20: (a) Upper strata of the valley fill in Foley Creek ( 350 m upstream of Foley mouth). Exposed is a mix of glaciofluvial and debris flow deposits. (b) Upper strata of the glaciofluvial valley fill in Foley Creek (∼1.2 km upstream of Foley mouth). The coarse fluvial beds are approximately 45 m above the modern channel. The majority of the boulders are most likely Mt. Barr granite, indicating the headwater provenance of the bulk of the sediments (c) Truncated remains of a fan deposit that once interfingered with glaciofluvial fill in lower Tamihi Creek.
44 Chapter 2. Hillslope and Tributary Sediment Stores described a landscape that appears to be in approximate equilibrium, showing rates of specific yield between 50 and 200 t/km2/yr across scales of less than 1 km2 to over 1 000 km2. They suggested that transitory departures from this pattern might be possible due to, for instance, climate cycles that induce an augmentation in rates of landsliding. Roering et al. [1999] indicate that the Oregon Coast Range may be in an equilibrium at the millennial scale, since rates of uplift are approximately balanced by the rates of inferred hillslope erosion. The rates of bedrock lowering established in the Oregon studies ranges from 0.05 to 0.15 mm/yr. The overall rates of sediment erosion established for Chilliwack Valley, using a model similar to Roering’s, are on the upper end of the scale, ranging from 0.10 to 0.16 mm/yr averaged over the whole basin. The higher rates are due to the large quantities of glacial tills on hillslopes that have been recruited to the network over the postglacial period. Much of the glacial contribution is derived from gully incision on till blankets or morainal deposits along valley walls. Rates of basin yield are lower than the rates of mobilization because of storage along the length of the system. Over the Holocene Epoch material has accumulated along the footslopes of the steeper 1st to 3rd order drainage segments. There is a decline in the net storage in 4th order catchments, as material is mobilized onward. At the scale of 5th order catchments and beyond, there is an increase in specific rates of sediment delivery due to remobilization of stores of glacigenic material that collected in the lower reaches of each catchment. These deposits continue to strongly influence transport conditions in the distal reaches of major valleys, roughly 20 to 200 km2 in size. In the next chapter, the estimates of post-glacial yield at the scale of 5th to 7th order catchments is explored in further detail at Chilliwack Lake.
45 Chapter 3
Chilliwack Lake
Having established approximate rates of sediment mobilization in the upper tributaries of the Chilliwack Valley, this chapter examines quantities of sediment yield at the next downstream step, that of the major 6th and 7th order valleys. Chilliwack Lake provides an opportunity to estimate summary rates of yield, since it has trapped nearly all of the post-glacial load arriving from Upper Chilliwack basin (187.5 km2), Depot Creek (57.7 km2) and Paleface Creek (37.9 km2). Establishing the rates of yield for fine and coarse sediment fractions at this scale provides some additional insights into the storage term in the sediment cascade between the hillslope scale and that of the tributary fluvial system. A significant term in Chilliwack Valley’s post-glacial sediment budget is the stores of outwash that accumulated in the lower tributary valleys. These valleys reached a maximum state of glaciofluvial aggradation prior to the retreat, at the valley outlet, of Fraser Ice (see Chapter 4). Following deglaciation and a fall in the valley mainstem base-level, these fills would have been rapidly incised, leaving only a few terraced remnants along the valley walls. In Chilliwack Lake, subaqueously-deposited glacial outwash sediments are evident in the seismic stratigraphy beneath the Holocene lacustrine sediments. The boundary between these two units marks the transition between delivery of outwash material during deglaciation and the development of graded, post-glacial fluvial systems in tributary watersheds. The thickness of the lacustrine deposit has been estimated using sonar and seismic profiling techniques; rates of deposition are verified using lake cores from the upper 3 m of lake sediments. Radiocarbon and paleo-magnetic dating of the lake cores reveals the chronology of deposition in the last 5 000 years. The lake core record highlights the pervasive influence of debris flow activity and large inputs of organic debris. Long-term rates of bedload accumulation are less certain, but may be estimated based on the geometry of three major deltas building out into the lake.
46 Chapter 3. Chilliwack Lake
3.1 Study Area
Chilliwack Lake is approximately 9.2 km in length, impounded behind a large moraine. The moraine is a vestige from the last glacial cycle and represents a terminal position of the Chilliwack Valley glacier [Clague and Luternauer, 1982]. Since its inception at the end of the last glaciation, the lake has been an effective sink for nearly all sediment delivered from the upper valley, and thus it provides an important record for the catchment sediment budget. According to the reservoir trap efficiency rating curves of Brune [1953], based on the ratio between reservoir capacity (860 ×106m3) and mean annual water inflow (630 ×106m3), very close to 100% of the total sediment delivery to the lake is trapped. Only a small fraction of fine sediment reaches the lake outlet. The sedimentary strata within the lake bed record the pace of fine sediment deposition since the retreat of the Chilliwack Valley glacier at 13.3 ka cal. B.P. The fan-deltas that have formed at the outlet of the upper Chilliwack basin and two major tributaries hold the total Holocene record of coarse sediment yield. The lake is surrounded by steep rugged valley walls and half a dozen small basins that deliver occasional debris flows directly to the lake. Outflow from the lake during nival floods averages 45 m3/s, though daily averages have been as high as 100 m3/s (HYDAT, 2007). Annual precipitation is on the order of 3 000 mm. Winters often have significant snowfall, but the lake surface has very seldom frozen over in the past century. The lake elevation is stable at 621 m a.s.l. At 11.6 km2 in surface area, Chilliwack Lake (Figure 3-1) is small in comparison with many other ‘large’ Cordilleran lakes (>10 km) that have been the subject of study [Gilbert, 1975; Eyles et al., 1990; Desloges and Gilbert, 1991; Eyles et al., 1991; Desloges and Gilbert, 1994; Eyles and Mullins, 1997; Gilbert et al., 1997; Desloges and Gilbert, 1998]. Its upstream contributing area is also proportionately much smaller than other large lakes, and upstream glacial sedimentary stores were not as extensive. Rates of sediment yield in British Columbia mountain lakes scale to some extent with the total glacierized area in the basin [Desloges and Gilbert, 1998]. Annual rates of deposition typically range from a few millimetres to about 2 cm sediment/year. Specific yields vary sig- nificantly among the sampled population, from 30-500 t/km2/year. Only a small percentage of Chilliwack Lake’s contributing area (2.7%) has permanent icefields or small glaciers, thus the rates of suspended sediment delivery are relatively low in comparison with other large lakes that have extensive glacial cover. The geometry of Chilliwack Lake is simple compared to larger lakes that have multiple basins, sills and bedrock-controlled topography. Gilbert et al. [2006] have divided large Cordilleran lakes into three distinct groups: (1)
47 Chapter 3. Chilliwack Lake
A B Silverhope Creek
Radium Paleface Creek Creek
C
D Centre Creek
US Border
Depot Creek
Little Chilliwack Bear Creek Creek
Figure 3-1: Chilliwack Lake, with TRIM digital terrain model. Lines A, B, C and D indicate seismic sections discussed below. those which contain only small amounts of sediment distributed as discontinuous deposits in the deepest parts of the lake, (2) those which received abundant sediment during late Pleistocene deglaciation but very much less during the Holocene Epoch and (3) those which contain thick deposits of sediment begun during deglaciation and continued through the Holocene. Chilliwack Lake evidently belongs in the second group, having a large, distinct package of outwash material that most likely accumulated in the immediate postglacial period, and relatively modest contributions thereafter. The pattern of sedimentation appears to be uniform and the history of the lake is fairly well understood. It thus presents a good case for developing a long-term lacustrine sedimentation model.
48 Chapter 3. Chilliwack Lake
Water Depth 80 m
100 m 250 m 90 m
100 m
110 m Paleface Fan
120 m
Figure 3-2: Figure showing mid-lake CHIRP data on the North end of ‘C’ transect, Figure 3-1, and the continuous drape of lacustrine beds leading up onto the edge of the Paleface fan delta. See site (b) in the next figure for the larger setting within the lake sediments.
3.2 Seismic Methodology
Profiling of the lake was undertaken using three different seismic methods in order to assess the geometry of lake deposits since the close of the Fraser glaciation. Sounding of the lake had previously been done by R. Gilbert and M. Church (pers. comm.) in 1989, using a Datasonics Bubble Pulse system. They were able to resolve a number of features that hinted at the sedimentary history of the lake, and they speculated about the possible origins of multiple recorded strata. A first survey was undertaken in September 2003 using a CHIRP II system. The CHIRP II has a single MASSA TR-75 transducer with a 4 kHz source and a CAP-6600 receiver. This provides a relatively low frequency signal and a high-resolution image of the upper sed- imentary strata. Penetration was typically on the order of 15-20 m. The imagery highlights the lacustrine bedding in the lake, which appears to be distinct and continuous. Figure 3-2 shows an example of how depositional layers drape evenly over the lake bed and over distal deltaic sediments from Paleface Creek, at the right. The sediments are interstratified with the alluvial fans and coarser rock fall deposits that project into the lake. Debris flow basins have built coarse-grained cones out into the lake. A second survey was undertaken during the same period using a Bubble Pulse SBP 510 system (source frequency = 400 Hz), the unit that was used in 1989. This time, however, a 24 channel hydrophone streamer was used, rather than the conventional Datasonics Eel.
49 Chapter 3. Chilliwack Lake
Hydrophone sensors in the streamer were located at 2 m intervals, allowing for a much broader multi-channel array than the Eel. Analog signals from the streamer were recorded with the Geometrics GEODE. Recording the shots with the GEODE allows the opportunity for multi- channel post-processing algorithms, stacking and migration. Finally, to sound out the geometry of the lower strata, a BOLT acoustic airgun was used with a streamer similar to the second survey. This was done in June 2004. Resolution from the airgun record provides only a rudimentary picture of finer structures (see a direct comparison of the two systems below in Figure 3-6), however it offers an excellent record of major boundaries within the lake sediments as well as the bedrock interface.
3.3 Interpretation of the seismic record
The limit of penetration of the CHIRP system shows the likely extent of finer-grained lacus- trine materials. The higher energy sources highlight the boundaries between lake sediments and underlying outwash material, as well as bedrock boundaries. Multiple reflections obscure the record in several places, however the presence of deeper structures in the lowest strata is confirmed by multiple surveys. Penetration was likely hindered in many areas by the presence of gas and organic layers that damped the energy of the seismic signal. The presence of these layers was confirmed in the lake cores, two of which contained thick (>10 cm) organic beds. Holocene lacustrine sediments attain an average thickness of approximately 9.5 m, more in locations near the lake deltas (up to 15 m), and in the most distal basin. This estimate is subject to errors inherent in modelling changes in seismic velocity with depth. Relatively clear waters are assumed. Velocity modelling from the CHIRP system was automated by the software and so it is difficult to ascertain the parameter sensitivity. Model parameters for velocity changes with depth from the airgun data are more explicit; however the vertical resolution of the data was too coarse to enable much refinement of depth estimates for the upper layer of lacustrine material. It is assumed that the relative precision of the CHIRP II record is such that the potential errors in estimates of sediment thickness are roughly ±0.5 m. Three major facies can be distinguished from the seismic records (Figure 3-3). It is spec- ulated that the lowest consists of advance outwash material, possibly with valley sediments that pre-date the advance of the Chilliwack alpine glacier. A middle facies would then corre- spond to sandy, subaqueously deposited late glacial outwash material sedimented during ice retreat, overlain by the third facies consisting of fine-grained, Holocene lacustrine sediments.
50 Chapter 3. Chilliwack Lake
The boundaries that separate the three groups are distinct, though there are numerous minor reflectors discernible within each unit. The zone beneath the Holocene lacustrine unit (as resolved by the airgun survey) is stratified but the facies appear less ordered. The dispersion of seismic energy within this unit indicates coarser material than lake sediments, consistent with sandier material. Figure 3-3 shows the air gun record from a longitudinal profile taken along the 9 km centreline of the lake, from the impounding moraine to the Upper Chilliwack River Delta. Unit ‘a’ represents the Holocene lacustrine record. In the upper section of the lake, distal portions of Paleface and Depot Creek deltas are revealed (‘d’ and ‘e’). The reflectors from unit ‘c’ disappear below the upward curve of the moraine at the distal end of the lake. The terminus of the Upper Chilliwack glacier likely remained in place against the moraine for an extended period, shedding outwash into the lower valley over the course of the final phase of Fraser glaciation. The lower boundary of unit ‘b’ appears to be conformable with unit ‘c’, suggesting that it was deposited as a sandy, subaqueous outwash following the retreat and downwasting of the Chilliwack ice lobe. Long, inclined reflectors originating from Paleface Fan are discernible, indicating interstratification of fan sediments with the outwash. There are at least two sub-units within ‘b’ up-valley of Paleface Creek, indicating more complex interactions among Upper Chilliwack, Depot and Paleface Creeks. The down-lake, distal fringe of the Depot Creek fan has a down-valley distal ridge morphology roughly 7 m high that appears to be built upon a structure deeper within the outwash package. The lowest zone (‘c’) appears to have large-scale disruption structures within it. However a reflector in the most distal cross section (Figure 3-4, see Figure 3-1, line ‘b’) shows a diagnostic “bow-tie” reflector that indicates a steep synform structure, consistent with a buried channel. It is interesting to note in Figure 3-4 that the disrupted structure is mostly confined to the center of the valley, and is surrounded by sediments that have undisturbed layering. The structure is picked up intermittently in the long profile section, often giving the impression of distorted structures in the lower unit. I assume this to be the Pleistocene valley floodplain that was buried by the advancing alpine glacier. The outwash that overlies the Pleistocene valley floodplain (Figure 3-4, layer ‘b’) constitutes the initial topography for the post-glacial period, and represents an important datum for the volumetric analysis and sediment budget that follows. By 13 300 ± 180 cal. B.P. the final strata of outwash were deposited on the sandur that extends from Chilliwack Lake Moraine to downstream of Slesse Creek [Saunders, 1985].
51 Chapter 3. Chilliwack Lake e Proximal d b Distance Along Lake Centreline (km) 2 4 6 8 a c Distal Composite seismic image of Chilliwack Lake (centreline transect, 9 km) showing major depositional units along the
0.1 0.2 0.3
ae Time (s) ravel T wo-Way T Figure 3-3: axial section. Five labelledoutwash material features are (c) discussed distortedportions in reflectors of the the within text: pre-Fraser Pleistocene glaciationseismic (a) strata, topography velocity (see continuous, (d) through lateral laminated Paleface the cross-section, Holocene Creek lower Figure lacustrine strata 3-4). delta, sediments, is and Vertical (b) likely exaggeration (e) sandy is somewhat Depot roughly higher 45x, Creek than though delta, in the showing lacustrine zone.
52 Chapter 3. Chilliwack Lake
Figure 3-4: Cross-section B (see Figure 3-1), the closest transect to the down-valley extent of Chilliwack Lake, and the deepest section (120 m water depth). Units a, b and c correspond to units labeled in the previous figure. Top figure shows the un-migrated data, with the characteristic ‘bow-tie’ structures. The migrated section, below, shows the synform structure at depth, though distortion is introduced to other parts of the trace. The valley walls are planar; curvature at the upper edges of the trace is due to turning of the boat and the array.
53 Chapter 3. Chilliwack Lake
After this phase, the glacier was de-coupled from the moraine, and began to retreat and/or break up. A lake would have quickly filled the resultant depression behind the moraine, and a regime of lacustrine sedimentation would have been established. Bedlevel at Paleface and Depot Creek would have fallen as the ice departed, and deltas would have begun building into the lake. Not long after, a delta would have begun prograding into the lake from the Upper Chilliwack River. Given the bounding geometry of the present lakehead delta floodplain, the upstream end of an early Holocene Chilliwack Lake may have been up to 4 km south of its present shoreline.
3.4 Fan Deltas
Upper Chilliwack River and the two other major tributaries to Chilliwack Lake have large fan deltas that have prograded into the lake. At the climax of the Fraser Glaciation, Depot and Paleface Creek catchments would have had glaciers that merged with the principal lobe of the Chilliwack glacier. At the close of the glacial period, an outwash fill would have built up in each basin, and has since been eroded and become incorporated into the depositional package at the mouth of each of the valleys. The fan deltas have continued building and prograding into Chilliwack Lake throughout the Holocene Epoch. These depositional pack- ages, then, represent approximately 13.3 ka of continuous accumulation of sand and gravel. The boundary between glacially derived bedload and Holocene delivery cannot be definitively resolved given the resolution of the seismic data. Figure 3-5 shows sonar bathymetry of the fan deltas and the location of the seismic transects. Water-borne seismic and land-based ground penetrating radar (GPR) transects reveal something of the architecture of the deltas, particularly where they grade from gravels to sands to lacustrine silts and clays. At the interface of sands and silts, upper sheets of sand ‘shade out’ the finer-grained strata below, obscuring the nature of the contact. Figure 3-6 shows two data sets from different profiling techniques carried out along the same transect path. Figure 3-6 (top) shows the image from the CHIRP survey, which high- lights the numerous fine-scale reflectors within the lacustrine beds. Note the dispersion with depth of the CHIRP signal, most likely attributable to the coarser outwash material. Figure 3-6 (lower) shows the boundaries between the lacustrine material, outwash, the lower valley fill, and a lower reflector that is assumed to be bedrock. In the more proximal section of the profile there is a package between Holocene delta sands and the bedrock reflector that is
54 Chapter 3. Chilliwack Lake CHWK-04 Creek Paleface 5 km
CHWK-02 eto ‘D’ Section 0 Depot Creek CHWK-03 Line Line Core Sonar Seismic Percussion Bathymetric Contour (5m) Location of subsurface surveys at Depot and Paleface Creek fan deltas. Underwater topography was surveyed using Upper Chilliwack Delta sonar on a 1003-6. m grid. Lake core locations and seismic trace paths are indicated. Section ’D’ refers to the seismic trace in Figure Figure 3-5:
55 Chapter 3. Chilliwack Lake
Distance from Shore (m) Delta 800 600 400 Slope
40m Sand-Silt Transition Lacustrine Deposits 60m Upper Outwash
- Depth (m) Boundary
CHIRP 80m
0.10 Upper Outwash Boundary
Outwash Fan
Bedrock(?) 0.20 Lower Boundary (?)
