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Home , Lillooet, Lillooet River, Meager Creek, Tantalus (Oahu)

Sedimentary Geology 176 (2005) 305–322 www.elsevier.com/locate/sedgeo

Impact of a volcano on sedimentation in River valley,

P.A. Frielea,T, J.J. Clagueb, K. Simpsonc, M. Stasiukc

aCordilleran Geoscience, 1021, Raven Drive, P.O. Box 612, Squamish, BC, V0N 3G0 bDepartment of Earth Sciences, University, Burnaby, BC, Canada V5A 1S6; Emeritus Scientist, Geological Survey of Canada, 101-605 Robson Street, , BC, Canada V6B 5J3 cGeological Survey of Canada, 101-605 Robson Street, Vancouver, BC, Canada V6B 5J3 Received 3 May 2004; received in revised form 15 December 2004; accepted 19 January 2005

Abstract

Lillooet River drains 3850 km2 of the rugged in southwestern British Columbia, including the slopes of a dormant Quaternary volcano at Mount Meager. A drilling program was conducted 32–65 km downstream from the volcano to search for evidence of anomalous sedimentation caused by volcanism or large landslides at Mount Meager. Drilling revealed an alluvial sequence consisting of river channel, bar, and overbank sediments interlayered with volcaniclastic units deposited by debris flows and hyperconcentrated flows. The sediments constitute the upper part of a prograded delta that filled a late lake. Calibrated radiocarbon ages obtained from drill core at 13 sites show that the average long-term floodplain aggradation rate is 4.4 mm aÀ1 and the average delta progradation rate is 6.0 m aÀ1. Aggradation and progradation rates, however, varied markedly over time. Large volumes of sediment were deposited in the valley following edifice collapse events and the eruption of Mount Meager volcano about 2360 years ago, causing pulses in delta progradation, with estimated rates to 150 m aÀ1 over 50-yr intervals. Two of the volcaniclastic units identified in drill core correlate with previously documented strong acoustic reflectors in at the downstream end of the basin. The Mount Meager massif constitutes only 2% of the drainage, but lithology counts of Lillooet River channel gravels indicate that a disproportionate percentage of the sediment is derived from the volcano. The data indicate that deposits of large debris flows are important elements of the sedimentary sequence and that Mount Meager dominates the sediment supply to Lillooet River. D 2005 Elsevier B.V. All rights reserved.

Keywords: Valley fill; Stratigraphy; Debris flow; Hyperconcentrated flow; Fiord-lake; Holocene; Lillooet River; Mount Meager; British Columbia

1. Introduction

T Corresponding author. Fax: +1 604 898 4742. Delta geomorphology, architecture, and sedimen- E-mail address: [email protected] (P.A. Friele). tary processes are governed by a host of factors,

0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.01.011 306 P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322 including the character of the receiving basin (bathy- Slaymaker (1972, 1977) summarized historic sedi- metry, water circulation, wave and current regimes, ment yield in the valley, whereas Jordan and Slay- and thermal and density stratification) and changes in maker (1991) and Slaymaker (1993) estimated the base level, sediment supply, and climate (Kostaschuk, long-term yield using a sediment budget approach. 1987; Smith, 1991; Kazuaki et al., 2001). In this Jordan and Slaymaker (1991) noted a 50% discrep- paper, we examine the contribution of a Quaternary ancy between the budgeted and historic sediment volcano to a deltaic valley fill in the Lillooet River yields and surmised that it might be due to a change in valley in southwestern British Columbia. We demon- sediment delivery to Lillooet River following the strate that instability on the volcano during the Little Ice Age or to underestimation of the frequency Holocene strongly influenced the evolution of the or magnitude of landslides at Mount Meager. delta. Jordan and Slaymaker (1991) further proposed a The sediment fill in the Lillooet River valley modification of the paraglacial sediment concept. records postglacial infilling of a 75-km-long fjord lake Paraglacial sedimentation involves large transfers of downstream from Mount Meager volcano (Fig. 1). sediment from uplands to river valleys during and

