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Home , Cheakamus River, Garibaldi Ranges, Mount Cayley, Mount Garibaldi, Squamish River, The Barrier

Geomorphology 53 (2003) 317–332 www.elsevier.com/locate/geomorph

Record of recent channel instability, Cheakamus Valley,

John J. Claguea,b,*, Robert J.W. Turnerb,1, Alberto V. Reyesa,2

a Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia, V5A 1S6 b Geological Survey of Canada, 101-605 Robson Street, , British Columbia, Canada V6B 5J3 Received 14 May 2002; received in revised form 3 October 2002; accepted 4 October 2002

Abstract

Rivers flowing from -clad Quaternary volcanoes in southwestern British Columbia have high sediment loads and anabranching and braided planforms. Their floodplains aggrade in response to recurrent large on the volcanoes and to advance of during periods of climate cooling. In this paper, we document channel instability and aggradation during the last 200 years in lower valley. Cheakamus River derives much of its flow and nearly all of its sediment from the massif, which includes a number of volcanic centres dominated by Mount Garibaldi . Stratigraphic analysis and radiocarbon and dendrochronological dating of recent floodplain sediments at North Vancouver Outdoor School in Cheakamus Valley show that Cheakamus River aggraded its floodplain about 1–2 m and buried a valley-floor forest in the early or mid 1800s. The aggradation was probably caused by a large (ca. 15–25 Â 106 m3) from the flank of Mount Garibaldi, 15 km north of our study site, in 1855 or 1856. Examination of historical aerial photographs dating back to 1947 indicates that channel instability triggered by this event persisted until the river was dyked in the late 1950s. Our observations are consistent with data from many other areas that suggest with large, but highly variable sediment loads may rapidly aggrade their floodplains following a large spike in sediment supply. Channel instability may persist for decades to centuries after the triggering event. Crown Copyright D 2002 Published by Elsevier Science B.V. All rights reserved.

Keywords: Floodplain; Aggradation; Channel instability; Stratigraphy; Little Ice Age British Columbia; Canada

1. Introduction

Rivers in British Columbia have responded in a complex manner to linked changes in sediment supply * Corresponding author. Department of Earth Sciences, Simon and climate on time scales ranging from years to Fraser University, Burnaby, British Columbia, Canada V5A 1S6. millennia (Church, 1981; Ryder and Church, 1986; Tel.: +1-604-291-4924; fax: +1-604-291-4198. Desloges and Church, 1987; Gottesfeld and Johnson- E-mail addresses: [email protected] (J.J. Clague), Gottesfeld, 1990; Jordan and Slaymaker, 1991; Ash- [email protected] (R.J.W. Turner), [email protected] (A.V. Reyes). more and Church, 2001). The principal responses have 1 Tel.: +1-604-666-4852; fax: +1-604 666-1124. been changes in river planform and aggradation and 2 Tel.: +1-604-291-3856; fax: +1-604-291-4198. degradation of channels and floodplains. The greatest

0169-555X/02/$ - see front matter. Crown Copyright D 2002 Published by Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-555X(02)00321-5 318 J.J. Clague et al. / Geomorphology 53 (2003) 317–332 changes since the end of the occurred early Ryder, 1972), although conditioned by glaciation, are during postglacial time when rivers rapidly aggraded really delayed, indirect responses to climatic change. their valleys in response to the transfer of large amounts They are more directly linked to sediment availability of sediment to valley floors (Church and Ryder, 1972; than to climate at the time of aggradation or degrada- Clague, 1986). The fills produced by this short-lived tion. phase of aggradation were incised over periods of Lesser changes in river planform and base level centuries to perhaps thousands of years in the early that are more directly linked to climatic change have after the supply of easily eroded drift became been documented in some river basins in British exhausted. These ‘‘paraglacial’’ effects (Church and Columbia and elsewhere (e.g., Meyer et al., 1992;

Fig. 1. Map of the Cheakamus and lower watersheds. Black areas are Quaternary volcanic rocks. The west flank of the Mount Garibaldi massif is the principal source of sediment to Cheakamus River. J.J. Clague et al. / Geomorphology 53 (2003) 317–332 319