Air Gun - Two-Way Travel Time (s)
Figure 3-6: Section D (see Figures 3-1 and 3-5). Two seismic techniques used on the same profile at the toe of Depot Creek Fan: CHIRP (above) and acoustic air gun (below). The CHIRP record shows a detailed picture of the uppermost unit visible in the air gun profile. The airgun record shows the deeper strata probably alluvium from the older Pleistocene fan that emerged from Depot Creek. Figure 3-7 shows a 3-dimensional perspective view of the long-profile seismic survey. The record shows that the uniform upper lacustrine layer overlies complex, interpenetrating layers of outwash sediments. The record at Paleface Creek shows curved reflectors that are likely outwash fans.
3.5 Ground Penetrating Radar Surveys
GPR surveys were conducted on the subaerial surface of the Paleface Creek fan delta in an attempt to discern the subsurface structure of the landform (Figure 3-8). All GPR data were collected with Simon Fraser University’s 400 V Sensors and Software pulseEKKO IV radar system. 50-MHz antennae were used with 2 m spacing and moved broadside perpendicular in 0.5 m intervals.
56 Chapter 3. Chilliwack Lake
Depot Creek Paleface Creek
Modern Sand Limit 3km
2km
1km
0km
Figure 3-7: Profile C (see Figure 3-1) with superimposed seismic record. Perspective view, facing south east, shows the relationship among the upper beds of lacustrine material, outwash beds, and several antiform reflectors at depth that likely indicate the structure of buried Pleistocence fans emerging from Paleface and Depot Creeks. The sand limit on the fans is inferred from the CHIRP record. Transverse seismic transects are indicated with black lines.
57 Chapter 3. Chilliwack Lake
Much of the subaerial portion of the fan is heavily forested, making GPR deployment difficult. Surveys were carried out on the sandy 100 m margin at the lake shore. The GPR transects indicate thick sandy foreset sequences. Not surprisingly, a lower bounding horizon to the outer deltaic strata could not be detected. The fan is built out over steep underlying topography, so the lower surface is beyond the ∼35 m limit of detection with GPR, likely on the order of 80-100 m depth (see Figure 3-9). The geometric model of the fan package remains an estimate based primarily on the water-borne seismic surveys. Estimates of the total coarse bedload sediment delivery in the Holocene Epoch are shown in Table 3.1. Volume estimates were established by subtracting ‘immediate post-glacial’ DEM surface from the contemporary one. The post-glacial surface was constructed using the seismic profiles, and incorporates errors due to seismic velocity calculations, and any possible misinterpretations of the seismic stratigraphy. This information then provides a basis for establishing the volumetric bounds of the total detrital load that has been delivered to the lake system since deglaciation. Rates of potential volumetric hillslope mobilization (developed in the previous Chapter) would appear to indicate that about 10% or less of the mobilized material reaches the terminal deposit. This is not surprising, given the storage within the hanging valleys and footslope accumulations along the length of the tributary catchments. It is difficult to provide an accurate assessment of deposition from the smaller lateral basins; it may be larger than indicated.
Table 3.1: Volumetric estimates for catchment erosion and fan deltas bedload delivery since deglacia- tion, not including outwash stores.
Contributing Est. Post Glacial Bedload Volume Area (km2) Erosion (Ch. 2; m3× 106) (m3× 106) Upper Chilliwack Delta 187.5 516-772 70-110 Depot Delta 57.7 167-241 24-28 Paleface Delta 37.9 102-155 14-18 Smaller Catchments 39.9 127-195 ∼8
The next section examines results from lake cores, allowing better estimation of grain size distribution, specific density, and rates of accumulation. Thus a time-integrated, mass-based estimate of yield can be established.
58 Chapter 3. Chilliwack Lake 100m 170m Gravel Limit Gravels, cobbles 1 is assumed. This makes the limit of 50m − 120m PF103 Sand 07 m ns . Sand Channel Sandy Abandonned Foresets PF100 Gravel 0m
100 200 300 400 500
70m m (ms) ime T ravel w-a T Two-Way b) Channel Active 50m 616900 5434000 616900 5433600 Fan Paleface PF101 PF100 Transect Sand Gravel of gravels
Approximate limit PF101 PF103 PF102
PF100 616500 5433500 616500 5434000
0m
GPR transects at Paleface fan delta. (a)Location map for the GPR transects. (b) Down-dip section. 0 m indicates 100 200 300 400 500
m (ms) ime Lake T Travel Two-Way 100m
c)
Chilliwack Lake Seismic Survey Line C5 a) Figure 3-8: the lake shore. (c) Strike section, moving NNE to SSW. A radiowave velocity of 0 penetration 35 m (500 ns). The lower bounds of the sediment package were not detected.
59 Chapter 3. Chilliwack Lake
Figure 3-9: Isopach diagrams, (a) Paleface and (b) Depot Creeks. A hypothetical post-glacial surface (top of outwash unit ‘c’, Figure 3-3), built with a 4th order polynomial grid, is subtracted from the modern topography of the lake fan deltas to yield an estimate of total coarse (sand and gravel) sediment accumulation from these catchments over the Holocene Epoch. The fines component of the fan deltas extends much further out into the lake and are estimated in a separate analysis.
60 Chapter 3. Chilliwack Lake
Chilliwack Lake (05)CHWK-06 (Distal 2) Core Locations Vibracore, ~3.5 m Bathymetry = 20m contours Deepest portion of lake 124m depth
(05)CHWK-04 (Paleface) Percussion core, ~2.4 m ~1km from mouth of Paleface CHWK-02 (05) (Depot) Creek, 101 m depth 120m Percussion core, ~1.8 m (05)CHWK-01 (Moraine) ~1km from mouth of Depot Percussion core, ~1.3 m Creek, 66 m depth Most distal site, 116 m depth
(05)CHWK-07 (Distal 3) 80m 100m 60m Vibracore, ~2 m 117 m depth
(05)CHWK-05 (Distal 1) Percussion core, ~1.9 m 117 m depth (05)CHWK-03 (Delta) Percussion Core, ~1.5 m 0 2 km ~1km from main delta, 64 m depth
Figure 3-10: Site map: Chilliwack Lake with the location and description of seven cores recovered from the lake.
3.6 Lake Cores
To supplement the reconstruction of the Holocene history of Chilliwack Lake, seven lake sediment cores - five percussion cores (CHWK-01 to 05) and two vibracores (CHWK-06 and 07) - were retrieved from Chilliwack Lake (Figure 3-10). CHWK-07 penetrated to a depth of 2 m, however the core barrel became twisted during penetration of the lake bottom, and it is thought that the recovered material may not be a truly vertical section; only limited analyses were done on this core. The cores were split lengthwise and photographed. As the lake cores dried, more pho- tographs were taken (after 2 days and 10 days) to capture any further contrast and details that might emerge from the lake stratigraphy. The cores were subjected to standard analyses to determine water content, sediment dry weight and specific density, organic content (loss on ignition), and mass magnetic susceptibility: 1.5 cc samples were taken at 5 cm intervals to measure water and organic content. Low-field susceptibility measurements were taken every 2 cm on each of the cores using a GF Instruments SM20 hand-help susceptibility meter with a 5 cm coil and a sensitivity of 10−6 SI. Oriented samples taken from the lake cores in small, 2 cc plastic cylinders at a 5 cm
61 Chapter 3. Chilliwack Lake sampling interval were submitted for analysis in the OLGA J-meter coercivity spectrometer at the Pacific Geoscience Centre, at Sidney, B.C.
3.6.1 Lake Core Descriptions
CHWK-01 was recovered on the lake-floor slope that rises on the moraine at the down- valley end of the lake. This core was relatively short (1.3 m) but effectively demonstrates the influence of debris flow activity in the lake over time. Seven thick (up to 3 cm) beds of relatively coarse sands (granules up to 2-3 mm) were evident, indicating the contributions from the terminal moraine and high-energy events originating from hillslopes adjacent to the lake. The longest cores came from the deepest sections of the lake. Two vibracores and one percussion core were pulled up near 120 m water depth. A small (20 g) wood chip recovered from the very bottom of the deepest core (3.46 m, CHWK-06) yielded an AMS radiocarbon date of 4 400 ±50 years B.P. (4 870 to 5 040 cal. B.P.). All of the cores have a rich record of forest fire events, indicating at least twenty episodes over the course of 5 000 years. Two percussion cores were taken on distal sections of the Paleface and Depot Creek fan- deltas (see Figure 3-5). One percussion core was extracted from the delta at the head of the lake. The core near Paleface Creek (CHWK-04) was the second-longest core recovered, with just over 2.4 m of silty material. The core taken nearest the head of the lake (CHWK-03) intercepted a thick organic bed, making it impossible to recover material beyond 1.5 metre depth. An organic bed at 84 cm depth yielded a conventional radiocarbon date of 180 ±40 years BP (roughly A.D. 1660-1800; radiocarbon calibration for this era is only loosely con- strained) which indicates very rapid rates of sedimentation.
Table 3.2: Radiocarbon ages from Chilliwack Lake.
Lab Code Core Depth Measured Age Calibrated Age2 (cm) (14C yr BP) (cal yr BP) Beta- 213922 CHWK-03 84 180 ± 40 280; 180; 150; 10; 0; (0-290) Beta- 213923 CHWK-06 346 4 400 ± 50 4970; (5040-4870) 2 — Intercept ages and age range in calendar years before AD 1950. The age ranges (in parentheses) represent 1σ error limits. Beta- 213922 was dated by conventional radiometric technique, Beta- 213923 was dated by AMS. The ages were determined using the INTCAL98 database.
Figure 3-11 and Figure 3-12 show the measured physical properties for CHWK-06 (the
62 Chapter 3. Chilliwack Lake distal lake basin) and among all cores. There does not appear to be any major temporal trend in the loss on ignition data, however there is a somewhat greater concentration of organic material in the more distal cores. Water content decreases with depth, likely due to compaction and dewatering of sedi- ments. Assuming the observed trend continues to greater depths, this may provide some explanation for the loss of seismic energy with depth in Chilliwack Lake. Sediments were generally massive, showing no indication of seasonal laminae or varves and very little visible variation with depth. Even with drying, the cores yielded little vertical variation other than beds with fine and/or coarse organics that were related to discrete sedimentation events.
3.6.2 Tephra and Other Disturbance Layers
A volcanic ash layer was found in two of the cores. Cores CHWK-04 and CHWK-06 yielded a single thick (3-5 mm) layer of ash at nearly identical depths (1.95 and 1.97 m). Fine volcanic glass from the ash was concentrated by density separation (methyl iodide, S.G. 2.2). This material was analyzed using a Cameca SX-50 scanning electron microprobe (M. Raudsepp, UBC EOS Electron Microbeam / X-Ray Diffraction Facility) with readings from individual shards indicating a likely provenance from Mt. St. Helens. 16 of 21 shards yielded readings that plot decisively within the Mt. St. Helens ‘Y’ Fe-K-Ca compositional field (Figure 3- 13). The remaining grains are widely scattered due to selection of impure glass shards or misidentification of grains. It is expected that this tephra must have been from the mid- Holocene Yn eruptive period. Published dates vary, but indicate a date of roughly 3 700 cal. yrs B.P. [Westgate, 1977; Luckman et al., 1986; Vogel et al., 1990; Mullineaux, 1996]. Slabs of sediment taken from the lake cores were submitted for X-ray analysis at UBC Hospital. Positive X-rays (4 mAs at 75 kV) reveal very fine low-density ash layers. The layers are often found above or interbedded with charcoal-laden layers (Figure 3-14). Presumably, burnt organic material continued to accumulate in the lake sediments for some time (decades) after the actual fire event [Hallett et al., 2003]. These event beds provide additional checks on the radiocarbon and paleomagnetic chronology (see below). Figure 3-14 shows positive X-ray and colour photographs of CHWK-04, -05 and -06. Three distinct events within CHWK-05 and -06 appear to be correlative, based on relative position and dating evidence. CHWK- 05 and -06 are separated by less than a kilometre. CHWK-04 is 2.15 km up-valley from CHWK-05 and registers at least one event that appears to be correlated with the other two.
63 Chapter 3. Chilliwack Lake
3 Dry Wt. Density (g/cm ) Water Content (%) Loss on Ignition (%) Susceptibility 1.2 1.4 0.45 0.6 0.75 0.05 0.1 0.15 0.5 1 1.5 0 0
50 50
100 100
150 150
200 200
250 250
300 300
350 350
Figure 3-11: CHWK-06 - Vibracore from the deepest section of Chilliwack Lake. Dark bands in the column at left indicate discrete beds with concentrations of coarser clastic material and/or fine organics. The upper portion of the core may have undergone some compaction during the coring operation, interrupting the otherwise coherent gradient of increasing density with depth.
3.6.3 Magnetic Parameters
Magnetic measurements were taken on the lake cores in order to identify any spatial or temporal trends in lithology and grain size that might signal changes in sedimentation regime. Measurements were also used to reveal the record of paleosecular variation in the earth’s magnetic field over time for use in paleomagnetic dating. Iron oxides from plutonic and volcanic sediment source areas are an important component in the detrital composition of Chilliwack Lake sediments. Bedload material from Upper Chilliwack Creek has a significant component of dacite rock from dikes and shallow pods that have cut across the Hannegan Volcanic Complex (D. Tucker, pers. comm.). Despite the
64 Chapter 3. Chilliwack Lake
Water Content (%) Loss on Ignition
35% 45% 55% 65% 75% 0 5% 10% 15% 20% 0 0
50 50
CHWK01 - Moraine Wall CHWK02 - Depot Creek 100 100 CHWK03 - Delta CHWK04 - Paleface CHWK05 - Mid Lake CHWK06 - Deep Lake 150 150 CHWK07 - Mid Lake (2) Depth (cm) 200 Depth (cm) 200
250 250
300 300
350 350
Figure 3-12: Cross-core comparisons of two parameters, water (%) and loss on ignition (LOI).
K2O
60 Glacier 30 Pk.
Bridge River
Mazama Mt. St. 50 40 Helens
FeO 50 30 Ca0
Figure 3-13: Ternary diagram showing compositional fields for a number of Holocene volcanic sources within the Cascades and Coast Mountains. Points (Xs) show microprobe readings taken on shards from the Chilliwack Lake tephra layer.
65 Chapter 3. Chilliwack Lake
Figure 3-14: Three cores from Chilliwack Lake show a number of fire episodes over time. From left to right: CHWK-06 (Distal 2), CHWK-05 (Distal 1), and CHWK-04 (Paleface). Positive X- Rays (4 mAs at 75 kV) reveal very fine low-density ash layers. Based on paleomagnetic dating and stratigraphic sequence, two event beds are correlated across two distal cores and a third from as far away as Palefacefan delta. The events span an approximate range of 1 350 to 1 750 cal. years B.P. relatively small map-area of this sediment source within the granitic Chilliwack Batholith, it has evidently provided abundant material to the upper catchment over time. A ternary diagram showing the lithological composition of coarse (> 16 mm) clastic channel material in the Upper Chilliwack mainstem is shown in Figure 3-15. Depot and Paleface Creeks both drain granitic terrain and evidently provide further enrichment of iron within the lacustrine sediments. The silt fraction from channel sediments in Upper Chilliwack River and cores in Chilliwack Lake were submitted for geochemical (ICP- MS) analysis (see Chapter 5). Results show that typical iron content within the silt fraction of the lake sediments is on the order of 3-5% (Figure 3-16). There is a general pattern of increasing iron concentration from the source river silts to the distal lake material. Figure
66 Chapter 3. Chilliwack Lake
100 Tributary Basins
20 80
40 Granitic (%) 60
orphic (%) m
Meta60 40
80 20 Mainstem Sediments
100 0 0 20 40 60 80 100 Volcanic (%)
Figure 3-15: Percentage of coarse (>16 mm) clastic lithologies found in active bars, Upper Chilliwack River (open squares) and major tributary sources (Bear and Indian Creeks, black diamonds). There is a disproportionate representation of volcanic lithologies in the channel, despite the mostly granitic source material provided by the major lower tributaries, which represent 41% of the upper catchment drainage area.