Fig. 1. A) Map of southern British Columbia showing the location of the Mount Meager volcanic complex (MMVC). B) Southwestern British Columbia, showing Highway 99 from Vancouver to Pemberton, and the Mount Meager volcanic complex. C) The study area showing the Lillooet River valley, and drill sites DHPV01-12 and SH1. The white bars delineate the four river reaches: 1- to Railroad Creek; 2- Railroad Creek to Ryan River; 3-Ryan River to Green River; 4 Green River to Lillooet Lake. Human settlement extends from the lower end of reach 1 (at DHPV09) to Lillooet Lake. P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322 307 immediately after deglaciation, causing rapid growth km2) is rugged, with up to 2800 m of local relief and of fans at tributary mouths (Ryder, 1971)and peaks up to 3000 m in elevation. About 15% of the subsequent aggradation of floodplains (Church and basin is glaciated. Most of the basin is underlain by Slaymaker, 1989). Jordan and Slaymaker (1991) resistant plutonic rocks (Woodsworth, 1977), but the suggested that Holocene sediment yield in the Mount Meager volcanic complex (Fig. 2), a dormant Lillooet River basin is episodic, with numerous Quaternary volcano, underlies about 2% in the basin pulses induced by volcanism, landslides, Neoglacia- headwaters. tion, and land-use change. Large parts of the Mount Meager volcanic complex Desloges and Gilbert (1994) studied sedimentation are hydrothermally altered (Read, 1979). The alter- rates in Lillooet Lake and suggested that fines (b63 ation, together with the steep slopes, result in high Am), which make up the lacustrine sediment pile, are rates of mass wasting and landslides. Four landslides produced most abundantly by glacial comminution of in excess of 1Â106 m3 occurred in the last century rock debris and that landslides are a relatively minor alone-in 1931, 1947, 1975, and 1998 (Bovis and source of fine sediment. They thus stress the Jakob, 2000; Carter, 1932; Croft, 1983; Evans, 1987; dominance of the paraglacial sediment source to Mokievsky-Zubok, 1977). The 1931 failure produced Lillooet Lake. a secondary debris flow that reached the mouth of Based on the history of other , Meager Creek (Fig. 2) and caused flood surges along episodic sedimentation should be recorded as a large Lillooet River (Decker et al., 1977, p. 161). Friele and number of discrete thick beds within the Holocene Clague (2004) documented flank collapses 8700 and sediment fill in Lillooet River valley. Radiocarbon 4400 years ago, involving at least 6Â108 m3 of ages from landslide debris exposed along Meager material on the south side of the volcanic complex. Creek and upper Lillooet River (Friele and Clague, Debris flows from these flank collapses traveled the 2004; Jordan, 1994; McNeely and McCuaig, 1991) length of Meager Creek into Lillooet River valley. In suggest that at least 12 large prehistoric landslides addition, Plinth Peak (Fig. 2) erupted explosively occurred in these valleys. In addition, an outburst about 2360 years ago (Clague et al., 1995; Nasmith et flood occurred shortly after the last eruption at al., 1967), producing volcanic and landslide deposits Mount Meager (Stasiuk et al., 1996). There probably that dammed upper Lillooet River (Stasiuk et al., have been other large landslides, but they have not 1996; Stewart, 2002). The dam failed, causing an been identified due to burial or erosion, while outburst flood that swept down the valley (Evans, smaller debris flows occur almost annually (Jakob, 1992; Stasiuk et al., 1996). This instability is well 1996). documented in the valleys adjacent to Mount Meager, By drilling the upper sediments of the Lillooet but its effects in the Lillooet valley to the south are valley fill, we test the episodic sediment yield model unknown because Lillooet River aggraded throughout proposed by Jordan and Slaymaker (1991). In this the Holocene (Jordan and Slaymaker, 1991), burying paper, we describe the architecture of the upper valley the evidence. fill and calculate rates of floodplain aggradation and Lillooet River is divisible into four reaches below delta-front progradation using radiocarbon ages on the mouth of Meager Creek (Fig. 1; after Jordan and fossil plant material recovered from drill cores. Slaymaker, 1991). The first, most northerly reach Finally, we compile the evidence to counter the notion extends 25 km downvalley from Meager Creek to that the pararglacial sediment supply dominates in the Railroad Creek. The river in this reach is braided Lillooet River basin. (Fig. 2), has a cobble bed (Fig. 3), and a gradient of 0.006. Gravelly colluvial fans from tributary valleys and rockslide deposits extend onto the active flood- 2. Setting plain, which is up to 1 km wide. The second reach extends 10 km downvalley from Railroad Creek to Lillooet River flows in a glacially modified river the Ryan River fan (Fig. 4). In this reach, the valley in the southern Coast Mountains of British channel is wandering (Desloges and Church, 1987) Columbia (Fig. 1). The Lillooet River basin (3850 to meandering and has a pebble-cobble bed (Fig. 3) 308 P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322

Fig. 2. View northwest up Lillooet River to the Mount Meager volcanic complex (Province of British Columbia airphoto BC563: 113; taken September 29, 1949). In the foreground is a 12-km length of reach 1, showing the braided channel of Lillooet River and confining debris fans. DGS indicates the fresh track of the 1931 debris flow that traveled the length of Meager Creek. The basin on the west flank of Pylon Peak is the source of two large landslides mentioned in the text (8700 and 4400 years ago). The valley just west of Mount Meager is the source of large landslides in 1932 and 1998. Plinth Peak is the source of the ca. 2400-year-old eruption.

Fig. 3. Bar-top texture and lithology trends in Lillooet River valley (after Kerr Wood Leidal, 2002). Reach breaks and drill core locations are also shown. P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322 309