Huisink, 1999). Some rivers in the Coast volcanic eruptions (Lipman and Mullineaux, 1981) of British Columbia aggraded their floodplains and may increase sediment delivery to rivers, causing acquired braided planforms during the Little Ice Age them to aggrade their valleys (Jordan and Slay- (Desloges, 1987; Desloges and Church, 1987; Got- maker, 1991). The perturbing event may be tesfeld and Johnson-Gottesfeld, 1990). These rivers short-lived, but its effects can persist for decades incised their floodplains and developed more anab- or even centuries. The cataclysmic eruptions of ranching planforms during the twentieth century, as Mount St. Helens in May 1980 and Mount Pinatubo climate warmed. Other researchers have documented in 1991, for example, triggered debris flows that climatically driven cycles of aggradation and degra- aggraded valleys many tens of kilometres from the dation at longer time scales. Huisink (1999),for volcanoes. The rivers that drain these volcanoes are example, reconstructed the complex response of the still adjusting to the eruptions (Simon, 1999; Hayes Maas River to alternating warm and cool intervals et al., 2002). during the Pleistocene–Holocene transition. Meyer This paper documents and attempts to explain a et al. (1992) attributed periods of late Holocene period of channel instability and aggradation in the alluvial fan growth and floodplain aggradation in Cheakamus River valley in southwestern British Yellowstone National Park to climatically driven Columbia. We date the perturbation and ascribe it to forest fire cycles. a large landslide at the head of a tributary of Chea- Other, more local factors can alter the equili- kamus River, 15 km north of our study area, in the brium of rivers, triggering base level and morpho- middle 1800s. We suggest that the river was still logical changes. Notably, large landslides (Hewitt, adjusting to this perturbation as late as the 1950s 1998), human disturbance (Knighton, 1989),and when it was dyked.

Fig. 2. Shaded-relief, digital elevation model showing localities mentioned in the paper. 320 J.J. Clague et al. / Geomorphology 53 (2003) 317–332

Fig. 3. Map of the study area showing the known extent of the buried forest, giant living red cedars, and large cut cedar and hemlock stumps. J.J. Clague et al. / Geomorphology 53 (2003) 317–332 321

2. Setting Garibaldi massif (Fig. 2). Mount Garibaldi is a Pleistocene volcano that last erupted 11,000– Cheakamus River drains 1070 km2 of the Coast 12,000 years ago during deglaciation of the region Mountains of southwestern British Columbia (Fig. 1). (Mathews, 1952a). The western slopes of the vol- It is a major tributary of Squamish River, which it cano are steep and prone to landsliding. Near the enters 9 km north of Squamish at the head of Howe head of Rubble Creek is a near-vertical cliff in Sound. Cheakamus River and its tributaries head in highly jointed . The cliff, which is named The alpine basins, many of which presently contain gla- Barrier, formed about 12,000 years ago when a ciers. flow erupted from a cone on the flank of Mount Of particular importance for our study are three Garibaldi and terminated against glacier ice filling tributaries of Cheakamus River, Rubble Creek, Cull- Cheakamus and Rubble Creek valleys (Mathews, iton Creek, and Cheekye River, which drain the 1952b). Large landslides from have west flank of the snow- and ice-covered Mount swept down Rubble Creek to Cheakamus River

Fig. 4. Lowermost Cheakamus River valley in 1947 (BC400-74) and 1996 (BCB96099-59); compare with Fig. 5. Note wide, braided and anabranching channel pattern in 1947. The active channel is narrower and more stable in 1996. 322 J.J. Clague et al. / Geomorphology 53 (2003) 317–332 many times in the Holocene, most recently in 1855 or 1856 (Moore and Mathews, 1978). The upper reach of Culliton Creek contains a similar cliff developed in fractured (Mathews, 1952b). This cliff, like The Barrier, is vulnerable to land- slides, although neither it nor the valley has been studied. Cheekye River heads in steep slopes devel- oped in late Pleistocene pyroclastic rocks. These slopes formed when the west half of Mount Gar- ibaldi volcano collapsed into Cheakamus Valley during deglaciation. A large fan (Cheekye fan, Fig. 2) has formed over Holocene time at the mouth of Cheekye River from hundreds of debris flows derived from the head of the basin. The most recent Cheekye to reach Cheakamus River occurred in 1958 (Jones, 1959). Our study area is located on the grounds of the North Vancouver Outdoor School (NVOS) about 4 km northeast of the confluence of Cheakamus and Squamish River, and 3 km north of Cheekye fan (Figs. 2 and 3). This part of Cheakamus valley is up to 850 m wide and is floored by a floodplain underlain by thick Quaternary sediments (Jordan- Knox et al., 2001). Bedrock slopes and the Cheekye fan constrict the floodplain to a width of about 100 m at the south, or downstream, end of the study area. The valley also narrows north of the mouth of Culliton Creek, and from there north to near Rubble Creek, Cheakamus River flows in a bedrock canyon (Cheakamus Canyon). A dam, built in 1955 to generate electricity, blocks Cheakamus River just north of Rubble Creek (Fig. 2).