3-16 shows that there is also a higher concentration of elements associated with magnetite, chromite, and other heavy minerals (Cr, Mg, Ti and V). A composite index of these elements, all of which have quite similar behaviour in the system, was generated by summing centered, log-ratio transformed values for individual elements (see Chapter 5). This pattern is mirrored to some extent in readings of magnetic susceptibility from each of the lake cores (Figure 3-17). There is a systematic increase in susceptibility from the delta to distal reaches. Bulk susceptibility is a proxy for both the relative concentration of magnetic minerals and the effective grain size. It would appear that grain-size variation exerts the strongest effect on the magnetic behaviour of the sediment. It is not expected that the slightly increasing concentration in iron exerts a primary effect on the magnetic readings, but it probably does enhance some of the down-lake contrast observed. Grain size information from the lake cores obtained from sedigraph analysis is shown in Figure 3-18. Fining and sorting processes are very effective beyond the upper lake delta. Some samples from CHWK-03 (taken roughly 670 m from the lake head) show up to 25% fine sand composition and a full range of silt sizes, while samples from Paleface (CHWK-04,
67 Chapter 3. Chilliwack Lake
1
0.6
0.2 1.4 km
4.7 km -0.2
Cr + Mg-0.6 + Ti + V 3.2 km 12.3 km Upper Chilliwack River CHWK03 - Delta -1 CHWK 02 - Depot Creek CHWK - 04 - Paleface Creek 8.9 km Distal Cores -1.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 Fe Figure 3-16: Graph showing the relative increase in iron and elements associated with heavy minerals and magnetic oxides within the silt fraction of channel sediments (Upper Chilliwack River) and lake sediments. Fe values range from 3 to 5%. Here they have been log-ratio transformed, centered on zero. Individual elements Cr, Mg, Ti and V have been similarly transformed, then summed, to create a composite index. Individually, V has the strongest down-valley gradient, and Ti the weakest.
0
50
100
150
200
Core Depth (cm) CHWK-03 - Delta 250 CHWK-02 - Depot CHWK-04 - Paleface CHWK-05 - Distal 1 300 CHWK-06 - Distal 2 CHWK-01 - End Moraine
350 9E-8 1E-7 1.1E-7 1.2E-7 1.3E-7 1.4E-7 1.5E-7 1.6E-7
cp- Bulk Magnetic Susceptibility
Figure 3-17: Susceptibility, measured at 5 cm intervals along the length of each core. There is an evident longitudinal gradient, with magnetic mineral concentration increasing down-valley.
68 Chapter 3. Chilliwack Lake
Grain Size (mm)
0.000977 0.0039 0.01563 0.0625 0.25 100 0
Coring Sites CHWK-02 - Depot CHWK-03 - Delta 75 CHWK-04 - Paleface 25 CHWK-06 - Distal 2
Clay 50 50
Silt Sand Percent Finer by Weight 25 75 Percent Coarser by Weight
0 100 -12 -10 -8 -6 -4 -2 Grain Size (y)
Figure 3-18: Sedigraph grain size curves from lake core sediment samples, Chilliwack Lake roughly 4 km from lake head) and beyond show material graded primarily in the 2-11 micron fraction (clay and very fine silts). Sediment at CHWK-06, 6.7 km from the upper lake delta, shows only a minor degree of further sorting.
3.6.4 Palaeomagnetism
A chronology was established for the deepest cores (CHWK-04, -05 and -06) using the vari- ation in magnetic inclination and declination. Curves established from the Chilliwack cores were compared with the closest available calibrated paleomagnetic curves, namely Fish Lake, CA [Verosub et al., 1986] and Mara Lake, BC [Turner, 1987]. The magnetic inclination and declination trends extracted from the Chilliwack cores were fit to the established data sets using a ‘least-error’ fitting technique. A master chronology was built from the deepest vibracore, CHWK-06 (3.5 m). Although the core wrapped itself in the tube no less than three complete revolutions, the torsion of the core sample appears to have been uniform, and declination data were recovered using a simple de-trending algorithm. All
69 Chapter 3. Chilliwack Lake inclination data were centered on zero. A program was coded for LabView, whereby the raw inclination and declination data were interpolated to a 200-element array. The user can then manually stretch the linked inclination and declination series into place by eye to match the Fish Lake or Mara Lake series, guided by a sum of square errors term. The fit that yielded the least error, and matched the tephra and 14C dates, was a starting point for further refinement. The minimum-error fits for all cores are superimposed in Figure 3-19. The average of these curves is the best estimate of the local paleomagnetic curve. Agreement in some periods is better than others, as shown by a standard deviation that was calculated for the inclination and declination estimates at each point in the time series. The final match with the Mara Lake and Fish Lake records is shown in Figure 3-19 at right. The Fish Lake series shows a much better fit, since that curve had the best initial fit during the curve-fitting process. Thus the final, averaged form of the Chilliwack curves is highly influenced by the Fish Lake data.
3.7 Rates of Sediment Accumulation in the Holocene Epoch
Based on the chronology developed from tephra and the paleomagnetic and radiocarbon data, it is possible to reconstruct the rates of sediment accumulation over time. Figure 3-20 shows the depth of sediment accumulated versus calendar year. The Paleface core (CHWK-04) record extends to approximately 3 600 cal. B.P. (2.4 m), and the core from the deepest portion of the lake (CHWK-06) extends to 5 200 cal. B.P. (3.5 m). Overall, there are relatively slow rates of accumulation in distal portions of the lake (average of 0.7 mm/year) and more rapid rates near the delta (5 mm/year). The overall interpretation is that rates of accumulation on the lake floor are relatively stable over the decadal to century scale. This is despite the evident variability of sediment delivery shown in numerous events beds, and among the varied geomorphic lake floor settings of the cores. The long-term gross rates of lacustrine sedimentation inferred from the lake cores are in good agreement with the interpretation of the CHIRP seismic record. The minimum expected thickness of the distal lacustrine package (above the outwash), based on accumulation rates of 0.7 mm/year, would be 9.45 m. The lake floor thickness varies somewhat, with the maximum distal thickness ranging up to 10 m, and accumulations near the fans reaching 15 m. The mean estimated thickness along the central transect of the lake is 9.8, with one standard deviation of ±1.6 m.
70 Chapter 3. Chilliwack Lake
Averaged Chilliwack Core ‘Agreement’ among cores Results shown with Fish and Mara Lake 0 0
-1000 CHWK-02 -1000 CHWK-04 CHWK-05 CHWK-06 Average Std. Deviation Inclination -2000 Declination -2000 Series Match Mara Lake Cal. Years B.P. Fish Lake Chilliwack
-3000 -3000
MSH’Y’ ~ 3600 Cal. B.P.
-4000 -4000 15 30 45 60 75 90 -90 -30 30 90 0 50 15 30 45 60 75 90 -90 -30 30 90 Inclination (F) Declination (q) Std. Deviation Inclination (F ) Declination (q)
Figure 3-19: Traces of magnetic inclination and declination from four cores are shown in the first panel, as well as their average. These are the raw (unsmoothed) data, interpolated onto a standardized 200-element array to facilitate cross-core statistics. Standard deviation among the magnetic readings are shown in the middle panel. The shorter core extends only to 2 500 B.P., and thus sample size diminishes prior to this date. The final panel shows Fish Lake and Mara Lake datasets compared with the Chilliwack series.
Following the assumption that the outwash surface is represented by the seismic reflector described in the previous section, an isopach map is shown in Figure 3-21 showing the thick- ness of the overlying Holocene lacustrine accumulation. The horizontal boundaries of the lacustrine sediment package can be accurately delineated at each of the transects, however it has to be estimated for intermediate areas. Assuming that the boundaries are delineated with an accuracy that is within ± 50 m, the surface area of the lacustrine deposit is 10.8 ± 0.45 km2. This is then combined with the expected vertical accuracy to produce the total volumetric error. A model of the expected vertical error term is shown in Figure 3-22a. The total bulk volume of Holocene fine sediment accumulation is 106 ± 13 ×106m3. A breakdown is shown in Table 3.3. The dry-weight density for most lakefloor sediments in the proximal zone ranges from 1.28 to 1.55 t/m3. In distal zones the density ranges from 1.18 to 1.44 t/m3 (Figure 3-
71 Chapter 3. Chilliwack Lake
0 CHWK02-Depot Creek CHWK03-Lake Delta -50 CHWK04-Paleface Creek 0.56 mm/yr. CHWK05-Distal 1 CHWK06-Distal 2 -100 Fire Disturbance Radiocarbon Date (cal yr. BP, 1 )ó
-150
-200 0.79 mm/yr. -250 Core Depth (cm)
-300
-350
-400 0 -1000 -2000 -3000 -4000 -5000 -6000 Calendar Date (B.P.)
Figure 3-20: Paleomagnetic chronology (calendar years) mapped to sediment core depth (cm). Red dots indicate fire disturbance events recorded in the deepest core. Elevated rates of sedimentation are inferred at the Upper Delta (CHWK-03) based on radiocarbon dating of an organic layer at 85 cm (180 ±40 years BP - roughly A.D. 1670-1800; radiocarbon calibration for this era is only loosely constrained). Before approximately 3 200 BP there appears to be a slightly elevated rate of accumulation. Reduced major axis regression within the distal cores indicates a significant change in slope (α=0.01), though there are fewer core samples deeper than 2 m to confirm this shift in trend.
22b). Organic material is estimated to constitute roughly 10% of the volume. Based on the relationship amongst density, depth and longitudinal position established from the lake cores, a simple three-dimensional model was developed to estimate the total mass of fine sediment in the lake. For the deeper portion of the deposit, it is expected that the average bulk density of the lake sediment has increased with time due to compaction and consolidation. It is assumed that the density profile roughly follows a logarithmic curve [Lane and Koelzer, 1943; Gill, 1988] and that the maximum density to be expected is near 1.65 t/m3 (further assuming constant grain size composition with depth). The total mass of fine lacustrine material is then calculated to be approximately 144 ± 18.6 ×106 Mg, indicating an average deposit density of 1.36 t/m3. Based on the above calculations, the long term (13 300 year) specific annual yield of fine lacustrine sediment for the whole contributing basin is 32 ± 4 t/km2/yr. Including the
72 Chapter 3. Chilliwack Lake
0.0m 3 6 9 12 15
Lacustrine Sediment Thickness (m)
Figure 3-21: Isopach map showing the inferred depth of lacustrine sediment since the end of outwash deposition and the onset of lacustrine conditions at Chilliwack Lake, based on seismic and sonar surveys. The seismic transects are overlaid in white.
(a) (b) 0 0
-2 100
-4 200
-6 300
-8 400 -10 Depth (cm) 500 CHWK-02 - Depot CHWK-03 - Delta
Depth of 1/2 m-12 strata CHWK-04 - Paleface 600 CHWK-05 - Distal 1 -14 CHWK-06 - Distal 2 ? -16 700
-18 800 0 1E+6 2E+6 3E+6 4E+6 5E+6 6E+6 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Volume (m3) Density (g/cm3)
Figure 3-22: (a) Volume of lacustrine sediment with depth. Uncertainty bounds are indicated by multiple profile lines. Model is based on seismic imaging of the sediments and follows assumptions outlined in the text. (b) Variation of specific dry weight density of sediments with depth in each of the lake cores. The logarithmic trend lines indicate the expected pattern of density with depth.
73 Chapter 3. Chilliwack Lake
Table 3.3: Sediment delivery to Chilliwack Lake: Volumetric estimates
Estimated Mean Density Mass Volume (×106m3) (Mg/m3)(×106 Mg) Medium Term (4 000 ka) Yield Deltaic Sediments ? Lacustrine Sediment 34.5± 2.5 1.48 51 Long Term (13 000 ka) Yield Deltaic Sediments 106 ± 13 1.8 190 Lacustrine Sediments 98.5 ± 6.5 1.48 146 Outwash Deposition 140 ± 20 ? Total Long Term 344.5 ± 24 bedload material from three fan deltas adds roughly another 30 t/km2/yr. In the last 2 000 years, the fine sediment yield has been about 21 t/km2/yr (Table 3.4), plus an unknown amount of bed material yield. This places it decidedly at the low end of the spectrum with respect to other large (>10 km2) Cordilleran lakes, though this is to be expected considering the small glacier coverage in the basin and relatively small upstream catchment area. Figure 3-23 shows Chilliwack Lake plotted within the growing dataset of lake information collected by researchers in the past few years.
Table 3.4: Mineral sediment delivery to Chilliwack Lake: Late Holocene (<2 000 years BP) estimates based on core chronology.
Average Deposition Inferred Specific Rate (mm/yr) Yield (t/km2/yr) Chilliwack Lake 0.48 ± 0.04 20.9 ± 2.0
There are no historical measurements of suspended sediment yield from the Upper Chill- iwack River. However, the medium term yield as inferred from the lake cores appears to be the same order of magnitude as the modern Silverhope Creek, a neighbouring catchment to the Chilliwack, with similar catchment area (350 km2) and physiography. Monitoring of suspended sediment there suggests a contemporary average specific fine sediment yield of 11.7 t/km2/yr, consistent with patterns of sediment yield in the Cordillera in post-Neoglacial time [Schieffer et al., 1999]. The resolution of the paleomagnetic curve averages 60 years between sampling points (5 cm). This temporal scale likely incorporates a significant range of variability, from multi- ple geomorphically effective floods to large scale changes in global circulation patterns (PDO,
74 Chapter 3. Chilliwack Lake
10000
Stave Meziadin 1000 Ape Berg Bowser Nostetuko Lillooet Mud Lake Hector Green 2 100 Chilko Chephren Maggie Quiniscoe Harrison 10 Chilliwack Moose ield (T/km /yr.) Glacier
Y Woods Clayoquot Pyramid 1 Kite
Klept Ash
0.1 Glacier Cover (%)
pecific Sediment
S Middle 85% 50% 0.01 Gallie 10% 0%
0.001 0.01 0.1 1 10 100 1000 10000 100000 Contributing Catchment Area (km2 )
Figure 3-23: Fine sediment yield for Chilliwack Lake shown in comparison to other lakes in British Columbia. Dots size indicates the proportion of basin glacial cover. Data collated from Gilbert et al. [1997]; Desloges and Gilbert [1998]; Schieffer et al. [1999]; Hodder et al. [2006].
ENSO). The presence of large, distinct beds that are presumably from discrete events empha- sizes the high variability of sediment delivery on the annual-to-decadal scale. At the century scale, however, much of the noise appears to be damped. Sedimentation rates appear stable, and do not show a prominent signal from Neoglacial advance, though a slight increase in sedimentation rates is hinted at prior to 3 200 BP. Further work is needed to assess this, but it appears that Neoglacial effects did not overwhelm the system and/or were buffered by storage effects upstream of the lake. Chilliwack Lake captures essentially the entire Holocene depositional record from 3 major basins. In the Strahler ordering scheme established in Chapter 2, Paleface, Depot and Upper Chilliwack Creeks are 5th, 6th and 7th order catchments, respectively. This is a spatial scale within the drainage where storage of material becomes relatively important. The total deposition in the lake, including the lower outwash layer, accounts for roughly 10-15% of the sediment estimated to have been mobilized within all contributing catchments.
75 Chapter 3. Chilliwack Lake
A majority of material eroded from hillslopes is left in storage between hillslope scales and the major tributary, or distal, fluvial domain.
76 Chapter 4
Evolution of Chilliwack Valley Mainstem
Most of the summary post-glacial sediment yield from Chilliwack Valley is contained in a large alluvial fan that extends in a semi-circular arc from Vedder Crossing, at the piedmont of the Cascade Range, to the Fraser River. Much of the sediment mass in Vedder Fan is composed of glacial material eroded from the glacially aggraded mainstem and the tributary valleys. Previous chapters have examined long-term sediment fluxes at the hillslope and tributary scales; here we look at the mass balance of post-glacial erosion and sedimentation at the scale of the mainstem valley. These are 7th and 8th order links according to the Strahler ordering system established in Chapter 2. In the first section of this chapter, the volume of the glacial valley fill is estimated by fitting a spline mesh across the remnant surfaces and calculating the bounding volume. The depth of valley fill that has been evacuated is over 80 m in places and the breadth is over 1 km. In the postglacial evolution of the valley, then, the lower tributary fills and mainstem deposits are a dominant component of the sediment budget. Given the uncertainty in the topography of the lower valley immediately following final retreat of the Fraser ice, maximum and minimum estimates are given. Many of the major valley depositional units were laid down in complex association with older deposits, and thus definitive landform interpretation and ascription of precise bounding volumes remains difficult. Nevertheless, an approximate estimate of the total volume evacuated from the lower valley can be made. The second part of this chapter examines the historical development of Vedder Fan at the outlet of the valley, using well logs and radiocarbon dates to infer the internal architecture of the fan and probable rates of accumulation. Sonic drilling work was undertaken by the City of Chilliwack in 2003-2006, and a rich database of historical well logs was assembled. The information has assisted in the development of a 3-dimensional chrono-stratigraphic model
77 Chapter 4. Evolution of Chilliwack Valley Mainstem of the fan. An idealized post-glacial Fraser Valley floor was reconstructed, and the volume of sediment deposited thereupon was estimated. A final section compares the eroded volumes from the major valley sediment sources with the total calculated fan volume.