Fig. 4. Aerial photograph of Lillooet River valley taken during the early stages of river training (Province of British Columbia airphoto BC396: 116; August 2, 1947). The Ryan River fan forms the boundary between reaches 2 and 3. Cutoff ages are from Decker et al. (1977). The locations of drill sites DHPV06 and DHPV12 are shown. Note the meandering channel planform of Lillooet River, with distal wetlands, back channels, and abandoned meander loops. and a gradient of 0.0025. The active floodplain is mouth of Kakila Creek (Fig. 1), and its level (196F4 1.5–2 km wide and is bounded by steep bedrock m above sea-level) is controlled by bouldery fan slopes. The third reach extends a further 15 km deposits at the outlet. The level was artificially downvalley to Green River. Prior to training, this lowered 2.5 m by dredging the outlet in 1956 (Gilbert, part of the river had an irregular meandering 1972; Nesbitt-Porter, 1985), thus its level prior to planform (Fig. 4). It has a sand to pebble gravel settlement was 198.5 m asl. bed (Fig. 3) and a gradient of 0.0009. The active The Lillooet River delta satisfies many of the floodplain in reach 3 is 1.5–2 km wide and extends criteria of the fan-foreset delta of Smith (1991). This to the valley walls. The fourth reach, between Green type of delta forms where a stream deposits sandy and River and Lillooet Lake, is 15 km long and has a gravelly bed load in a deep body of water. However, straight to sinuous channel, a sand bed, and a the gradient of Lillooet River is too low (0.0006) and gradient of 0.0006. Reach 4 terminates at the modern the bed material is too fine (mainly sand) for this term delta of Lillooet River. to be properly applied to the Lillooet delta. It is thus Channel widths are 120–200 m, average channel more appropriately termed a lake-head foreset delta depths are 3–6 m, and thalwag depths are 7–9 m in (Kostaschuk, 1987). reaches 2, 3, and 4. Width-to-depth ratios range from The delta is rapidly extending into Lillooet Lake. 20 to 60 (hydraulic geometry from Kerr Wood Leidal, Detailed ground surveys and air photo analysis 2002). Local relief on the floodplain ranges from 1 to (Gilbert, 1972, 1975) indicate an average progradation 4m(B.C. Ministry of Environment, 1990). Vegetation rate of 7–8 m aÀ1 between 1858 and 1948. In the early prior to settlement consisted of forest and wetlands 1950s, the lowermost 30 km of the river was (Decker et al., 1977; Teversham and Slaymaker, straightened and dyked. These measures, along with 1977). dredging, steepened the channel gradient, causing Lillooet Lake is 24 km long, 1–2 km wide, and up incision and bank erosion. Thus, in the period 1948– to 140 m deep (Desloges and Gilbert, 1994). The lake 1969, progradation rates increased markedly to 20–30 is impounded behind a paraglacial alluvial fan at the maÀ1. From 1969 to the present, progradation rates 310 P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322 have averaged 15 m aÀ1 (Jordan and Slaymaker, 4. Drilling rationale and methods 1991; Kerr Wood Leidal, 2002). 4.1. Rationale

3. Valley fill sediments Assuming a conservative, long-term progradation rate of 5 m aÀ1, the delta front was 50 km upstream Lillooet River is typical of the coarse-grained 10000 years ago, soon after the valley was deglaci- streams described by Brierley (1989) and Miall ated (Friele and Clague, 2002). This puts the head of (1996). Common environments on the floodplains of Lillooet Lake somewhere within reach 1 (Fig. 2) at the such rivers include channels, point bars, levees, beginning of the Holocene. Drilling sites within abandoned meander loops, oxbow lakes, and swamps reaches 1, 2, and 3 (Figs. 1 and Figs. 3) are thus (Fig. 4). A typical alluvial sequence consists of within the area of the lake that was infilled during the upward-fining channel fills interlayered with overbank Holocene. Only the subaerial part of the valley fill was sediments that fine away from channels (Table 1). targeted (i.e. materials above mean lake level, or 198.5 Alluvial fans extend into the Lillooet valley from m asl). It was not possible to drill the 100–140 m thick tributary streams underlain by plutonic rocks. Fans subaqueous sequence because of the cost. Moreover, from large basins (N15 km2) are mainly fluvial in we recognized that radiocarbon ages on proximal origin, whereas those from smaller basins have been deltaic sediments would be difficult to interpret due to formed partly or largely by debris flow activity subaqueous landsliding (Gilbert, 1972). (DeScally et al., 2001). We chose target depths for drilling on the basis of Volcanic debris flows triggered by eruptions the estimated thickness of topset deposits, assuming (Crandell, 1971; Scott et al., 1992; Vallance and that mean lake level has not changed since the Scott, 1997) or flank collapse (Siebert, 2002) may middle Holocene. This assumption is justified on the leave valley-wide deposits many tens of kilometers following grounds. First, recent studies of well-dated from their source (Iverson et al., 1998). The Lillooet alluvial fans in British Columbia indicate they River valley fill sequence thus could include deposits formed quickly during and immediately after degla- of debris flows and hyperconcentrated flows derived ciation (Lian and Hickin, 1996; Friele and Clague, from Mount Meager (Table 1). 2002). Accordingly, the Kakila Creek fan was