4.1 Initial Conditions
Before introducing the post-glacial sediment budget of the lower valley, a brief overview of the regional glacial history is presented. Radiocarbon dates in the text that follows are in 14C years, in keeping with normal practice in Quaternary studies, whereas later discussions use calibrated ages in order to properly estimate rates of sediment flux. The Puget Lobe of the Cordilleran Ice Sheet reached its maximum extent 14 500-15 000 14C years ago [Easterbrook, 1992; Porter and Swanson, 1998; Booth et al., 2003]. By 13 000 14C BP, a large calving embayment began to develop in the Strait of Georgia and as ice retreated, glaciomarine drift and other marine deposits accumulated over a landscape that had not yet rebounded from isostatic depression [Clague et al., 1997]. Armstrong et al. [1965] designated this interval as the Everson Interstade, ending at about 12 000 14C years BP. Relative sea levels lowered substantially - up to 200 m in some parts of the Fraser Lowland - as a result of isostatic rebound of the deglaciating land surface [Hutchinson et al., 2004]. Relative sea level fell from 150 m elevation to about −15 m between 12 000 14C yr BP to 9 900 14C yr BP. There was then a slow rise to near the modern level some 3 000 years later. There were at least two late readvances of the ice sheet in the Lower Fraser Valley (Figure 4-1), marking the Sumas Stade. The number of the advances and their timing remain unclear [Clague et al., 1997; Kovanen and Easterbrook, 2002]. With the advance of the Sumas ice, glacial sub-lobes extended into the lower Coquihalla, Hatzic and Chilliwack Valleys. During this period a large train of outwash extended from Chilliwack Lake, past Slesse Creek to a short distance downstream of Borden Creek (Figure 4-1a). The sandur appears to have prograded downvalley into a large and long-lived lake that was ponded by Fraser Ice. The upper valley glacier had retreated east (upstream) of Slesse Creek by 11 900 14C years BP. Fraser ice that entered the lower valley advanced up-valley toward Tamihi Creek and downstream toward the Columbia Valley. Evidence suggests that erosion of lower Chilliwack Valley mainstem sediments, and the building of Vedder Fan, began immediately following the recession of the Fraser ice lobe in the Lower Mainland [see Easterbrook, 1971; Saunders, 1985, Figure 4-1c] . Ice-free conditions
78 Chapter 4. Evolution of Chilliwack Valley Mainstem in the lower valley commenced after approximately 11 000 14C years B.P., based on the radiocarbon chronology at Tamihi moraine developed by Saunders et al. [1987] and Clague et al. [1997]. After this date, Tamihi moraine was abandoned and ice gradually disappeared above Vedder Crossing. The date of the final withdrawal of the ice sheet from the Fraser Valley is not well constrained; Kovanen and Easterbrook have proposed two readvances of the Fraser ice between 11 400 and 10 250 14C years (their Phases SIII and SIV), introducing the possibility that the ice sheet remained in the Fraser Valley for some time after the deposition of material at Tamihi Moraine. As the ice retreated from the low divide of metasedimentary rock between the Chilliwack River and the Fraser Valley, the Chilliwack River would have begun to flow northward to the Fraser River. With the retreat of the ice, the effects of the baselevel fall in Chilliwack Valley would have propagated rapidly upstream. Much of the initial downcutting in Chilliwack tributaries was accomplished in the early part of the Holocene Epoch. High sediment loads would have resulted from mass movement on valley slopes laden with glacial debris. Material was initially eroded from the valley floor of Liumchen Creek, later followed by Tamihi Creek, Slesse and Foley Creeks. There is a sequence of terraces between Foley Creek and Slesse Creek that record the stages of incision in the channel. Evidence at Post Creek and upper Foley Creek emphasizes that channels were sometimes shaped by catastrophic flooding - j¨okulhlaups- as ice dams released large volumes of meltwater that had collected in proglacial lakes and valleys [Goff and Hicock, 1995]. In the Fraser Lowland, Cameron [1989] discussed the possibility of a wasting ice lobe sitting in the Sumas Valley in the final phase of deglaciation (Figure 4-1d), leaving a large slackwater basin that would eventually become Sumas Lake. To the southwest, the Nooksack River began building an alluvial fan toward the Sumas Valley (Figure 4-1e). The Chilliwack fan built outward, prograding into Sumas Lake, and also possibly contributing to the north- ward deflection of the Fraser River [Cameron, 1989]. The distal edges of the fan interfinger with floodplain sediments of the Fraser River. The initial topographic state for the post-glacial sediment budget investigation, and the routing model that follows in Chapter 6, is considered to be the landscape as it was immedi- ately prior to the onset of baselevel fall (Figure 4-1d). At that point the Fraser lobe no longer obstructed the valley at Vedder Crossing, and the alpine Chilliwack glacier had receded to the headwaters of the valley. Constraints on the exact date are poor, but for the purposes of estimating rates of sediment evacuation in the sediment budget, a date of 11 100 14C years,
79 Chapter 4. Evolution of Chilliwack Valley Mainstem 15 km 0 Fan Vedder (d) (e) (f) City of Chilliwack Lake Sumas Valley Sumas Fan Sumas [1997]. C years B.P. (d) A remnant lobe of ice in Sumas Valley, following ice 14 Valley Clague et al. Chilliwack Tamihi Moraine [1989] and Lake Cultus (a) (b) (c) Cameron C years B.P. (e) Sumas Lake fills the depression left by the ice, basin fills throughout Holocene (f) The Sumas Ice 14 Valley Columbia [1987], Six-part deglacial history of Chilliwack Valley: (a)-(c) depicts the retreat phase of the final stage of the Sumas ice Saunders et al. modern landscape: Cityfrom of Chilliwack and Sumas Valley. Chilliwack River has been channelized into the Vedder Canal. Adapted retreat, 10 500 - 10 000 Figure 4-1: lobe, active floodplain outlined in green, 11 200 to 10 500
80 Chapter 4. Evolution of Chilliwack Valley Mainstem or 13 000 calendar years, is used. A 500 year error in this date would introduce a 4% error in the whole Holocene sediment flux data.
4.2 Mid Valley Fill
Because many remnants of the mid-valley glacial topography remain intact, it is possible to estimate the total volume of sediment evacuated from Larson Sandur, which extended from present day Chilliwack Lake to a short distance downstream from Borden Creek. Several terrace surfaces representing different historical stages of valley evolution were rendered in ArcGIS using data from field reconnaissance and published data, in order to recreate the end- Pleistocene topography. Subtracting the DEM grid representing modern topography from this yields approximate volumes of evacuated material. Modern fan deposition and other accumulations were accounted for in the resulting volume, to establish the net erosion. Figure 4-2 shows an isopach diagram of the total volume estimate to have been evacuated from Larson Sandur and the valley fills in lower Slesse, Foley and Chipmunk Creeks. Some erosion from the surface intact remnant topography is assumed. The total volume eroded here (Table 4.1 does not include the glacio-lacustrine unit that underlies the sandur, as this is tabulated in the following section.
Table 4.1: Net bulk volume eroded from the mainstem between Chilliwack Lake and Borden Creek
Reach Eroded Sandur Vol.(m3x106) Chilliwack Lake to Slesse 15.3 ±2.1 Lower Slesse Valley 5.0 ±0.7 Slesse to Tamihi 25.3 ±5.2 Sum 45.6 ±5.7
The reach extending from Slesse Creek to Ranger Run (located 100 m west of the edge of Figure 4-2) consists of floodplain that has been constructed from the reworked mass of sediment originating from Foley Creek, Larson Sandur and Slesse Creek. There were likely several generations of paraglacial floodplain that have progressively coarsened with succeeding transport episodes. There is now a broad (up to 1 km wide) floodplain from which material is recruited by a laterally active channel. Figure 4-3 shows the surveyed extents of terraces between Foley and Tamihi Creeks. Terrace surfaces on the North and South sides of the valley were surveyed using Paulin altimeters. The surveys revealed only sparse evidence of high terraces within the sandur
81 Chapter 4. Evolution of Chilliwack Valley Mainstem
590000.000000 593000.000000 596000.000000 599000.000000 602000.000000 Chipmunk Creek Foley .000000 Creek .000000 5441000 5441000
h nc
.000000 e .000000 B ’s on 5438000 rs 5438000 a L Net Erosion Depth (m) 0 - 10 10.1 - 20
.000000 20.1 - 30 .000000 30.1 - 40 40.1 - 50 5435000 5435000 Slesse 50.1 - 60 Creek 60.1 - 70 70.1 - 80 01 2 4 Kilometers 80.1 - 90 90.1 - 106 .000000 .000000 5432000 5432000
590000.000000 593000.000000 596000.000000 599000.000000 602000.000000
Figure 4-2: Isopachs of the estimated net erosion from the mid-valley, at Slesse confluence. The maximum erosion depth is 106 m. Shallow erosion is shown across the intact remnants of Larson’s Bench, after a pre-erosional surface was fit to it. remnants, but there are numerous terrace flights at lower elevation. It is expected that initial downcutting occurred quite quickly, and once the channel became sufficiently armoured, it took longer to incise and rework the sediment supplied from within the deposit and from upstream (see Chapter 6).
4.3 Glacio-Lacustrine Deposition
Exposures of laminated clay- and silt-dominated sequences are found throughout the main- stem drainage (Figure 4-4), recording the existence of at least two, most likely several, lakes over the course of the late Pleistocene Epoch. Glaciolacustrine facies are quite variable, mak-
82 Chapter 4. Evolution of Chilliwack Valley Mainstem
Foley 400m Chipmunk Creek Creek
Slesse Borden Creek Creek ench n’s B Larso 300m
Sandur/ Delta
Outwash Surface Upper Terraces 200m Mid Terraces Lacustrine Deposits Lower Terraces Water Level
10 15 20 25 30
Distance Upstream from Vedder Crossing (km)
Figure 4-3: Longitudinal survey of terraces between Foley and Tamihi Creeks. Degradation appears to be hinged on a knick point close to the outlet of Foley Creek. Terrace flights have been grouped as high (red), middle (light blue) and low (green). Points surveyed on the terraces are coloured accordingly; points on the modern river are coloured dark blue. River topography is taken from the BC TRIM DEM breaklines ing definitive interpretation of lacustrine sequences quite difficult. Numerous interpretations have been made regarding the extent and interconnection of lakes [Chubb, 1966; Munshaw, 1978; Clague and Luternauer, 1982; Hicock et al., 1982; Saunders et al., 1987]. A glaciolacustrine sequence is found in the lower 10 km of the valley (see Figure 4-6, Hicock et al. [1982]), up to 16 m thick, and overlying outwash gravels from the advance phase of the Fraser glaciation (20 to 21 ka 14C B.P.). Exposures of a second major sequence, representing several episodes of glaciolacustrine sedimentation, extend from a short distance (approx. 1 km) upstream of the Slesse Creek confluence to near the Tamihi Creek confluence [Clague and Luternauer, 1982; Saunders, 1985; Saunders et al., 1987]. Present exposures in lower Slesse Creek show the lake extended some distance up Slesse Valley and have been dated to 11 900 ka 14C B.P. [Saunders et al., 1987]. Well logs from the Slesse Hatchery (Provincial Well Log Database: 23325 & 23326) indicate that the laminated silts extend to
83 Chapter 4. Evolution of Chilliwack Valley Mainstem
Figure 4-4: Fine sediment source material is delivered episodically from deep glacio-lacustrine de- posits in the mid-valley reaches. over 70 m depth . Dates recovered by [Saunders et al., 1987] further downstream along the mainstem near the Tamihi confluence indicate a similar age: 11 600 ka 14C B.P. Assuming valley-wide extents for the above sequences, the total volume of eroded glacio- lacustrine clays and silts in the mainstem and Slesse drainages is at least 122 ±26 × 106 m3, a significant portion of the lower valley budget. In recent times, individual episodes of erosion in the glaciolacustrine material are estimated to deliver over 104 m3 to the channel [Thomson, 1999]. One source area in particular was estimated to have delivered as much as 1.8 x 106 m3 of material [Thurber Engineering, 1997]. These are some of the most prominent slope movements along the valley mainstem.
84 Chapter 4. Evolution of Chilliwack Valley Mainstem
Figure 4-5: Larson’s Bench is incised by a late Holocene channel. The formerly active layer of the incising channel overlies sandy deltaic fill on the left; lacustrine clay on the right.
4.4 Lower Valley Fill
The Lower Chilliwack Valley has a complex history of glacio-fluvial, glacio-lacustrine and glacial till deposition. The most prominent deposits in the lower valley are the Ryder Lake Upland and the Tolmie Upland (Figure 4-6). In post-glacial time, these thick moraine deposits have been subject both to river incision and to diffusive slope processes that have carved a network of large gullies into their flanks. The sediment mass has been largely stabilized under the influence of vegetation, however there are still shallow failures that continue to shed material to the floodplain below. In the early postglacial evolution of the valley, the upland moraine complexes have been a very important sediment source to the mainstem due to the direct mode of delivery to the channel (no intervening storage) and the large volumes of material delivered. The Ryder Lake Upland comprises a thick sequence of early Fraser Glaciation outwash sediments overlain by silts and a compact massive diamicton, most likely subglacial till. The upper sequence of sediments is fine-textured and contain an assortment of sub-angular to sub-rounded, often striated clasts. Quartzite lithologies found in the Ryder Lake Upland suggests a Fraser Valley provenance [Saunders et al., 1987], although the contribution of Chilliwack source material is not entirely clear (See Chapter 5). The Tolmie Upland is part of the ‘Upper Terrace’ sequence identified by Saunders [1985]
85 Chapter 4. Evolution of Chilliwack Valley Mainstem
(‘L2b’ surface). The genesis of the Tolmie Upland is uncertain, however, assuming that the gullies within the deposit have been eroded in post-glacial time, the minimum volume eroded from the source area has been estimated to be 80 ±13 × 106 m3. Saunders also identified a large failure in the same terrace sequence, which deposited a large quantity of material in the river valley. The net erosion there is estimated to be 29 ±4.4 × 106 m3.
4.4.1 Ryder Lake Upland Moraine Complex
According to Saunders [1985], the Ryder Lake Upland was a principal deposition site for the Fraser Valley ice lobe, and it is likely the largest single source area in the postglacial sediment budget. The depth of the deposit is over 100 m in places, based on water well records and exposures (Figure 4-6). The compaction of the material indicates little or no secondary movement occurred after till formation. Lodgement and meltout processes were likely largely responsible for their deposition. Near the up-valley extent of the Ryder Lake Upland, there is the residual crest of a large moraine [Saunders et al., 1987]. Two important problems in determining the exact magnitude of this contribution are establishing the sequence of events that unfolded during glaciation, and the true historical bounding geometry of the deposit. The first point has been addressed by Saunders et al. [1987], who have described the terrace geomorphology of the lower valley. In their recon- struction of late-glacial valley conditions, the Fraser Lowland ice mass advanced into the valley and deposited large quantities of till. A subsequent readvance of the ice overrode and compacted the till, and left the end moraine near Tamihi Creek. The mass of subglacial till extended from Tamihi Moraine in the east to a point roughly 7 km downstream, where a delta was built out toward a large lake in the modern Cultus Lake basin. A river floodplain 250-300 m wide was built along the southern margin of the glacier. Based on this reconstruction of the Ryder Lake Upland till, the second problem (bounding geometry) is dealt with here as three possible scenarios: (i) a relatively deep deposit, making
Figure 4-6 (facing page): Cross section showing the available evidence (exposures and well logs) to describe the Pleistocene and post-glacial valley fill. The modern channel is demarcated with a dashed line. Upper planform map shows detailed terrain mapping [Armstrong, 1980; Ryder, J.M. and Associates, 1995] that outlines major lacustrine and morainal strata. Ryder Lake Upland is depicted with a veneer of aeolian material and thick till accumulations beneath. Points on the plan map indicate exposures and well logs that are graphed in the section detail. Vertical exaggeration is 20x. There is assumed to be considerably more lateral complexity within the valley’s deposits (and eroded volume) than can be reconstructed with this evidence.
86 Chapter 4. Evolution of Chilliwack Valley Mainstem
Creek
Outwash
at Pierce
Exposure
Bluffs
?
Campground
a/Sandur
Upper Valley
Delt
fs
Gray Bluf
et al.
Silt Clay Diamicton
1997
20000
Clague
11 400 11 700 11 900
1966
Chubb,
Borden
Well Logs - Legend
Glacio-Lacustrine
(Multiple Episodes)
Bedrock Gravel Sand
outh Cen.
Y
all
Valley W Modern River Profile
20 190
fs
CH-3
Tolmie Upland
Bluf
Tolmie
TSON-9
WA
WATSON-10
ance Along Transect (m)
35877
CH-1B (SS)
Crest of
et al.
35900
1 300
11 500 11 200 11 800 11 600 1
11 200
10000
1997
amihi Moraine
Dist
T
Clague
CH-2 (LS) Lower Valley Transect
28078
34830
Ryder Upland
Older Silt
Sequence
Ryder Creek
56031 Advance Outwash
Veneer
59530
32336
Thin Aeolian
32308
57836
?
Till Deposit
CH-1A (RC)
Ryder Upland
37909
34588
51506
CH-1
21 400 21 600
0
0
400 200 Elevation a.s.l. (m) 87 Chapter 4. Evolution of Chilliwack Valley Mainstem
Table 4.2: Estimated bulk erosion volumes for three different assumed topographic configurations in the Lower Chilliwack Valley in post-glacial time. The true value is assumed to be intermediate between I & II. The bounds of minimum net erosion are shown in Figure 4-8
Scenario Description Estimated Net Erosion (m3x106) I Valley-wide fill 1 350 II Intermediate fill 855 III Minimum net erosion 309 a valley-wide fill at the elevation of the ‘middle terrace’ of Saunders et al. [1987], (ii) a deposit of intermediate depth, closer to the mid-elevations of the lower valley, and (iii) a minimum estimate of Lower Valley erosion, closer to the valley floor. The associated volumes for each of these configurations is shown in Table 4.2. Cross-sections showing the extent of these three scenarios are shown in Figure 4-7. The third scenario appears less likely, based on the geometry of the subglacial surface on Ryder Lake Upland and Tamihi Moraine remnants, but it represents the minimum estimate of the net erosion from lateral valley deposits. Figure 4-8 shows the eroded volume from the valley sidewalls, based on a reconstruction of the deposit with a spline mesh surface (grey zone in Figure 4-7a). The wall angle of least-eroded portions of both the Ryder Lake Upland and the southern valley wall show a similar angle of repose. Fluvial reworking has likely undercut the slopes repeatedly, as the river cut down through the deposit and sediment mass was reworked by Chilliwack River. It is assumed that the lower terrace surface that dams Cultus Lake, sitting approximately 20 m above the modern Chilliwack River floodplain, was once a continuous surface across the broad lower reach of the river above Vedder Crossing (Saunders et al. [1987], see Figure 4-1). This represents between 62 ± 8 m3x106 of gravelly material. Additional quantities of sediment delivered over the course of the Holocene from the Columbia Valley have mostly been intercepted by Cultus Lake. Compared to some of the fluxes detailed in the previous chapter, a dominant proportion of the Holocene sediment budget is derived from the former glacial valley fill. At least 1 km3 of material, possibly as much as 1.6 km3, has eroded from the valley mainstem, representing roughly 60-75% of the sediment budget. From the perspective of the modern sediment trans- port rates, the fluxes required to evacuate that volume would have been much higher than modern rates. The volume of outwash material estimated to have been evacuated from lower Slesse Creek alone, for instance, represents over 1 000 years of gravel bedload transport from
88 Chapter 4. Evolution of Chilliwack Valley Mainstem
Extents of the late glacial ice mass, 400 Chilliwack Valley
Glacio- fluvial surface Ryder Lake Upland L3 ‘Middle’ Ice Terrace I 200 II L4 ‘Lower’ Terrace Elevation (m a.s.l.) III Modern Floor North ? South 0 1 2 3 4
(a)
Tamihi Moraine 400 Ryder Upland
L3 Surface I Lake Elevation Ice II 200 Moraine III
Longitudinal Valley Section, Looking North 0 1 2 3 Cross Section Distance (km) (b)
Figure 4-7: Reconstructed geometry of the major valley landforms in Lower Chilliwack Valley, downstream of Tamihi Creek. (a) shows a composite set of cross sections of the valley near Ryder Creek, 3 km downstream from the crest of Tamihi Moraine. The shaded area represents the zone of minimum net erosion of glacial material (scenario III, see Figure 4-8 and text). (b) is a longitudinal valley cross-section looking North, encompassing the former locality of Tamihi moraine. A mid-valley lake elevation is shown, based on the assumed elevation of the delta front at the distal end of the Larson’s Bench sandur. Cross-section lines from Ryder Upland showing the maximum height of the remnant moraine are in the background.
89 Chapter 4. Evolution of Chilliwack Valley Mainstem
120 m 90 60 30
Deposition 0 -30 -60
Erosion -90 -120 m
Figure 4-8: Scenario III, with minimum bulk volume erosion from valley sidewall sediment sources, including the Ryder Lake Upland. Contours of erosion and deposition are in increments of 15 m. Total erosion is 344 ±45 × 106 m3. Deposition evident at the base of Ryder Upland is approximately 35 × 106 m3. the entire Chilliwack Basin (roughly ∼50 000 m3 yr−1).
4.5 Vedder Fan
The alluvial fan at the outlet of the Chilliwack Valley, known as Vedder Fan, is a Holocene landform that has built out over the outwash and floodplain surface of the Fraser Valley (Figure 4-9). In post-glacial time, distributary rivers have delivered a mixture of coarse sediment from the Chilliwack Valley. The switching of the channel back and forth through the action of avulsions and reactivation of secondary channels has gradually built up a semi- circular cone of sand and gravels that is more than 35 m thick near the apex. The Chilliwack River was in the process of reworking and incising through accumulated glaciofluvial fill in the lower valley when the ice retreated and construction of the fan was
90 Chapter 4. Evolution of Chilliwack Valley Mainstem
Chilliwack Elevation Mountain Valley Floor 1 - 4 4 - 6 6 - 8 8 - 10 10 - 12 12 - 13 13 - 15 15 - 17 17 - 19 19 - 21 21 - 23 23 - 25 25 - 28 28 - 30 30 - 32 32 - 35
Vedder Mountain
- 00.5 1 2 Kilometers
Figure 4-9: Figure showing a band of elevations between 1 and 35 m a.s.l. in Sumas Valley, Vedder Fan and the Fraser River. The bounds of Figure 4-14, showing City of Chilliwack Drilling work (2006), are highlighted. Note the semi-circular geometry of the Vedder Fan, and the relatively low- lying surrounding topography. TRIM BC digital elevation data. initiated. The lowest possible elevation of the fan apex is on the bedrock sill of a ‘V-notch’ in the range front, only a few metres below the present channel - roughly 30 m above sea level. If the glaciofluvial fill above Vedder Crossing was intact at the time that fan-building began, it is possible that the apex could have been up to 10 m higher, as discussed by McLean [1980]. Based on interpretation of the modern radial profile of the fan, it is quite likely that there has been some complex reworking of the upper region. Material from the apex has been incised and redistributed out along the fan, much in the manner described by Schumm [1977]. Judging from historical maps of the fan area (Figure 4-10, top), the Thuwlman / Lhqueleq / Qwelkwaltem channel complex (Chilliwack / Luckakuck / Atchelitz, see Bowman [1992]; Schaepe [1999]) was a multi-threaded wandering gravel bed river. There was a distributary
91 Chapter 4. Evolution of Chilliwack Valley Mainstem network of slough channels that would have been reactivated periodically as the river avulsed from one side of the fan to the other in the course of aggradation. The Sto:lo Indian word for the river at this time was Tswelmuh (Th’ewlmel, Schaepe [1999]), the term Tsel meaning to “go away”. This is an appropriate name for a river that changes its course, as the Chilliwack system often does. The historical Chilliwack River developed a network of small channels that drained west- ward to Sumas Lake [Orchard, 1983]. Sumas Lake was a large body of water that changed its extent seasonally, depending principally on the flows in the Fraser River. At low water, the lake was 3 m deep, with a length of 10 km and width of 6 km. During spring freshets and winter rains the lake swelled to a length of over 25 km and a depth of 10 m [Thom and Cameron, 1996]. The last of the lake waters were drained in 1924 to make way for agricultural lands. Cameron [1989] notes that silt and sand facies recovered from drill cores within the Sumas valley are quite variable, indicating a high energy environment within the lake basin at times. The lake was bounded by Nooksack/Sumas River fan deposits to the southwest and the Vedder Fan to the northeast (Figure 4-1e).
4.6 Architecture of the Vedder Fan
The basement of most of the Chilliwack/Sardis area and the Sumas Valley is a thick blue clay unit [Halstead, 1986; Cameron, 1989]. None of the drilling at the fan apex attained depths that approached this horizon. Logs from a petroleum test hole place the lower limit of gravels in the area at 24 m below grade (−12 m a.s.l.) near mid-fan, roughly 4.3 km out from Vedder Crossing. Gravels are underlain by sand to 43 m below sea level, and then by clay to 400 m b.s.l. A deep borehole further east (11.6 km from Vedder Crossing, Bos Trout Farm) found a similar stratigraphy: sand and gravel to 14 m b.s.l., sand to 52 m b.s.l., and then layers of clay, silts and fine sands to bedrock at 489 m b.s.l. Presumably the sands are glacial outwash and deltaic material, overlying the deep glacio-marine or marine fill that underlies much of the Fraser Valley [see Armstrong, 1984]. The bounds of the Vedder Fan have been delimited by Armstrong [1980], based on surface expression of sediments and an evaluation of drilling logs. This work has been supplemented by Levson et al. [1996] and Monahan and Levson [2003], who have provided greater detail in the course of reviewing available well records and geotechnical drilling reports to assess the earthquake hazard for the Chilliwack area. Maps and sections from these reports have
92 Chapter 4. Evolution of Chilliwack Valley Mainstem
Thuwélman (Chilliwack) Qwelkwaltem (Atchelitz) Lhé queleq (Luckakuck)
Th’éwálmel (Vedder)
Sumas Lake
Chilliwack Mountain
Vedder Canal
Figure 4-10: Historical planform of Chilliwack River north of Vedder Crossing, ca. 1891 (top) and 1991 (bottom). An interlinked network of channels (four of the larger threads are labelled) alternately occupied and abandonned various section of the fan as it evolved. The historical figure was derived from an ordnance survey of the Chilliwack area. The modern map was generated from BC TRIM mapping. 93 Chapter 4. Evolution of Chilliwack Valley Mainstem provided the foundation for the work that follows. The pattern of fan growth is probably similar to the model proposed by Blair and MacPherson (1994, Stage 1, Figure 4-11). These authors have proposed a sequence of fan development that begins with a system dominated by mass-wasting processes, gradually evolving into a low-gradient fluvial system. Stage 1 in the case of the Chilliwack would have been heavily influenced by the very large pulse of glacial material that was delivered from the catchment in the early stages of development.
Figure 4-11: Model of alluvial fan growth, after Blair and McPherson [1994]. The lower bounding surface slopes slightly upward, as deposition keeps pace with a slowly rising base level. In the case of Vedder Fan this corresponds to active deposition on the Fraser Valley floor over the course of the Holocene.
It is assumed that the construction of Vedder Fan proceeded in a regime of minimal tectonic influence, and that subsidence only occurs by compaction under the weight of the sediment delivered. The shape of the nascent fan may have been influenced by stagnant ice in the Fraser Valley; it is possible that a significant proportion of the fan material is stored in the remnant basin that became Sumas Lake. Near-surface (<5 m) apex facies were exposed in section at a number of construction sites around the town of Sardis. An example is shown in Figure 4-12, where the upper 5 m (approx.) of a section displays a coarse base of fluvial gravels with sandy fill above. The gravel packages are most commonly massive, with abundant sands in matrix-supported units. Drill holes and exposures close to the apex and mid-fan show that deep fills of bedded uniform sands are not restricted to more distal sections but can be found throughout the fan head as well (Figure 4-12 and 4-13). Thick sand layers are typically the product of overbank sedimentation and infilling of sloughs. Minor silt layers are common throughout. The outcrops shown in the figure are similar to exposed banks in the lower portion of the Chilliwack Valley, above Vedder Crossing, where the river has maintained a wandering to
94 Chapter 4. Evolution of Chilliwack Valley Mainstem braided habit. The Vedder Fan has a relatively low gradient by comparison with other larger paraglacial fans in BC. The modern proximal fan slope is 0.022; channel slopes in the Vedder River range from 0.0046 in the first 3 km below Vedder Crossing to 0.00035 in Vedder Canal.
4.7 Well-log database
A database of well logs was developed using records from the British Columbia Ministry of Water, Land and Air Protection (WLAP), and the extensive collection of geotechnical logs compiled by Levson et al. [1996]. Further additions were obtained from Emerson Groundwater and the City of Chilliwack [Emerson, 2003]; the latter maintains a network of monitoring wells and has carried out a number of drilling projects in the last ten years. Automated text recognition and lithology coding of the logs was accomplished using the LDBuilder software package that was developed by a research group led by N. Schuurman and D. Allen, Simon Fraser University. The lithology units were reviewed to ensure con- sistency. More than two thirds of the database was eventually discarded, leaving 255 holes that met criteria of depth (at least 6 m deep) and detail needed to develop a representative 3-dimensional model. Information was imported into RockWorks to enable 3-dimensional interpolation of the lithology units. Rockworks employs an inverse-distance weighting algorithm with a horizontal bias to interpolate among the lithologic units in the bore holes. It further uses a randomized blending technique to simulate the likely horizontal transitions between boreholes. 3-D pixels (known as voxels) used in the model measured 150 x 150 x 0.5 m. Relatively few deep holes have been drilled within the proximal, gravelly areas of Vedder Fan due to difficult drilling conditions. The subsurface information database was enriched considerably by sonic drilling carried out in 2003-2006 in order to improve monitoring of the City’s drinking water supply. Drilling efforts were focused near the fan apex; Figure 4-14 shows the locations of the drill holes. All of the cores from the drilling campaign were made available for inspection and sampling. The cores were generally deep, achieving depths of 20 m on average. The deepest cores extended to over 50 m depth.
4.8 Apex Gravels - Core Descriptions
Material from the drill cores was classified according to sediment calibre and whether the unit was matrix- or clast- supported. A small number of framework gravel deposits appeared
95 Chapter 4. Evolution of Chilliwack Valley Mainstem
Figure 4-12: Gravel quarry near Vedder Road at Watson. Looking westward: flow was from left to right. Photo credit Vic Galay, Northwest Hydraulic Consultants.
Figure 4-13: Gravel quarry near Vedder Road at Watson. Photo is looking southward, and the direction of flow was out of the page. Photo credit Vic Galay, Northwest Hydraulic Consultants. 96 Chapter 4. Evolution of Chilliwack Valley Mainstem
572,500 573,000 573,500 574,000 574,500 575,000 575,500 576,000
00.25 0.5 1 1.5 5,441,000 Kilometers
UNSW-06 MW-1999-7
MW-2005-07 5,440,500
MW-2003-1
BC-11424 5,440,000 MW-2003-3 MW-2003-2 PW-1964-3 MW-2003-4 MW-1995-4
BC-75340 MW-2006-2 MW-2006-1 MW-2006-4 5,439,500
MW-2005-2 Vedder
Crossing 5,439,000
² 5,438,500
Figure 4-14: Locations of City of Chilliwack drilling operations, 2003-2006 (1995 aerial photogra- phy). See Figure 4-9 for map location with respect to the larger fan area. in the drill cores, though it was sometimes uncertain whether some of the matrix had been partly washed out in the course of removing the sediment from the core barrel. The sonic coring tends to disrupt sedimentary structure and clast orientation, and so classification remains somewhat tentative. Nevertheless, the recovered samples generally showed good representation of the sedimentary layers, and yielded valuable information such as patterns of bedding and vertical fining, lithology, and the nature of active geomorphic processes on the fan over time. This 1-dimensional view does not provide a diagnostic reconstruction of the fan history, but it does provide a few hints as to the evolution of the fan over time. Cobbles were recovered from a few units, but clasts coarser than 256 mm were generally quite rare. The barrel of the sonic core was 6 inches (152.4 mm) in diameter; perhaps two or three stones were recovered that exceed that diameter (the drill effectively bored through larger stones rather than displacing them, so that their presence, if not their full dimensions,
97 Chapter 4. Evolution of Chilliwack Valley Mainstem
0.063 mm 0.250 mm 1 mm 4 mm 16 mm 64 mm 75 Sand Gravel
50 Percent Finer 25
0 -4 -2 0 2 4 6 Grain Size (Y)
Figure 4-15: Grain size distribution from a sampling of units recovered from sonic drilling. Repre- sentative fractions > 32 mm could not be effectively recovered from the core samples, but the fractions that were examined effectively show the bimodal nature of the fan deposits. was reasonably well represented). Figure 4-15 shows a plot of the grain size distribution from a number of drill cores. Sands consistently had a modal size close to 0.5 mm. Much like the modern channel gravels on the fan (see Chapter 6), there is a bimodal character to most of the gravels with a deficit of material in the range of 1-8 mm. Figure 4-16 shows a cross-section of Vedder Fan based on the cores from City of Chilliwack drilling (Figure 4-14), as well as a number of other well logs (which were not viewed or sampled) within three kilometres of the fan apex, arranged as a cross section. Higher energy river environments (coarse, well-sorted matrix-depleted gravels) account for roughly 8% of the total examined core length. Approximately 35% of the record is dominated by low- energy environments that exhibit graded sands and silts with occasional, unsorted gravel. The remainder consists of intermediate states with generally abundant matrix and varying degrees of sorting that represents a mix of bed material load and washload. Sands with occasional gravel in the lowest unit encountered (MW03-1, MW06-1 and 4) have a slightly darker hue and a relatively greater proportion of quartzite clasts than other units encountered. Pebbles found in the sediment are not otherwise much different from Chilliwack lithological makeup (similar granitic, volcanic and metasedimentary types). This unit is interpreted to be chiefly composed of reworked tills. Based on the dating evidence recovered from the cores, this is probably part of the initial pulse of till evacuated from the
98 Chapter 4. Evolution of Chilliwack Valley Mainstem
Table 4.3: Radiocarbon ages from drilling at Vedder Fan. All samples were dated by conventional radiometric technique. Intercept ages and age range in calendar years before AD 1950. The age ranges in parentheses represent 1σ error limits. The ages were determined using the INTCAL98 database.
Lab Code Core Depth Measured Age Calibrated Age (m) (14C yr BP) (cal yr BP) Beta-213924 Twin Rinks (S.End) 16.5 8 800 ± 60 9 880 (9 710-9 920) Beta-215942 Unsworth Road 11.0 5 420 ± 40 6 250; 6 200 (6 190-6 280) Beta-215943 Watson & Tyson, MW05 27.4 9 020 ± 100 10 190 (9960-10230) Beta-215944 Garrison Crossing, MW06 36.6 9 700 ± 60 11 160 (11 120-11 190) valley. Material at depth (below −10 m a.s.l.) that is closer than 1 km to the apex is presumed to be part of the till package as well. However, there are some uniform framework gravel facies in MW05-07 (see Figure 4-17a) with well-rounded clasts that would suggest active sorting processes, and thus perhaps a lower-gradient fluvial system. The lithologies encountered here are typical of the modern upper Chilliwack River, rather than Fraser provenance.