Table 1 Criteria for interpretation of facies encountered in drill core Facies Depositional Structure Lithology Texture Reference environment Channel fill Active channel Clast-supported, b50% volcanic Coarse silt to cobbles, Brierley, 1989; massive to stratified but dominantly very Gilbert, 1972; Kerr (cross-bedded, rippled) fine sand to granule Wood Leidal, 2002. gravel; clasts of mud and peat due to bank collapse. Overbank Floodplain Horizontally laminated Not studied Fine to very fine sand, Brierley, 1989; to bedded silt, and clay interbedded Miall, 1996. with peat. Hyper-concentrated Channel and Clast-supported, massive N50% volcanic Poorly-sorted, sandy Pierson and Scott, 1985; flow overbank to weakly bedded, granule gravel; may have Smith, 1986 normally graded cap of wood debris or pumice. Debris flow Channel and Massive to weakly N75% volcanic; Diamicton Pierson, 1985; overbank stratified, matrix includes Jordan, 1994 supported hydrothermally altered clasts. P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322 311 probably built across Lillooet Lake during the early Additional data were obtained during a geotech- Holocene, implying that mean lake level (ca. 198.5 nical investigation for a new school at Pemberton m asl) has changed little since then. Second, the (Signal Hill; site SH1, Fig. 1) in 2002. A rotary auger Lizzie Creek fan-delta today forms a sill that divides was used to drill four 10–15-m holes, and a cable-tool Lillooet Lake into separate subaqueous basins drilling system was used to drill one 110-m-deep (Desloges and Gilbert, 1994), and other, now-buried, cased well. These drilling techniques yielded basic valley-side fan-deltas in Lillooet River valley may lithological information and material for radiocarbon have done the same. However, the surfaces of fans dating. emanating from large tributary basins, such as those of Green, Birkenhead, and Ryan rivers, intersect paleo-lake level before reaching the opposite valley 5. Stratigraphy side. This observation indicates that the Holocene lake was a continuous water body and not several Facies interpretations are based on the source smaller lakes separated by fans. The pre-1950s lake material summarized in Table 1. Drill cores were surface elevation (198.5 m asl) is thus taken as the grouped according to their location in the valley. Four level of the paleo-lake over its entire length. cores (DHPV09, DHPV11, DHPV06, and DHPV12) are arranged in a downvalley transect 34–50 km from 4.2. Field methods Mount Meager (Fig. 5). Three cores (DHPV03, DHPV04, and DHPV05) form a cross-valley transect Twelve sites along the axis of the valley were (Fig. 6) about 42 km downvalley from Mount Meager. drilled to depths of 10–40 m (Fig. 1). About 260 m of The stratigraphy at these seven sites differs in detail, core were recovered, logged, and sampled. A conven- but all sites share a volcanic debris flow marker bed. tional dry auger fitted with a 10-cm split spoon In contrast, volcaniclastic deposits were not encoun- sampler was used at sites DHPV01, DHPV02, tered in three drill holes (DHPV02, DHPV08, and DHPV03, DHPV04, DHPV05, and DHPV06 (Fig. DHPV01), 53–57 km from Mount Meager (Fig. 7). 1). Drilling was done in 1.5-m (5-ft) flights. Most However, a pumice-rich, sandy pebble gravel marker retrieved cores occupied no more than about 60% of bed occurs at or just above mean lake level at the the sampler. Due to poor core recovery, we switched southernmost drill sites (DHPV07 and DHPV10), 63– to sonic drilling technology for sites DHPV07, 65 km from Mount Meager (Fig. 8). DHPV08, DHPV09, DHPV10, DHPV11, and DHPV12 (Fig. 1). This system yielded continuous 5.1. Channel deposits core in 3-m flights. Core recovery was good, except in cobble gravel, which was encountered only at the Fining-upward sequences with cobble or pebble most northerly site (DHPV09). gravel at the base and medium to coarse sand at Cores were extruded into plastic sleeves and then the top are interpreted to be channel gravel and split and logged in the field. One half of each core was point bar deposits. Cobble gravel beds up to 6 m archived at the Vancouver office of the Geological thick form the channel facies at site DHPV09. Survey of Canada. Sediment texture, structures, clast Overbank facies are uncommon at this site, which shape and lithology, plant material, and contacts were may reflect channel instability in reach 2 and the described for strata as thin as 0.5 cm. Representative upper part of reach 3. Channel units at other sites cores and important features and contacts were consist of pebble gravel beds tens of centimeters photographed. Samples were collected for radiocar- thick. Lags with mud ball clasts were noted at sites bon dating, pebble lithology, and clay mineral and DHPV03 and DHPV01. The lags occur at the base grain size analyses. Surface elevations were estimated of medium to coarse sand beds in sequences from British Columbia Ministry of Environment ranging up to 6 m thick. The core at site DHPV08 floodplain maps (B.C. Ministry of Environment, contains three sequences that grade from channel 1990), which have a 1-m contour interval and spot facies at the base to overbank facies at the top elevations with decimeter accuracy. (Fig. 7). 312 P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322

Fig. 5. Core logs for sites DHPV09, DHPV11, DHPV06, and DHPV12 (see Fig. 1 for locations).

5.2. Overbank deposits at sites DHPV03, DHPV04, DHPV05, DHPV06, DHPV11, and DHPV12. These sediments are inter- Laminated fines and peat are common at all drill preted to be hyperconcentrated flow deposits (Table sites. Clastic strata range from millimeters to tens of 1). centimeters thick, and peat beds are up to several Hyperconcentrated flow deposits in Lillooet valley metres thick. These deposits are commonly interbed- should contain a greater proportion of volcanic clasts ded with fine to very coarse sand and stringers of fine than normal river bedload. In the reaches of interest, gravel, which we interpret to be sand sheets. Most lithology counts of pebbles on bar surfaces of the overbank units are 2–5 m thick, suggesting consid- modern river yielded an average of 25% volcanic erable channel stability during aggradation. The clasts and 75% basement clasts (Fig. 3). Lithology maximum logged thickness is 8 m (sites DHPV11, counts of selected samples of very coarse sand and DHPV06, DHPV02, and DHPV07). pebbles (1–25 mm size class) from core (Table 2) indicate that beds interpreted a-priori as normal 5.3. Hyperconcentrated flow deposits channel deposits contain 25–50% volcanic clasts, whereas those interpreted as hyperconcentrated flow Massive to weakly stratified beds of poorly sorted, deposits contain 50–100% volcanic clasts. The higher volcanic-rich coarse sand and fine gravel from a few percentage of volcanic clasts in some channel gravel decimeters thick to about 4 m thick were encountered beds in cores, in comparison to modern channel P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322 313