4.9 Chronology and Volumetric Estimation
Wood was found in four different cores at the base of sand or silty units that were evidently low energy environments. Enough wood was recovered to enable conventional radiocarbon analysis. The wood at Twin Rinks (MW03-4) was recovered at a depth of 18 m below grade, however the drill hole collar was located on the floor of an old gravel quarry, adding approximately 7 m to the inferred stratigraphic horizon (see Figure 4-16). The depth and date of wood from this unit is consistent with woody material recovered from MW05-7 sited 725 m away (Figure 4-14). Dates from wood at these sites were (2σ) 9 865 ±285 and 10 125 ±245 cal. B.P. (Table 4.3). A deep hole at Chilliwack Area Services Unit (across the street, 285 m from MW05-7) yielded wood at 8 m below sea level that was dated at 11 035 ±175 cal. B.P. Isochron curves representing temporally bounded surfaces in the deposit are shown on the cross-section diagram (Figure 4-16). Their elevation is assumed to be constant within the radial swath of the fan, though this is clearly a simplification. Sequential paleo-surfaces of Chilliwack Fan were generated as a series of axially symmetric conical grids that are based on the isochrons presented in Figure 4-16. The distal slope of each surface is smoothly graded to intercept inferred elevations of the Sumas Valley and
99 Chapter 4. Evolution of Chilliwack Valley Mainstem
Gravel Limit
-06 (elev. 21 m)
Unsworth Road UNSW Gravel Sand Clay 14-C Date Water Wells +64mm 16 mm 8 mm 2 mm
10 000 BP+/- Tyson
Estimated Modal Grain Size atson &
Modern Topography (TRIM) 6230 BP+/- (elev. 27m) Sand
Gravel Corner of W
Silt/Clay 2005-07 Gravel w/
Framework sand matrix
(elev. 28 m) Area Services Unit
e 2006-1 . 28 m) i
1035 BP +/- (elev 1
f c
Near Cheam Centre
MW03-1
(elev.23 m)
win Rinks
. 24 m) T
MW03-4
(elev
win Rinks
T
. 29.63m) MW03-2
d b
Distance from Vedder Crossing (km)
(elev
45215 Keith Wilson Rd.
PW64-3
(elev. 31 m)
5633 Laura Cres.
1424
1 Darker, Sandy Diamict edder Rd.
1 2 3
(elev. 30.46 m)
E. of 5619 V
MW95-4 (elev. 34 m)
Garrison Crossing 2006-4
g
. 35 m)
(elev
CFB Property 2005-2 a Cross-section of Vedder Fan, based on examination of sonic core cuttings and assembled well logs. Letters refer 0
40
30 20 10 -40 -10 -20 -30 Elevation (m a.s.l.) Figure 4-16: to locations of photosexamined. in Dashed Figure lines 4-17. indicateapproximate, interpolated unless Narrower logs surfaces significant figures with with are an solid indicated. associated colours date are based database on wells radiocarbon with samples. minimal detail Most and elevations are have not been
100 Chapter 4. Evolution of Chilliwack Valley Mainstem i Silt Beds h g Lower Strata f e d More Matrix c 20 cm b Photos of sonic core material, illustrating facies assemblages that are indicated in Figure 4-16. Wood dated at 10 125 Framework Beds a 245 cal. years B.P. was recovered from the silt unit shown in (i) Figure 4-17: ±
101 Chapter 4. Evolution of Chilliwack Valley Mainstem the Fraser River floodplain. There is some evidence that supports placing an upper distal bounding surface of the fan at 6 800 cal. B.P. near 5 m a.s.l. Cameron (1989) found Mazama ash (approx. 7 680 cal. B.P.) at 0 and at −4 m a.s.l. in the Sumas Valley. A drill log on the northern distal edge of Vedder Fan records the presence of volcanic ash at 6 m a.s.l., though it is not explicitly identified as Mazama. Wood at 12 m a.s.l. was recovered on Unsworth Road, approximately 2.95 km northwest of Vedder Crossing, that was dated at 6 230 ±60 years cal. B.P. The lowest bounding surface is an approximation of Fraser Valley topography circa 11 000 B.P. The potential error inherent in defining the vertical extents of this boundary is much larger than in the horizontal (i.e. the horizontal extents are quite large relative to the vertical). If, for example, the elevation of the bounding surface was consistently overestimated by 2 m, the resultant volumetric error would be roughly 10% of the total volume. The horizontal boundaries present some difficulties as well, in that the lateral extent of the Fraser floodplain is not well constrained as the Vedder Fan builds. An accurate estimate of the lower bounds of the fan is required, and the model presented here is based on the best available evidence. The base of the unit generally conforms to boundaries that have been proposed or inferred by Cameron (1989, ‘Unit 4’), Levson et al. [1996] and Emerson [2003] and does appear to be in good agreement with most of the well logs in the database. The radial geometry of the fan may be approximated as a level, symmetric radial cone, rotated through 180 degrees. An averaged profile of the fan was generated by taking 30 radial profiles from apex to distal edge across the TRIM data in even 5.3 degree increments, extend- ing 6 km down slope. An exponential curve was fit to the multiple topographic measurements taken at successive radii from Vedder Crossing (Figure 4-18). The conical regression surface was subtracted from the modern topography. The east side showed slightly more positive residuals (i.e. topography is generally lower) and the west side showed more negative. The gross discrepancy is a matter of perhaps 16 ×106 m3 on either side - not insubstantial, but small compared to the total volume. On the whole, errors on the regression cone cancel out to a residual error of 5 million m3 or 0.22% of the total volume.
4.9.1 Isopach Diagrams
Using the established geometry of the fan, with lower bounds near −12 m a.s.l., a 3D model was constructed using the well database described above. The possibility of a basin extending
102 Chapter 4. Evolution of Chilliwack Valley Mainstem
45
-0.0001989x 40 y = 36.21e
35
30
25
20 Elevation a.s.l. (m) 15
10
5 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 Distance from Vedder Crossing (m)
Figure 4-18: Regression curve through multiple radial sections of Vedder Fan to −30 m depth, within the bounds of the former Sumas Lake, is considered as well. Three major lithologic categories (gravel, sand, silt/clay) were distilled from many log entries, to develop a general picture of the fan architecture and the relative thickness of each major category (Figure 4-19). Gravel units include some proportion of sand, while sand units represent relatively uniform sand. The spatial density and depth of subsurface data for the study area are not optimal for this form of analysis, and a number of interpolation effects are evident. The results shown here should be considered highly generalized. A series of diagrams show the approximate thickness of lacustrine, sand and gravel units between the posited lower bounding surface of the fan and the modern topography. This volume is 40 m thick near Vedder Crossing, and tapers to less than 20 m at distal regions. The boundary of historical Sumas Lake is shown in the diagrams. Here sand and silt deposits are depicted as having a thickness of up to 40 m; based on drilling work and radiocarbon analysis by Cameron [1989], accumulations of Chilliwack-derived material may be greater. Gravel (Figure 4-19b) is predictably quite thick at the apex of the fan, and there appears to be a lobe of coarse material extending to the northeast. The bounds of this lobe are tentative, since there are relatively few deep holes to the north and northwest to constrain the true gravel thickness there. Silt and clay units (Figure 4-19c) are concentrated in four different zones, labeled on the diagram: (1) Sumas Lake deposits (2) Fraser silts and clays (3) a small but deep deposit on the southwest distal edge of Vedder Fan, and (4) a lacustrine zone in eastern Chilliwack.
103 Chapter 4. Evolution of Chilliwack Valley Mainstem
Surficial Geology - after Levsonet al. , 1995 a) Proximal Distal Fan Fan
Mid- Lacustrine Fan Sediments
Isopach Diagrams of Vedder Fan from Borehole Logs Depth of Fill 1m 10m 20m 30m 40m
b) c)
4
2
1 3 Gravel Silt/Clay d) e)
0.00030
0.00015
0.00
Well Depth (m)
Sand Confidence
Figure 4-19: Isopach diagrams of gravel, sand and lacustrine material within the stratigraphic bounds of Vedder Fan and surrounding area. Bounds of former Sumas Lake are shown in black. See text for description of numbered localities in 4-19c.
104 Chapter 4. Evolution of Chilliwack Valley Mainstem
There is apparently a thick clay lens (6-8 m thick) at the center middle section of Vedder Fan, but this is based on a two relatively isolated core logs; there is an insufficient density of holes nearby to adequately assess the extent of this deposit. The pattern of sandy units (Figure 4-19d) shows a prominent swath of thick deposits across most of the mid- to distal fan. Shallower sand deposits are also evident at what must have been the delta of Sumas Lake. Sumas Lake itself has quite thick accumulations of sand. Spatial density of the holes varies from roughly 20 /km2 to 0.2 /km2. A map showing the point density in the model at different points across the fan is shown in Figure 4-19e. The depth of the well log is indicated by point size. The number of well points that fall within a 1.5 km radius are weighted by their depth, then totaled and divided by the area of the circle, giving a spatially distributed index of well log coverage, and thus confidence in the subsurface model. Based on the assumed extent of the Chilliwack Fan, volumes were calculated for gravel, sand and silt/clay fractions. Typical bulk densities for these materials are assumed and density differentials with depth are taken into account. Volume has been stratified into different historical periods detailed in Figure 4-20, and resulting estimates are shown in Table 4.4. Table 4.4: Volumetric estimates of Vedder Fan composition (m3 × 106) based on well log records. Values are cumulative over the time intervals indicated. The overall expected accuracy of the estimates is ±16%, based on the range of possible spatial extents and depths of the fan. Values in brackets indicate additional volumes attributable to the Sumas Lake Basin.
Period Gravel Sand Clay/Silt to 11 035 BP 14.5 9.4 8.7 to 10 000 BP 90.1 36.7 21.5 to 6 230 BP 696 383 436 to Present 1 020 555 (765) 768 (872)
4.10 Discussion
The summary volume of Vedder Fan has been examined in order to better constrain the output term for the larger sediment budget model. The bulk volumetric output of gravel, sand and silt/clay fractions appears to balance most of the net eroded bulk volume that has been established in the previous chapters, to a first approximation. The chronostratigraphic model shows an initially heightened phase of growth, followed by reduced aggradation on the
105 Chapter 4. Evolution of Chilliwack Valley Mainstem
2.8E+9
Sumas Lake 2.4E+9 Deposition (?) ) 3 2E+9
1.6E+9 Silt / Clay
1.2E+9 Sand 8E+8 Cumulative Volume (m
4E+8 Gravel
0 -12000 -10000 -8000 -6000 -4000 -2000 0 Calendar Years B.P.
Figure 4-20: Cumulative volumetric growth of Vedder Fan from 11 035 BP to present, based on the assumed stratigraphic relationships indicated in Figure 4-16. Error bounds indicate 16% error assumed in the calculations. fan. A large proportion of the volumetric output from the valley consists of sandy material derived from the deep glacial deposits in the lower and middle reaches of Chilliwack Valley. This would appear to constitute much of the lower fan stores. The sand and gravel volume in Vedder Fan is estimated to be 1.57±0.25×109 m3or roughly 3.0 ×109 t, assuming some compaction and a bulk density of 1.9. Since deglaciation, this averages out to a bed material yield of 121±19 ×103 m3/yr. The long term average since 6 200 cal. B.P. appears to be 78.8±9 ×103 m3/yr, a figure that is higher than the mean, but within the range of modern estimates made by McLean [1980]; Martin [1992] and Ham [1996] (Table 4.5).
Table 4.5: Bed material yield below Vedder Crossing - lowest, mean and highest annual averages over the course of study periods for three investigations using different methodologies
Bed Material Load Estimate Low (m3/yr) Mean (m3/year) High (m3/yr) McLean (1980), 1940-1976 58 000 ± 76 000 ± Martin (1992) 1981-1990 19 000 ±8 670 36 600 ±5 600 156 800 ±16 700 Ham (1996) 1952-1991 4 900 ±3 900 54 600 ±10 000 178 000 ±10 000
The net volume of glacial material (outwash and till) that has been eroded from the valley mainstem and lower tributaries was estimated to be 1.4 ×109 m3 (middle range estimate, see above). This represents roughly 90% of the fan volume, suggesting that either the estimated
106 Chapter 4. Evolution of Chilliwack Valley Mainstem volume eroded from the lower valley is high or that transfer of material to the Fraser is higher than anticipated. I expect the answer is a combination of the two; there is large uncertainty in the bounds of post-glacial topography in the lower valley. The contribution of glacial material is nevertheless the most significant portion of the early fan’s development, the river load has been gradually supplemented with more modern weathering products over post-glacial time. For a comparison among scales, the total volumetric post-glacial yield to Chilliwack Lake, including outwash deposition, was 344.5 ± 24 ×106 m3. The specific bulk rate of yield is thus 79 ± 9 m3/km2/yr over 13 000 years. The total bulk volume estimated for Vedder Fan indicates a post-glacial specific rate of yield almost twice that of Chilliwack Lake: 146 ± 23 m3/km2/yr for the entire basin, due mostly to erosion of very large glacial stores in the lower valley. Examination of the long-term fine sediment yield component from both Chilliwack Lake and the lower valley show a similar disparity. The amount of fine sediment identified in Vedder Fan would suggest approximate bulk specific yields of 48 m3/km2/yr (67 t/km2/yr) over the long term. The Holocene lacustrine specific yield at Chilliwack Lake was 21.8 m3/km2/yr (32 t/km2/yr). The deep glacio-lacustrine deposits in the mid-valley represent several centuries, quite likely millennia, of fine sediment accumulation that continues to be eroded from the valley. The Upper Chilliwack basin would have had considerable glacigenic material, but not nearly as much as the lower valley. Monitoring of the fine sediment load at Vedder Crossing shows the contemporary yield is 106 ±60 t/km2/yr [Church et al., 1989], somewhat higher than the inferred long-term average. This is possibly due to the effects of land use within the valley. The averaging effect over the latter 6 200 years of the Holocene leaves no capacity to resolve the contributions from distinct episodes of accelerated sedimentation such as the events of the Neoglacial. Nevertheless, the transport estimates presented here assist in verifying that modern rates of sediment yield are not significantly different from those of the past, and that the aggradational process on the fan may be effectively modelled based on the bulk transport rating relations that have been established in the historic hydroclimatic regime.
107 Chapter 5
Characterization of Valley Sediments
5.1 Introduction
Previous chapters explored the total volumetric exchange that has occurred in the Chilliwack drainage network since the close of Fraser Glaciation. This chapter examines the character of the valley mainstem, in particular, the sedimentology and geochemistry of channel sediments. These investigations illustrate the longitudinal gradients of sediment calibre and fine sediment chemistry, which provide some important constraints on rates of mixing among the many sediment sources in the valley. Sediment characteristics such as grain sorting, clast lithology, and major and trace element chemistry reveal the present meta-equilibrium state of the system, which continues to evolve on a scale of centuries to millennia. It is beyond the scope of this thesis to characterize the full range of depositional environments in the valley, but some important patterns emerge from representative sites. Tributaries deposit sediment into a relatively confined valley, and have built extensive fan deposits that are inter-stratified with mainstem alluvium. Sediment is also recruited to the mainstem from large deposits of floodplain and glacigenic material along the length of the channel. The calibre and lithology of materials supplied to the mainstem vary according to the source, and there tends to be a dispersal trail of both coarse clastic material and fine sediments downstream of each major source or tributary junction. This chapter endeavours to examine the relative influence of the major tributaries upon the river below Chilliwack Lake, as observed in the patterns and gradients of grain size, lithology and geochemistry. As outlined in previous sections, the middle mainstem section and lower tributaries of Chilliwack Valley have been in a prolonged state of degradation; incision of the post-glacial deposits has proceeded to a depth of over 80 m in the mainstem, and tens of metres in the
108 Chapter 5. Characterization of Valley Sediments lower tributaries. In the middle mainstem the river has abandoned its broad glacial outwash floodplain, carving itself a narrow canyon within Pleistocene deltaic and lacustrine beds. A fining trend is evident in the valley [McLean, 1980] though the pattern is subject to frequent interruptions by lateral inputs from individual tributaries. With the extensive reworking of the alluvial deposits and the incorporation of glacial cobbles and boulders, a relatively coarse lag has developed on the bed of Chilliwack River. The lag deposit has an important influence on the evolution of the channel long-profile in that it increases the hydraulic roughness of the channel, and may also contribute to increased concavity of the long profile (see Chapter 6). Steep, bouldery riffles can be found on the downstream edges of some tributary fans; coarse accumulations of boulders tend to develop up to 1 km downstream of the major tributary fans. Much of the material coarser than the 128-256 mm (i.e. boulder) fraction appears to have been effectively trapped within the valley (escaping eventually after weathering or abrasion), while the finer grades are carried beyond the valley to the piedmont fan. Thus there is a marked partitioning of the sediment budget with respect to differential sediment mobility acting over long periods.
5.2 Sampling of Tributary and Mainstem Gravels
Surface (Wolman) and subsurface (bulk volume) sampling was carried out in Chilliwack Valley in July and August, 2004. Bars were typically sampled at low flows when the maximum surface area was accessible for sampling. Representative surface and subsurface sampling of gravel bars of the mainstem and lower tributaries is wrought with a number of difficulties. Bars represent a palimpsest of different depositional environments [Kellerhals and Church, 1978; Bluck, 1982; Shaw and Kellerhals, 1982; Rice, 1996], from sheets of gravel to blankets of fines laid down in waning flows to broad fields of static armour deposited by much larger, historical flows. The variability across the width of the broad, wandering channels of Chilliwack Valley is often considerable. Ideally, surface sampling sites are large enough to sample 400 stones in a 12 m2 (0.5 m grid) area, yet remain within a homogenous facies unit. Following recommendations of Shaw and Kellerhals [1982], Rice [1996], Bunte and Abt [2001] and others, sites were selected that represented the coarsest active environment that could be isolated, irrespective of location on the bar surface, i.e. not necessarily at bar heads. The grain size distribution is invariably
109 Chapter 5. Characterization of Valley Sediments influenced by relict materials. Clearly armoured areas were avoided, though lag boulders and cobbles were frequently present, potentially unduly skewing the estimate of the surface composition. Some bars have material that was finer than the ambient river substrate, due to the deposition of transient bedload gravel sheets. Some sites were clearly influenced by engineering works that have either disrupted the substrate or have changed local flow conditions. Despite reasonable measures taken to target active channel gravels, the grain size distribution measured is not always exclusively the product of modern channel processes. Grain size samples were taken from 18 sites in two major tributaries (Foley and Slesse), 12 sites in the Upper Chilliwack River and 32 sites along the maintem below Chilliwack Lake. Photographs were taken of all bar facies, using a 1 m2 photo template for scale. Grid-by-number (Wolman) samples were made using a 30 m tape measure. Stones that fell under each metre increment (greater spacings were used in the presence of larger caliber material) were identified by lithology and sized using a square-hole gravel sizing template with 1/2Ψ intervals. The finest clast size category was 8 mm, and patches of finer material were noted as ’fines’. For optimal characterization of the overall grain size distribution, up to 400 observations were recorded where sufficient bar area was exposed. Sampling grids were not always square, but an attempt was made to follow a grid pattern that matched the boundaries of the targeted depositional unit with minimized longitudinal or transverse bias. A representative sample of 3-4 kg of surface material, including fines and grains up to 22 mm was collected by hand. This was returned to the lab for sieving and determination of the ’fines’ content of the surface fraction and refinement of the overall distribution. A hybrid grid-based surface grain size distribution was generated following the procedures developed by Fripp and Diplas [1993] and outlined in Bunte and Abt [2001]. Axial measurements of boulders larger than 256 mm were made in order to assess the distribution of the coarse population of clasts within the sampling unit. This information was used to improve the representation of coarse material in the upper tail of the size-distribution curve. At most sites, subsurface bulk (volume-by-weight) samples were collected subsequent to Wolman sampling. The surface layer was removed to a depth of the largest stone in the active surface layer of the sampled unit. Material from the subsurface was then excavated until 750 to 1200 kg were collected (the largest sample was 1800 kg). The largest stones on the bar averaged about 40 kg or roughly 4% of the total sample weight. All except for three exceptionally coarse sites met the 5% precision criterion (for gravels with clasts greater than 128 mm) specified by Church et al. [1987].
110 Chapter 5. Characterization of Valley Sediments
Figure 5-1: Sampling bar material in the lower Chilliwack Valley (site # 111-04)
Subsurface samples were sieved in 1/2 Ψ intervals in the field to 16 mm (Figure 5-1). The remaining finer material was split and a small sample (3-4 kg) was returned to the laboratory for further analysis. Weight corrections were applied to the bulk sample measurements based on the water content of the subsamples. Corrections were typically on the order of 4-6%. Sampling frequency of bar material was better than 2-3 sites per kilometre in lower valley reaches, and as seldom as one site in a 4 km stretch through the canyon where access was difficult and exposures of bar material were infrequent. Figure 5-2 shows the locations of gravel sampling sites in the valley. Two sites were chosen to assess how representative a subsurface grain size grading at a site may be, relative to upstream and downstream links. The sites chosen were (1) just upstream of the Tamihi Bridge (113-04), and (2) a large bar complex upstream of Borden Creek (135-04). Replicate samples were taken from within the same facies unit, although the duplicate site was invariably somewhat finer. The pit was dug to the usual depth, and results showed similar curves. Figure 5-3 shows the differences among each grain size fraction for 4
111 Chapter 5. Characterization of Valley Sediments 135-04 135-04 ! > ! > ! > ! > > > 134-04 134-04 142-04 142-04 ! > ! > 131-04 169-04 ! > ! > ! > 403 128-04 128-04 404 125-04 Geochem Wolman Bulk - Wolman Wolman - Geochem Bulk - Wolman - Geochem 125-04 FOL12 E E E E ! > > E ( Sampling Points 406 ! > ! > ! > 405 122-04 122-04 ETRC#3 ETRC#3 166-04 E E ( E E E E ! > ! > FOL10 118-04 118-04 ( FOL6 ! > ! > 163-04 ETRC#1 163-04 ETRC#1 93-04 ETRC#2 ETRC#2 409-04 FOL5 410-04 ! > ! > 424-04 E E 424-04 E 422-04 E E E 172-04 E E 422-04 418 AIR1 E 418 E 114-04 114-04 408-04 423-04 ! > ! > 423-04 AIR3 E E 407-04 ! > E ( ! > E ! > 148-04 113-04 ! > ( 148-04 E E E 113-04 E E ! > E ! > 419 138-04 419 ! > 082-04 082-04 ( 420 420 150-04 150-04 ! > ! > ! > FOL8 FOL9 95-04 421-04 95-04 421-04 FOL12 ! > ( FOL13 162-04 E E FOL14 106-04 106-04 ! > 153-04 413-04 Meters 414-04 176-04 175-04 99-04 > 99-04 87-04 ! > 87-04 ! > 144-04 > ! > ! > E E ! > 156-04 107-04 107-04 ! > ! > 88-04 ! > 88-04 96-04 96-04 112-04 190-04 112-04 97-04 97-04 ! > 91-04 ( ( 91-04 > 180-04 > > > ! > ! > 135-04 92-04 ! > 92-04 ! > 83-04 98-04 83-04 ! > ! > > 98-04 625 90-04 90-04 134-04 85-04 85-04 Sampling sites within Chilliwack Valley. (a) Lower Valley and (b) Upstream of the Slesse Creek confluence. Open 142-04 0 1,250 2,500 3,750 5,000 ! > Figure 5-2: circles indicate surface sampling only, and filled circles indicated both surface and bulk sampling.
112 Chapter 5. Characterization of Valley Sediments
16 32 64 128 256 10%
Duplicate U/S - 093-04 U/S - 163-04 6% U/S - 123-04 D/S - 095-04 D/S - 094-04 D/S - 107-04
2%
ference
-2%
Percent Dif
-6%
Upstream of Tamihi Bridge
-10% 4 5 6 7 8 Grain Size (Y) 16 32 64 128 256 10%
Duplicate U/S - 153-04 U/S - 156-04 6% U/S - 180-04 D/S - 129-04 D/S - 126-04 D/S - 123-04
2%
ference
-2%
Percent Dif
-6%
Upstream of Borden Creek
-10% 4 5 6 7 8 Grain Size (Y)
Figure 5-3: Duplicate samples taken at (a) just upstream of the Tamihi Bridge (113-04), and (b) a large bar complex upstream of Borden Creek (135-04). Lines indicate the relative difference among samples for each size fraction, relative to the first of the two duplicates (‘0’ datum). Samples are not significantly different within each sedimentary link, but show a closer affinity than between links. to 7.5 Ψ, relative to the first sample of the two duplicates. The two sets of duplicate samples were taken at the upstream and downstream boundaries of the sedimentary link that spans the length of river between Slesse Creek and Tamihi Creek on the mainstem. There were 6 bulk samples taken along intervening bars, at an average spacing of 1.5 km. Figure 5-3(a) shows the duplicate sample taken upstream of Tamihi bridge. All fractions for the duplicate sample tend to plot within the range of variance for samples taken upstream (i.e. within the sedimentary link). The samples taken upstream of Borden Creek show more of a mixture, however, here also, there appears to be more of an affinity for the within-link mixtures, particularly in the finer range. Thus, while bulk samples generally show a more stable down-valley fining pattern than surface samples, it must be kept in mind that the at-a-site variability may be at least as great
113 Chapter 5. Characterization of Valley Sediments as the within-link variability, particularly for the coarser (>64 mm) range of grain sizes. The cumulative grain size distributions for tributary and mainstem gravels are shown in Figure 5-4 (a-e). This includes samples from Slesse Creek, Foley Creek and Upper Chilliwack River. Within the mainstem samples, it can be seen clearly that the surface samples overall show much greater variability than the subsurface samples, particularly in the gravel range. The range of standard deviation (σ) values for the individual surface samples shows a wider range than the subsurface samples (Figure 5-5). Many surface samples are better sorted than the subsurface and show much greater skewness and kurtosis. This effect is reduced when the fine fraction (<2 mm) is trimmed from the distribution, and the fractions re-normalized (Figure 5-5(b)). The bed grain size distributions show relatively poor sorting (σ = 0.65 to 2.2), which is well within the range of sorting coefficients reported from other gravel bed river studies [see Ferguson and Paola, 1997]. Bulk samples of glacial deposits were taken at several sites in the valley for an approxi- mate characterization in the sediment routing model (Chapter 6) and for contrast with the channel sediments. Not surprisingly, the texture is highly variable and therefore difficult to characterize at the basin scale. There tends to be a richer sand component, with a mode in the 1/2 to 2 mm in range, often with a secondary mode in the coarser fractions. Examples of several deposits are shown in Figure 5-4(e) for comparison with channel sediments. As fluvial networks develop over glacial parent material, selective transport carries away sands and silts leaving a concentration of coarser clasts, illustrating their relative ubiquity within the deposits. Their true percentage is nevertheless quite difficult to accurately characterize from these relatively small bulk samples. When compared to channel sediments, the till deposits sampled have similar variance, but lower skewness and kurtosis, indicating a lack of fluvial sorting (Figure 5-5). Positive skewness indicates a deficit of finer material (<1 mm). Only a few of the tills show any sign of a distribution skewed toward the finer material; all of the fluvial material in tributaries and the mainstem are skewed to the coarser end. The highest skewness and kurtosis values are attributable to samples taken higher in the catchments where a lag of bouldery material emphasizes the coarse mode or even introduces a pronounced third mode to the distribution.
5.2.1 Fining Patterns
The geometric mean grain size of surface (composite Wolman) samples from alluvial reaches along the length of the mainstem ranges from 28 to 200 mm. The spatial scale of sampling is
114 Chapter 5. Characterization of Valley Sediments
a) Mainstem - Wolman Sample b) Mainstem - Bulk Sample
100% 0% 100% 0%
90% 10%
80% 20% 80% 20%
70% 30%
60% 40% 60% 40%
50% Silt Sand Gravel Boulders 50%
40% 60% 40% 60% Percent Finer Percent Finer Percent Coarser Percent Coarser 30% 70%
20% 80% 20% 80% Cobbles 10% 90%
0 100% 0 100% -6 -4 -2 0 2 4 6 8 10 12 -6 -4 -2 0 2 4 6 8 10 12 Grain Size (Y) Grain Size (Y)
c) Tributaries - Wolman Sample d) Tributaries - Bulk Sample
100% 0% 100% 0%
80% 20% 80% 20% Foley Ck.
60% 40% 60% 40%
40% 60% 40% 60% Percent Finer Percent Finer Percent Coarser Percent Coarser
20% 80% 20% 80% Slesse Ck.
0 100% 0 100% -6 -4 -2 0 2 4 6 8 10 12 -6 -4 -2 0 2 4 6 8 10 12 Grain Size (Y) Grain Size (Y)
e) Till Sources - Bulk Sample
100% 0%
80% 20%
60% 40%
40% 60% Percent Finer Percent Coarser
20% 80%
0 100% -6 -4 -2 0 2 4 6 8 10 12 Grain Size (Y)
Figure 5-4: Grain size distributions for bulk and Wolman samples within the Chilliwack mainstem (a,b), Tributaries (c,d) and Glacial Tills (e).
115 Chapter 5. Characterization of Valley Sediments
Mean - Standard Dev.(σ) Skewness - Kurtosis
a) 4 12 Channel Subsurface Channel Surface Till / Glacial Sources
3 9
2 6
Kurtosis Standard Deviation
1 3
Full Distribution Channel Subsurface Channel Surface Till / Glacial Sources
0 0 -4 0 4 8 -1 0 1 2 3 Mean Grain Size (Y) Skewness
b) 4 12
Channel Surface Channel Surface Channel Subsurface Channel Subsurface Till / Glacial Sources Till / Glacial Sources 3 9
2 6
Kurtosis Standard Deviation 1 3
Truncated Distribution
0 0 -4 0 4 8 -1 0 1 2 3 Mean Grain Size (Y) Skewness
Figure 5-5: Mean, standard deviation, skewness and kurtosis of gravels (Ψ scale) sampled in Chilli- wack Valley. (a) shows the statistical moments of the full distribution, including fines, (b) shows the same statistics with the distribution truncated at 2 mm (no fines).
116 Chapter 5. Characterization of Valley Sediments sufficient to provide only an approximate picture of the fining pattern between the tributary sources (Figure 5-6). Despite scatter, there is a coherent pattern of downstream fining, particularly within major sedimentary links. The grain size distributions in the middle and upper portions of the mainstem below Chilliwack Lake are largely controlled by lateral inputs from tributaries and Pleistocene deposits, however once the channel passes the coarse-grained tributary junction with Slesse Creek, a more persistent fining pattern develops. The fining pattern among various grain sizes can be best appreciated when viewed as individual cumulative fractions, plotted along the length of the valley (see Figure 5-6a). There is minimal influence exerted on the subsurface fining pattern by coarse tributary inputs and other sediment sources when compared to the gradient for the surface grain size distribution. The surface pattern also shows a somewhat steeper rate of fining. There is a step-change in the surface pattern at Slesse Creek, where the material becomes considerably coarser.
The fining coefficient (α in the equation D/Do=exp(-αx), accounting here for both sorting and abrasion) for surface material between Chilliwack Lake and Slesse Creek is 0.020. The fining rate steepens between Slesse Creek and Vedder Canal, and is estimated to be 0.081 for the surface D50 and be 0.033 for the subsurface D50. The discrepancy is much less for the
D84: 0.036 for surface and 0.034 for subsurface. These fining rates are all significantly less for the degrading mainstem channel than for the aggrading Vedder Fan, where rates are on the order of 0.2 [Ferguson et al., 2001]. Figure 5-6(c) shows the same section of the lower valley, this time with the mid-reaches of Slesse Creek (14-25 km) shown. Here it can be seen that there is coarse material in the lower reach of Slesse Creek that feeds into the mainstem, explaining the step-change in the previous graph. Material upstream of the bridge at Lower Slesse Creek is dominated by boulders, with patchy collections of gravel; thus a representative grain-size distribution is difficult to obtain upstream of 16 km. The subsurface grain size trend shows a relatively stable fining pattern between Foley
Figure 5-6 (facing page): Downstream fining along Chilliwack Valley Mainstem. (a) shows the cumulative fractions of the subsurface sediments from Chilliwack Lake to the end of the gravels in Vedder Canal. Samples from Vedder Crossing to the canal were taken by Y. Martin and BC Environment (1989-1991). (b) shows the fining pattern for surface samples. No samples were taken beyond Vedder Crossing. (c) is similar to (b), except the fining gradient is shown from the mid-reaches of Slesse Creek (grey shading) to Vedder Crossing. The comparison shows clearly the geographic source terrain of the coarse material in the mid-reaches of Chilliwack River. Dashed lines indicate regression fits to the data downstream of Slesse Creek.
117 Chapter 5. Characterization of Valley Sediments
Centre Nesakwatch Foley 100% Chipmunk Slesse Borden Tamihi Liumchen Vedder 2 mm 4 mm 8 mm 75% 16 mm 32 mm
50% 64 mm
Percent Coarser 25% 128 mm
256 mm 0 0 8 16 24 32 40 48 56 64 Distance Downstream from Chilliwack Lake (km)
100% 2 mm 4 mm 8 mm 16 mm 75% 32 mm
64 mm No Data 50% Collected
Percent Coarser 25% 128 mm
256 mm 0 0 8 16 24 32 40 48 56 64 Distance Downstream from Chilliwack Lake (km)
100% 2 mm 4 mm 8 mm 16 mm 75% 32 mm
50% No Data 64 mm Collected
Percent Coarser 25% 128 mm
256 mm 0 8 16 24 32 40 48 56 64 Distance Downstream from Slesse Headwaters (km)
118 Chapter 5. Characterization of Valley Sediments
Creek and roughly 4 km below Vedder Crossing, with an evident spike just below Slesse Creek, where a bouldery section interrupts the pattern. There is another spike at 40.8 km, which may possibly be attributable to engineering works on one of the bars that was sampled. The coarse distribution at 45.3 km is due to a large lag deposit below Liumchen Creek. Material below Vedder Crossing was collected by Martin [1992], who sampled 19 bars along the length of the Vedder River and canal. The data were supplemented in Ferguson et al. [2001] with samples from the Vedder Canal in 1998 [Bloomer, 2000] and seven subsurface samples taken by BC Ministry of Environment personnel after a major flood in 1989. This distribution has potentially changed slightly in the intervening twelve years, as the gravel from the Chilliwack River progrades into the canal. The overall pattern shows the rapid diminution of grain sizes at the distal edge of Vedder Fan. Figure 5-7 shows the percentage of sand content (material < 2 mm) in surface and sub- surface gravel samples. There is a gradual increase in sand content moving downstream, likely related to recruitment of sandy bank material and Pleistocene deposits, as well as clast abrasion. There is more scatter in the relation for surface samples, which is not surprising given the considerable variability of surface facies. The higher surface sand content tends to be found in the mid- to lower reaches.