Fig. 6. Core logs for sites DHPV03, DHPV04, and DHPV05 (see Fig. 1 for locations). gravel, may be due to reworking of prehistoric, hyperconcentrated flow unit at site DHPV06 yielded a volcanic debris flow and hyperconcentrated flow radiocarbon age of 2480F40 14C yr BP (2406–2789 deposits. No historic volcanic debris flows have cal yr BP; Table 3). traveled significantly far down Lillooet River valley, A 2.5-m-thick hyperconcentrated flow unit at site but several have done so earlier in the Holocene DHPV12 rests on channel gravel, which in turn (Friele and Clague, 2004). sharply overlies floodplain sediments dated at Scattered, rounded pumice grains up to pebble-size 2000F60 14C yr BP (1923–2046 cal yr BP; Table were noted throughout hyperconcentrated flow units 3). The dated floodplain sediments were eroded before in cores DHPV05 and DHPV06. In addition, the the overlying channel gravel was deposited and surface of the hyperconcentrated unit at DHPV05 is provide only a poorly constrained maximum age for capped by 30–50 cm of poorly sorted angular pumice the hyperconcentrated flow at this site. The hyper- clasts ranging up to cobble size, interpreted to be float concentrated flow unit is thus appreciably younger (Pierson and Scott, 1985). A wood fragment from the than 2000 years and is tentatively correlated with a 314 P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322

Fig. 7. Core logs for sites DHPV02, DHPV08, and DHPV01 (see Fig. 1 for locations).

900-year-old, valley-filling debris-flow deposit in 50 km downvalley from Mount Meager (Figs. 5 and upper Lillooet River valley (Jordan, 1994). Figs. 6). Three samples of wood fragments collected Thick (2–6 m) hyperconcentrated flow units were from the diamicton and one from peat on the surface encountered at the base of the drill holes at sites of the deposit provide a bracketed age between DHPV04 and DHPV11. An age of 3230F70 14Cyr 2570F40 14C yr BP and 2690F50 14Cyr BP (Table BP in sand 5 m above the hyperconcentrated flow unit 3). Based on these limiting dates, the deposit is 2540– at site DHPV04 and an age of 4550F90 14CyrBP 2970 cal yr old. Its age, volcanic provenance and from within the unit at site DHPV11 indicate that the massive, matrix-supported structure indicate a debris event occurred sometime between about 3300 and flow origin (Table 1). A strong acoustic reflector 13– 5500 cal yr ago (Table 3). The flow may be associated 32 m below the floor of Lillooet Lake, which has been with a large flank collapse of Pylon Peak into Meager estimated to be 2495F670 years old based on Creek valley about 4400 cal yr ago (Friele and sedimentation rates (Desloges and Gilbert, 1994), Clague, 2004). probably correlates with the debris flow. The debris flow overrode overbank sediments at 5.4. Debris flow deposits four sites (DHPV03, DHPV04, DHPV11, and DHPV12), indicating that it was not confined to the A layer of volcanic diamicton 2–4.5 m thick and at river channel. The diamicton lies on channel gravel at least 1000 m wide was encountered in drill holes 34– sites DHPV09, and DHPV05, suggesting that it was P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322 315

Fig. 8. Core logs for sites DHPV07, DHPV10 and SH1 (see Fig. 1 for locations). flowing down the old river channel at those sites. The at the base of the debris flow, 42–50 km downvalley lower part of the diamicton includes gravel at from the source. These observations indicate that the DHPV06 and rip-ups of silty floodplain sediments at debris flow likely traveled farther downvalley than we DHPV12 (Fig. 9), suggesting significant shear stress have documented through drilling.

Table 2 Lithology of selected gravel samples Drill hole Depth (cm) Lithology (%) (1–25 mm size class) Facies interpretation Volcanic Basement Miscellaneous DHPV11 480–500 51 40 9 Hyperconcentrated flow DHPV11 650–670 64 24 12 Hyperconcentrated flow DHPV11 750–770 47 32 21 Hyperconcentrated flow DHPV11 850–870 88 8 4 Hyperconcentrated flow DHPV11 2415–2438 65 27 8 Hyperconcentrated flow DHPV11 2600–2620 75 18 7 Hyperconcentrated flow DHPV11 2980–3000 46 42 12 Hyperconcentrated flow DHPV04 710–730 69 21 10 Hyperconcentrated flow DHPV04 2081–2100 72 23 5 Hyperconcentrated flow DHPV08 300–460 48 39 13 Channel DHPV08 900–1000 37 55 8 Channel DHPV08 1680–1830 49 42 9 Channel DHPV02 760–910 35 58 7 Channel DHPV02 1220–1370 23 71 6 Channel 316 P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322

Table 3 Radiocarbon ages Drill hole Latitude Longitude Depth Material Enclosing facies Lab numbera Radiocarbon Calendric age (yr BP)c 14 b (N) (W) (cm) age ( C yr BP) Mode Range DHPV09 508 31.314V 1238 04.359V 2920 Wood Debris flow OS-36552 6370F35 7340 7297–7466 fragment DHPV09 508 31.314V 1238 04.359V 3100–3200 Wood Debris flow OS-36556 6250F30 7260 7076–7302 fragment DHPV03 508 29.779V 1228 57.990V 490–500 Peat Overbank Beta-166050 1960F40 1960 1872–2043 DHPV03 508 29.779V 1228 57.990V 790 Wood Debris flow Beta-166057 2690F50 2860 2794–2966 fragment DHPV03 508 29.779V 1228 57.990V 2440 Peat Overbank Beta-166053 5130F60 5910 5785–6041 DHPV04 508 29.550V 1228 58.131V 1040 Twig Debris flow Beta-166058 3970F40 4460 4347–4575 DHPV04 508 29.550V 1228 58.131V 1500 Wood Channel Beta-166051 3230F70 3480 3322–3684 fragment DHPV05 508 30.048V 1228 58.053V 540 Peat Overbank Beta-166052 1780F60 1750 1590–1916 DHPV05 508 30.048V 1228 58.053V 680 Charcoal Paleosol Beta-166059 2570F40 2790 2544–2809 DHPV11 508 29.518V 1228 58.082V 1620 Wood Overbank GSC-6647 2950F50 3150 3047–3285 fragment DHPV11 508 29.518V 1228 58.082V 1810 Peat Overbank GSC-6646 3590F60 3930 3828–4028 DHPV11 508 29.518V 1228 58.082V 2410 Wood Hyperconcentrated GSC-6645 4550F90 5240 5096–5490 fragment flow DHPV06 508 27.674V 1228 56.155V 600 Peat Overbank Beta-166165 1250F60 1250 1045–1341 DHPV06 508 27.674V 1228 56.155V 886 Wood Paleosol Beta-166166 2480F40 2630 2406–2789 fragment DHPV06 508 27.674V 1228 56.155V 1960 Wood Overbank Beta-166054 3870F40 4370 4200–4466 fragment DHPV12 508 26.057V 1228 54.550V 800 Peat Overbank GSC-6654 2000F60 1990 1928–2050 DHPV12 508 26.057V 1228 54.550V 1110 Wood Debris flow GSC-6648 2720F80 2870 2802–2968 fragment DHPV02 508 24.239V 1228 53.147V 1510 Wood Overbank Beta-166049 2840F80 3020 2826–3260 fragment DHPV02 508 24.239V 1228 53.147V 1800 Wood Overbank GSC-6651 3320F60 3610 3518–3686 fragment DHPV02 508 24.239V 1228 53.147V 1840 Wood Overbank GSC-6650 3700F60 4080 3978–4196 fragment DHPV02 508 24.239V 1228 53.147V 1970 Wood Overbank GSC-6649 3820F60 4270 4143–4399 fragment DHPV02 508 24.239V 1228 53.147V 2260 Wood Overbank Beta-166056 4410F40 5020 4914–5324 fragment DHPV01 508 22.654V 1228 51.734V 1040 Peat Overbank GSC-6653 1220F50 1180 1111–1307 DHPV01 508 22.654V 1228 51.734V 1210 Peat Overbank GSC-6652 1990F60 1990 1923–2046 DHPV01 508 22.654V 1228 51.734V 1900 Wood Channel Beta-166055 2510F40 2660 2411–2796 fragment SH1 508 17.00V 1228 50.00V 680 Wood Overbank Beta-139037 1860F70 1830 1661–2015 fragment SH1 508 17.00V 1228 50.00V 1200 Wood Channel GSC-6546 2680F60 2820 2799–2897 fragment SH1 508 17.00V 1228 50.00V 1680 Wood Foreset GSC-6575 3100F80 3370 3260–3433 fragment a Beta–Beta Analytic, GSC-Geological Survey of Canada, OS-. b Laboratory-reported uncertainties are 1j for Beta and OS ages and 2j for GSC ages. Ages are normalized to y13C=À25.0x PDB. Datum is AD 1950. c Determined from atmospheric decadal dataset of Stuiver et al. (1998) using the program CALIB 4.0.2. The range represents the 95% confidence limit calculated with an error multiplier of 1. The mode is the average of the part of the calibration probability distribution that explains more than 50% of the variance. Datum is AD 2000. P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322 317

A second volcanic diamicton was encountered at 23–24 m depth at site DHPV09. We interpret it to be a debris flow deposit on the basis of the criteria discussed above. The diamicton is capped by channel gravel, indicating that its surface was eroded. Its original thickness is thus unknown. We estimate the age of the diamicton to be 4300–4530 cal yr based on an extrapolation of an aggradation rate of 4.4 mm aÀ1 from underlying, radiocarbon-dated deposits (see section 6.1). The debris flow that deposited the diamicton probably resulted from a flank collapse at Pylon Peak (Fig. 3) about 4400 years ago (Friele and Clague, 2004). A third volcanic diamicton, similar to those described above, was encountered at 28.2–36.2 m depth at site DHPV09. It was at least 8 m thick when deposited and has a maximum age of 6250F30 14Cyr BP (7076–7302 cal yr BP). No landslide or eruption of this age has been documented in the valleys of Lillooet River or Meager Creek.

6. Floodplain evolution

6.1. Aggradation rates

Average aggradation rates were estimated from radiocarbon ages obtained from drill core (Table 4). Each radiocarbon age was calibrated (Stuiver et al., 1998), and the modal value of the probability distribution was used as the most probable age of the sample (Table 3). Average rates were calculated from dated depths to the surface at each site, as well as between samples. In some cases, two or more dated samples came from a single unit in a core, allowing an average aggradation rate to be determined for a Fig. 9. Silt rip-up at the base of diamicton (Dmm) in core DHPV12. specific facies, for example overbank fines. Uncer- The diamicton overlies floodplain silt (Fl) and sand (Ss). tainties in radiocarbon ages range from 30 to 90 years, thus aggradation values have possible errors of up to No evidence, however, has been found for the 20%. This error, however, is within the standard debris flow at drill sites south of site DHPV12. Sites deviation of the mean of all estimates (Table 4). No DHPV02, DHPV08, and DHPV01 are located in a attempt was made to correct for sediment compaction. relatively narrow section of the valley, occupied by Average long-term (1000–7200 years) aggradation both Ryan and Lillooet rivers (Fig. 4), where the rates range from 1.4 to 6.0 mm aÀ1; the mean and potential for erosion is higher. However, the debris standard deviation are 4.4F1.3 mm aÀ1 (n=18) (Table flow may have become confined to the old river 4). Average medium-term (100–1000 years) aggrada- channel in this area, making it more difficult to tion rates range from 1.0 to 6.5 mm aÀ1, with a mean locate. and standard deviation of 3.4F2.3 mm aÀ1 (n=5). 318 P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322