5.3 Lithology and Geochemistry
As part of the sediment budget investigations, clast lithology was recorded in the course of collecting surface Wolman samples. There is a systematic decrease in the granitic (granodi- orite) content of surface sediments from headwaters to valley mouth. The geochemistry of fine, interstitial sediments from the subsurface was also analysed in order to establish if some partitioning of the budget could be established by developing a mixing model based on major and minor element “fingerprints”. Given that the geological contrast between headwater ar- eas and the lower valley is relatively high, it was anticipated that some measure of the rates of downstream sediment mixing could be obtained.
5.3.1 Coarse Clast Lithology
Granitic clasts provide a good marker lithology along the length of the valley below Chilli- wack Lake (see Physiography section, Chapter 1). There is an exponential decline in the concentration of coarse granitic clasts within the channel from headwaters to piedmont.
119 Chapter 5. Characterization of Valley Sediments
40%
Subsurface Sand Surface Sand
30%
Vedder Crossing
20%
10%
0 0 10 20 30 40 50 60 Distance Downstream from Chilliwack Lake (km)
Figure 5-7: Percentage sand (<2 mm) in surface and subsurface deposits, plotted along the length of Chilliwack River.
Metasedimentary lithologies such as phyllites, slates and cherts are the most voluminous additional lithologies downstream, followed by various volcanics, metamorphic rocks, includ- ing an assortment of greenstone, gneiss, conglomerate. Figure 5-8a shows the pattern of dilution of coarse clastic granitic material along the mainstem. The dispersal pattern of granite is primarily due to the renewal of the sediment load by clasts of Chilliwack and Cultus Group lithologies and the exchange of granitic material within the storage reservoirs along the length of the drainage over time. The percentage of upstream granitic terrain, versus metamorphic terrain, was calculated at each step along the length of the mainstem, using ArcView’s Flow Accumulation algorithm. Area accumulation begins at Chilliwack Lake and in the headwaters of the lower valley tributaries. At each tributary junction, there may be a positive or negative step in relative percentage of granitic terrain upstream, and downstream from Foley Creek the balance is less than 50%. The longitudinal pattern of granite percentage in the channel shows a consistent renewal of granitic clast composition near each tributary junction, followed by downstream attenuation. Despite a decrease in the balance of upstream granitic terrain at tributaries such as Foley
120 Chapter 5. Characterization of Valley Sediments
100% a) 128+ mm
75% Foley
Nesakwatch Slesse Borden Tamihi Liumchen
50% Non-Granite
32-90+ mm Centre
Upstream Fractional Area Slesse Creek 25%
Percent Granite Composition 8-32+ mm Foley Creek
Granite 0 0 8 16 24 32 40 48 56 Distance Downstream from Chilliwack Lake (km)
b) 100% 100
Tamihi
Centre
Slesse
Borden 75% Grain size diminution Liumchen 80 á = 0.04
50% 60
Foley
Lag Fraction
Mobile Fraction
Grain Size (mm) 25% 40
Nesakwatch 0 20 0 8 16 24 32 40 48 56 Distance Downstream from Chilliwack Lake (km)
Figure 5-8: (a) Downstream variation in percentage granite composition of streambed sediments. The size composition of the total granite percentage is broken into 3 classes: 8-22 mm, 32-90 mm and 128 mm and larger. Relatively few granite pebbles were recorded, while a large number of boulders are evident. The dark line indicates the relative percentage of granitic terrain upstream (not including the drainage above Chilliwack Lake). (b) Shows the percentage of granitic clasts in the 32-90 mm category relative to both upstream granitic terrain (black) and percentage of granitic boulders (light gray). This is an index of how much mobile granitic material we might expect to see in the channel versus the observed. See text for discussion. The Sternberg relation for diminution of grain size from 90 mm to 32 mm is shown (dotted line, axis to the right).
121 Chapter 5. Characterization of Valley Sediments and Tamihi, there is still a recharge of granitic material. Most of the granitic clasts counted belong to a lag population (128 mm and greater), while the size fraction that is associated with coarse active bedload (32-90 + mm) is relatively depleted in granites. The finer fractions (8-32 + mm) form a smaller percentage yet. Smaller clasts are more mobile, and thus move through the system at a greater pace. The cumulative percentages from the three size classes are plotted in Figure 5-8a. The factors that influence the basin-wide lithological patterns are numerous and complex. In glacial terrain, the reworking of mainstem material such as glacial outwash, floodplains, fans and tills complicates the picture further. There is a large accumulation of granitic boulders at the outlet of Foley Creek, for instance, that is derived primarily from an exposure of relict sandur that is closely coupled with the mainstem. The volume of remnant glacial deposits and floodplain storage is considerable in relation to volumes of sediment transfer and therefore the dispersion pattern is persistent and not subject to transitory perturbations. The balance of granitic terrain upstream appears to explain most of the lithologic compo- sition in the coarser fractions. That is, granitic boulders appear on the bed roughly in pro- portion to the relative area of granitic terrain upstream. The behaviour of the 32-90 + mm fraction is more complex. Figure 5-8b shows the percentage of granite in this size range, nor- malized by the boulder range (gray line) and by the percentage of granitic terrain upstream (black), which are roughly similar. This provides an approximate index of how much mobile granitic material we might expect to see in the channel versus the observed. The graph reveals a trend of relative depletion above Slesse Creek, and then of enrich- ment downstream to Vedder Crossing. Fining processes may be responsible for the relative diminution of headwater granites in the upper portion of the mainstem between Chilliwack Lake and Slesse Creek. Sampling of surface gravels suggests a Sternberg coefficient of 0.02 for surface D50 within that reach, and 0.081 downstream of Slesse Creek (see above). If a 90 mm clast were worn down to less than 32 mm in 25 km, the Sternberg coefficient would be just over 0.04 which is high for the river above Slesse Creek, but within range of observed rates in the system. Downstream of Slesse Creek, the relative supply of granite material is replenished, presumably due in part to reactivation of glacial material from the numerous sources along the length of the mainstem.
122 Chapter 5. Characterization of Valley Sediments
5.4 Silt Geochemistry
The geochemistry of modern channel subsurface interstitial silts shows patterns that are similar to those found in the above analysis, particularly with respect to the interaction among sediments derived from headwater areas and those from tributary sources. The influence of tributaries sources on the mainstem geochemistry is relatively strong, particularly upstream of the Slesse Creek confluence. Below Slesse Creek, the geochemical patterns become more stable, again emphasizing the importance of the reworking of stored material along the valley mainstem. The silt fraction (< 63µ) was chosen for analysis since it travels in suspension and is gen- erally well mixed within downstream sedimentary reservoirs. Many studies have shown that the geochemical composition of silts offers a reasonably integrated picture of the fine sedi- ment sources upstream (Cullers et al. [1987]; Shilts [1995]; Collins et al. [1997]; Dirszowsky [2004]). The capacity for differentiating upstream contributions of course depends strongly on the contrast in bedrock and parent materials found within each catchment. Some sources may tend to be over-represented, while others with weaker contrast may not be detected.
5.4.1 Methods
In the course of sampling subsurface material in bars, sandy material was collected by hand from just below the active layer and put aside for geochemical analysis. Additional samples were collected at intervals along the mainstem, in tributaries, and in a number of prominent sediment source areas such as the glaciolacustrine exposures of the lower valley. Samples collected from sediment source areas were taken from undisturbed, unweathered C-horizon (parent material) deposits. At several sites, 4-5 samples were collected within a span of approximately three to four channel-widths in order to examine the natural variability of the geochemical signature in different fluvial environments. A total of 20 of 89 samples were collected as field duplicates. Samples were oven dried at 100◦C for 8 hours, then disaggregated (where necessary) and sieved to 63µ. Eleven blind duplicates were prepared for submission to different assay laboratories in order to assess possible error inherent in laboratory work. Samples were submitted to a suite of geochemical analyses using inductively coupled plasma mass spectrometry (ICP-MS) at both ACME labs and ALS Chemex (Vancouver). ALS Chemex offered a 27 element package (ME-ICP61), and ACME provided 37 elements in
123 Chapter 5. Characterization of Valley Sediments their Group 1EX package. Both laboratories performed a total digestion of the samples by
4-acid (HNO3-HClO4-HF-HCl) techniques. Element determination was done using ICP-MS. Control reference standards and analytical duplicates were routinely inserted into sample suites to monitor and assess accuracy and precision of laboratory analytical results. Upon examination of the results, Ag, Be, Bi, Cd, Mo, Sb and W were discarded outright, as they were close to the detection limit and did not offer good discriminating potential. From an initial dataset of 27 element variables, 20 are then left for analysis. The performance of these variables was analysed for both analytical and sampling errors. Using the method of Thompson and Howarth [1978], the analytical error was estimated from 11 duplicate sediment samples that were submitted to different laboratories. Precision was better than ±10% at the 95% confidence level for most elements. Cr, Mn, Ti and V were close to that precision threshold, and As, Co, Mo, and S were definitely over that limit. Field duplicates from three different sampling sites (Center Creek, Tamihi Creek and the mainstem below Liumchen Creek) were examined in order to establish the range of within-site variability. One-way analysis of variance (ANOVA) was used to verify whether the variability of the element concentrations within sampling sites was greater than between the sites. The null hypothesis (no difference between sites) was rejected for all elements except Pb (α=0.05). As, Cu, and P generally had low F scores. Post-hoc multiple comparisons using Scheff´e’stest indicate that Mg, Mn, Ni, and Sr remained the best discriminating elements for these sites, having significantly different means at the α=0.01 level. Ca, Co, and Na all failed to reject the null hypothesis of differing means at α=0.10. Element concentration data were first log-transformed and centered to a zero mean to correct for the characteristic skew of geochemical data. Correlation among the elements was then examined (see Table 5.1). There are strong (>0.7) positive associations amongst metal elements Fe, Co, Cr, Mg, Ni, V and Zn. Metals and iron oxide minerals are introduced in distinctive proportions from the various source terranes in the Chilliwack Valley, particularly the mid to lower valley tributaries such as Foley, Slesse, Tamihi and Liumchen. Potassium is an effective proxy for plutonic felsic materials in the valley, particularly orthoclase feldspar, a very common mineral in granitic terrain. Basins within the Chilliwack Batholith such as Centre Creek and the Upper Chilliwack Valley show greater concentrations of K, Al, Na and P. Positive, though weaker, associations can be found among these elements.
124 Chapter 5. Characterization of Valley Sediments
Table 5.1: Correlation scores among 19 elements in fine grained (< 63µ) sediments (both active channel and hillslope sources) Chilliwack Valley. Values greater than 0.65 or less than -0.65 are highlighted.
Al As Ba Ca Co Cr Cu Fe K Al 1.000 As 0.182 1.000 Ba 0.337 0.222 1.000 Ca -0.114 -0.179 -0.470 1.000 Co -0.008 0.493 -0.011 0.042 1.000 Cr -0.322 0.248 -0.049 0.250 0.665 1.000 Cu 0.152 0.339 0.279 -0.062 -0.031 0.004 1.000 Fe 0.227 0.559 0.359 0.083 0.518 0.568 0.370 1.000 K 0.680 -0.010 0.341 -0.181 -0.432 -0.701 0.116 -0.202 1.000 Mg 0.122 0.476 0.018 0.341 0.772 0.773 0.130 0.703 -0.336 Mn 0.441 0.489 0.365 0.038 0.333 0.140 0.447 0.665 0.228 Na 0.366 -0.371 -0.226 0.403 -0.613 -0.487 0.123 -0.316 0.533 Ni -0.257 0.311 0.011 0.180 0.772 0.945 -0.037 0.512 -0.588 P 0.467 0.248 0.050 0.341 0.031 -0.054 0.324 0.379 0.408 Pb 0.384 0.253 0.277 -0.344 -0.157 -0.492 0.384 -0.149 0.652 Sr 0.079 -0.327 -0.367 0.456 -0.387 -0.343 -0.237 -0.525 0.394 Ti 0.064 0.351 0.382 0.058 0.324 0.554 0.222 0.787 -0.318 V 0.032 0.389 0.247 0.141 0.524 0.666 0.276 0.910 -0.454 Zn 0.363 0.505 0.653 -0.420 0.139 -0.014 0.760 0.485 0.240
Mg Mn Na Ni P Pb Sr T V Zn Al As Ba Ca Co Cr Cu Fe K Mg 1.000 Mn 0.476 1.000 Na -0.289 0.000 1.000 Ni 0.817 0.209 -0.538 1.000 P 0.292 0.524 0.439 -0.057 1.000 Pb -0.221 0.179 0.183 -0.380 0.211 1.000 Sr -0.297 -0.243 0.596 -0.291 0.136 0.213 1.000 Ti 0.522 0.523 -0.236 0.444 0.245 -0.367 -0.573 1.000 V 0.674 0.456 -0.382 0.551 0.249 -0.352 -0.602 0.785 1.000 Zn 0.171 0.602 -0.174 0.018 0.236 0.503 -0.466 0.388 0.337 1.000
125 Chapter 5. Characterization of Valley Sediments
5.4.2 Factor Analysis
Two trends appear to be at work in the geochemical data: a ‘metals’ component and an ‘aluminosilicate’ component. A bivariate plot showing Fe vs. K (Figure 5-9) gives the best initial separation of the various deposits in the Chilliwack Valley. Borden Creek lithologies do not appear to have significant intrusive igneous minerals or iron oxide concentrations. A variety of multivariate techniques were explored to uncover the underlying structure of the element variables in a framework that was consistent with the lithological environment of Chilliwack Valley. R-mode factor analysis using principal components was used because results could be interpreted in a clear geological sense. Given the poor discriminating ability of many trace elements (described above), an opti- mum set of 11 elements was used for the factor analysis: Al, Ba, Ca, Fe, K, Mn, Na, P, Ti, V, Zn. It was found that three factors account for the majority (∼80%) of common variance in the data (Table 5.2). The first factor (“Fe-factor”) strongly weights the metal-associated elements: Fe, Mn, Ti and V. The second factor (“K-factor”) applies a large, negative weight to the aluminosilicate component: Al, K, Na, and P. The third factor places largest weights on Ba, Ca and Zn, though the geological interpretation of this factor is not as straightforward. A Varimax rotation of the scores emphasizes the elements further. The first two factors correspond reasonably well to the two major lithologic groups in the valley: metasedimentary rocks of the Chilliwack and Cultus Groups versus granitic rocks from the Chilliwack Batholith. Minerals containing elements of the metals group (Fe, Mn, Ti and V) can be found in both lithologic groups, though concentrations appear to be systematically higher in the metasedimentary terrain, particularly Slesse Creek. Figure 5-10(a) shows the excellent separation of the bedrock sources based on the sediment factor scores. 5-10(b) shows a directional plot of the factor scores. Figure 5-10(c) shows the geochemical data plotted along the two factor axes. Samples from Chilliwack Valley mainstem channel plot along the diagonal axis, showing the balanced influence of the two sources on the composition of the fine sediment. The tributary influence in the upper mainstem is stronger than it is further downstream, below Slesse Creek. Samples taken from above Slesse Creek (tied by a line and shown with their chainage in kilometres above Vedder Crossing, Slesse = 24.1 km) tend to show points that sway toward the compositional field of their upstream source basin. In the lower valley, an equilibrium develops along the unit ratio line, with points moving from proximal to distal. Samples with higher element concentrations plot closer to the ‘proximal’ end of the spectrum.
126 Chapter 5. Characterization of Valley Sediments
2
Batholith Tributaries Chwk/Cultus Group Tributaries Mainstem Alluvium Pleistocene Deposits 1 2 1.5 K (%) 3
1 4
0.5 2 4 6 8 Fe (%)
Figure 5-9: Bivariate plot showing the major geochemical domains among channel sediments and glacial deposits in the Chilliwack Valley. Among the raw element data, K and Fe provide the best discriminating potential. Fine channel sediments from batholith sources (upper left) and Chilliwack Group or Cultus Group sources (lower right) are reasonably distinct. Mainstem alluvium plots as an intermediate field, and glacial deposits plot to the lower left. A few seemingly anomalous points are evident: (1) a sample taken from glaciolacustrine bluffs at the Slesse/Chilliwack confluence. There is a strong affinity here for headwater lithologies (2) two samples taken from the Slesse Park landslide, which show a concentrated (proximal) mix of the two source terranes. (3) is a distinct sandy bed from Slesse Park landslide that clearly has a strong indication of Slesse source material. There is also a ‘mainstem alluvium’ point here that was sampled downstream of the Nesakwatch Creek confluence, illustrating its strong influence on mainstem sediment composition. (4) is from Borden Creek, whose source terrain appears relatively high in silica and low in indicator elements.
127 Chapter 5. Characterization of Valley Sediments
a) b) 3 1
Alluvium Batholith 1 Enclave V Chwk Group Elbow 0 Ti YAC Ca
K-Factor K-Factor Fe Ba Zn -1 Mn Na P Al K -3 -1 6 4 2 0 -2 -4 -6 1 0 -1 Fe-Factor Fe - Factor 1.5 Chilliwack/Cultus Group Distal c) (Metamorphic) Source Pleistocene Deposits Liumchen &