Table 4 thicknesses of debris flow deposits in the Lillooet Long- and medium-term aggradation rates determined from valley fill range from 2 to 8 m. Hyperconcentrated calibrated radiocarbon ages flows may deposit sheets up to 1 m thick in overbank Site Span Rate Comment settings and up to 6 m thick in river channels. In (cal yr) (mm aÀ1) contrast, rainfall and nival floods produce deposits up Long-term rates (1000À7200 years) to only about 1 m thick, and these deposits thin DHPV09 7260 5.0 Base of diamict to surface DHPV03 5910 4.1 Bottom sample to surface rapidly away from river channels. DHPV03 3950 4.9 Between top and bottom samples 6.2. Progradation rates DHPV04 3480 4.3 Bottom sample to surface DHPV05 2790 2.5 Bottom sample to surface Modal values of calibrated radiocarbon ages (Table DHPV05 1040 1.4 Between top and bottom samples 3) were used to estimate the position of the Lillooet DHPV11 5240 5.7 Base of flow to surface delta front at different times (Fig. 10). It was assumed DHPV11 2090 3.8 Between top and bottom that the elevation of the lake surface throughout the samples Holocene was the same as that immediately prior to DHPV06 4370 4.5 Bottom sample to surface dredging (198.5 m asl), as discussed previously. At a DHPV06 3120 4.4 Between top and bottom samples few sites, radiocarbon samples were taken from mean DHPV12 2825 4.6 Base of diamict to surface lake level and could be used directly to define delta DHPV02 5020 4.5 Bottom sample to surface front positions. However, at most sites, the deepest DHPV02 2000 3.8 Between top and bottom samples were above mean lake level. In these cases, it samples was assumed that the sample represents the age of a DHPV02 1250 3.7 First and fourth samples; over-bank unit particular paleo-floodplain surface graded to mean DHPV01 2660 7.1 Bottom sample to surface lake level. The corresponding delta front position was DHPV01 1440 6.0 Between top and bottom determined by projecting the dated sample down- samples valley to mean lake level along the slope of the SH1 1830 3.7 Top sample to surface modern floodplain surface. An assumption implicit in SH1 2820 4.3 Second sample to surface Average 4.4F1.3 this analysis is that the slope of the paleo-floodplain is similar to the modern one. The assumption appears to Medium-term rates (100À1000 years) be reasonable because the long-term aggradation rate DHPV11 780 2.5 Upper two samples; of 4.4 mm aÀ1 results in a rate of delta progradation of single peat unit 7.3 m aÀ1, which is the historical rate prior to river DHPV02 590 5.1 First and second samples; over-bank unit training (see Fig. 6 in Jordan and Slaymaker, 1991). DHPV02 470 0.9 Second and third samples; There are other possible sources of error in this over-bank unit analysis. The largest amount of potential error stems DHPV02 190 6.5 Third and fourth samples; from the unevenness of the floodplain surface. Along over-bank unit a line perpendicular to the valley axis, floodplain DHPV01 810 2.0 First and second samples; single peat unit relief can range up to 3 m, and the range from the Average 3.4F2.3 channel thalwag to the levee top is about 8 m. On a surface with a slope of 0.0009 and with a difference of Medium-term rates are derived from peat and inter- 4 m in sample elevations, the possible error in a down- bedded peat and mud units. The higher long-term valley projection is 4.5 km. This error can be reduced average rate is explained by the fact that it incorpo- by considering the depositional environment of the rates debris and hyperconcentrated flow units. dated samples and adjusting sample elevations Rapid aggradation occurs during extreme events accordingly. Samples from overbank fines require no such as overbank floods, hyperconcentrated flows, adjustment in elevation, whereas those from channel and debris flows. Debris flows can deposit valley- gravels must be adjusted upward. Additional error wide sheets metres thick over valley lengths of many may be introduced if the dated sample is not the same tens of kilometers in minutes to hours. Recorded age as the enclosing sediment. Radiocarbon ages on P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322 319

Fig. 10. Longitudinal section of Lillooet River valley, showing radiocarbon ages used to reconstruct the position of the Lillooet Lake delta front at different times during the Holocene. The radiocarbon ages allowed calculation of long-term, incremental, and pulse progradation rates. delicate plant remains in paleosols and peat probably sample and the sample from overbank sediments at represent the true age of the sediments, whereas those DHPV03 to lake level. A radiocarbon age was on wood fragments in gravel or diamicton must be obtained at the contact between gravel and laminated considered maxima. sand at site DHPV06, probably on a bar surface With these caveats, we have calculated delta front setting. This sample was not adjusted. Its trace, positions at each of the sites as follows. At site projected downvalley, intersects a statistically equiv- DHPV09, we obtained radiocarbon ages on two alent radiocarbon age on overbank fines at site samples from the basal diamicton. The diamicton DHPV02. The mean of these two ages was projected rests on overbank fine sediments. We transferred the to mean lake level. A dated sample from a channel lag maximum age of the diamicton vertically down to the deposit at site DHPV01 was adjusted 4 m upward and top of the overbank sediments and then along the projected downvalley to mean lake level. Lastly, the modern surface gradient downvalley to mean lake upper sample from site SH1, collected from overbank level. Two deep samples were dated at sites DHPV03 sediments, was projected directly to mean lake level. and DHPV11, one from a hyperconcentrated flow unit This exercise yielded five delta front positions and another from overbank fine sediments. The (Fig. 10) and four intervals over which progradation hyperconcentrated flow unit at DHPV11 rests con- rates could be calculated. Estimated average progra- formably on overbank sediments, so we transferred dation rates are 3 m aÀ1 from 7260 to 5575 cal yr BP, the age of that sample to the top of the overbank unit, 7maÀ1 from 5575 to 4320 cal yr BP, 4 m aÀ1 from and then projected the mean depths and ages of that 4320 to 2660 cal yr BP, and 13 m aÀ1 from 2660– 320 P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322

1830 cal yr BP. The long-term average for the period years is 105 m aÀ1. The pulse rate for the landslide 7260–1830 cal yr BP is 6 m aÀ1, similar to the and eruption is about 150 m aÀ1 (Fig. 10). average historic rate prior to river training. The pumice bed at DHPV07 (Fig. 8), similar to that at DHPV10, occurs at a depth of 11.2–11.5 m and 7. Sedimentation in Lillooet lake comprises 10 cm of openwork, rounded granule to fine pebble pumice overlying 15 cm of pumice-rich, The Mount Meager volcanic complex appears to be very coarse sand to granule gravel. The rounded the dominant source of sediment deposited in Lillooet grains indicate that the material was transported as Lake. Both of the strong acoustic reflectors identified wash load rather than float, and the presence of by Desloges and Gilbert (1994) may stem from events pumice at lake level within channel deposits places at Mount Meager. Desloges and Gilbert (1994) the delta front at this site at the time of the eruption correctly attributed the 2495F670-year-old reflector about 2400 years ago. to volcanism. They concluded, however, that the Incremental progradation rates range from 3 to 13 upper acoustic reflector, at 6–13 m depth in the lake maÀ1. The average rate during the interval 4840– sediments and estimated to be 890F240 years old, 3850 cal yr BP is twice that during the interval 7260– was caused by an increase in the magnitude and 5575 cal yr BP. The period with the higher rate frequency of floods during the Little Ice Age. includes a flank collapse of Pylon Peak into Meager Unbeknown to them, a 900-year-old, valley-filling Creek valley about 4400 cal yr BP (Friele and Clague, debris flow covering 1.9 km2 of upper Lillooet River 2004), and is likely recorded by the middle diamicton valley had been documented by Jordan (1994). This at site DHPV09 and by the deepest hyperconcentrated event is the more likely source of the upper reflector. flow units at sites DHPV03 and DHPV11, based on We argue that volcanic debris is the single most the estimated ages of these deposits. The average important source of fine sediment in the Lillooet River progradation rate increases over threefold from 4320– basin. Debris flow deposits at Mount Meager contain 2660 cal yr BP to 2660–1830 cal yr BP. The latter 25–50% silt and 3–10% clay by weight, and the clay- period includes a large debris flow and the eruption of size fraction is dominantly phyllosilicate minerals Plinth Peak 2360 years ago. The landslide is recorded (Friele and Clague, 2004). Detailed examination of at sites DHPV09, DHPV03, DHPV04, DHPV05, the clay mineralogy of Lillooet Lake sediments could DHPV11, DHPV06, and DHPV12 by a valley-filling confirm that Mount Meager is the dominant sediment sheet of diamicton that is 2–4 m thick in reach 2 and source in the basin. the upstream part of reach 3. The eruption and associated outburst flood are recorded by a hyper- concentrated flow deposits at sites DHPV03, 8. Conclusions DHPV04, DHPV05, DHPV11, and DHPV06. Event, or pulse, progradation rates were much Lillooet valley was occupied by a 75-km-long fiord larger than suggested by the averages cited above lake at the end of the Pleistocene and, over the last (Fig. 10). Much of the sediment delivered to the delta 10,000 years, has advanced its delta about 50 km front may have arrived during large debris flows and downvalley through the study area to the present head hyperconcentrated flows (Vallance and Scott, 1997), of Lillooet Lake. Assuming an average yield of 400 B, or in a few decades following these events as the calculated using a sediment budget approach (Slay- deposits were reworked by Lillooet River (Major et maker, 1993), Lillooet River has deposited on the al., 2000). If the progradation rate preceding an event order of 15 km3 of sediment over this period. is viewed as a background rate, akin to base flow in a Average rates of floodplain aggradation and pro- hydrograph, it can be subtracted from the incremental gradation based on calibrated radiocarbon ages from rate encompassing the event. The resultant bevent drill core are, respectively, 4.4 mm aÀ1 and 6 m aÀ1, dischargeQ can then be adjusted to a more appropriate but large landslide events caused sediment pulsing. delivery period, say 50 years. In the case of the 4400 The long-term average progradation rate is similar to year old landslide, the adjusted pulse rate over 50 the pre-river training rate, but short-term rates, P.A. 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