3. Methods

Historic changes in the planform of Cheakamus River at NVOS were documented by comparing British Columbia Government aerial photographs dating from 1947 to 1996. Additional data on river channel positions were obtained from a 1:5000-scale topographic map of the valley floor, prepared by the British Columbia Ministry of Environment, and a less detailed planimetric map, prepared by Carl Halvorson, an employee of NVOS. Seven trenches were dug at NVOS (Fig. 3) to examine the sediments underlying the Cheakamus Fig. 5. Map showing the active floodplain of Cheakamus River in River floodplain. A 20 Â 20 cm grid was placed on 1947 and 1996; compare with Fig. 4. J.J. Clague et al. / Geomorphology 53 (2003) 317–332 323 one or two walls of each trench, and the sediments verified by visual comparison of prominent narrow were mapped, described in detail, and sampled for rings. particle-size analysis and radiocarbon dating. The chronology presented in this paper is based on radiocarbon ages, tree ring counts, and ring-width 4. Evidence of past channel instability measurements. Nine samples of charcoal and wood were radiocarbon dated at the Geological Survey of The lower, alluvial reach of Cheakamus River is Canada Radiocarbon Laboratory (Ottawa) and Iso- dyked to prevent the valley floor from being inun- Trace Laboratory (Toronto). The samples were recov- dated during . Prior to dyking, the active ered from sediments exposed in the trenches and from floodplain was broad and the river had a mainly the stems of trees that are partially buried in these braided channel planform. We present evidence here sediments. Ring widths of living and dead trees were for historical channel instability and for a period of made to the nearest 0.005 mm using a Velmex-type floodplain aggradation immediately before the his- stage with stereo-microscope. We attempted to cross- torical period. date samples into a master chronology based on their relationships to each other and to a dated old-growth 4.1. Historical channel changes red cedar at NVOS. Initial cross dating was done using the International Tree Ring Data Bank (ITRDB) Cheakamus River had a dominantly braided plan- program COFECHA (Holmes, 1993), with 50-year form at NVOS in 1947, with local anabranching segments lagged by 25 years. Significant correlations channels separated by vegetated islands (Figs. 4 and were flagged by COFECHA, and the cross dating was 5; terminology after Nanson and Knighton, 1996). At

Fig. 6. Stem of a western red cedar rooted approximately 1 m below the present ground surface. Compare the straight stem of the buried tree with the flaring buttress of the living cedar at the left. 324 J.J. Clague et al. / Geomorphology 53 (2003) 317–332 that time, the active floodplain, delineated on air- and 100 m wide (Fig. 3). The stems are burned and photos by an absence of vegetation, was up to 300 m severely decayed above the present ground surface, wide and consisted of several intermittently active but their buried root boles are fresh and unweathered. channels. Between 1947 and the late 1950s, when A living forest comprising western red cedar, west- the river was dyked, the channel shifted up to 300 m ern hemlock (Tsuga heterophylla), red alder (Alnus in both east–west and north–south directions. The rubra), and vine maple (Acer circinatum) occurs in active floodplain also appears to have narrowed over the area of the buried trees (Fig. 7a). Incremental cores this period, in most places to about 100 m. After the were extracted at breast height from 15 of the largest river was dyked, the channel ceased to wander and it living conifers to estimate the age of the forest. The became fixed in its present location (Fig. 5). oldest of these trees is a western red cedar 135 years old. The ecesis interval, or time required for coloni- 4.2. Prehistoric channel instability and aggradation zation of disturbed surfaces, for red cedar in this environment is poorly defined, but probably does Remnants of a prehistoric buried forest are pre- notexceed10years(Huff, 1995; Daniels et al., served along the west side of Cheakamus Valley near 1997; K. Lertzman, personal communication, 2002). NVOS. The old forest is manifested by a large number These data indicate that the modern floodplain surface of in situ, vertical stems of buried western red cedar in the area of the buried forest dates back to at least trees (Thuja plicata) (Fig. 6). The trees are up to 1.6 m 5–10 years before AD 1868, when the 135-year-old in diameter and occur over an area at least 1.5 km long red cedar began to grow.

Fig. 7. (A) Second-growth forest growing on the floodplain of Cheakamus River near trench 5. (B) Old-growth cedar at NVOS. J.J. Clague et al. / Geomorphology 53 (2003) 317–332 325

Fig. 8. Histograms of maximum circumferences of old-growth living trees, old-growth cut trees, second-growth living trees, and buried trees at NVOS. With a few exceptions the living and cut old-growth trees are south of the southernmost buried tree. Measured trees are western red cedar (T. plicata) and western hemlock (T. heterophylla).

Three very old living red cedars are present just growth red cedars are 600–1000 years old, based on a south of the area of the buried forest (Figs. 3 and 7b). comparison of their circumferences with those of the The centres of the three trees have rotted away; thus it blown-down red cedar and the much smaller living is not possible to determine the ages of the trees. red cedars in the area of the buried forest to the north However, they are well over 410 years old, which is (Fig. 8). Cut stumps of large, old-growth hemlocks the approximate age of a smaller ring-dated red cedar and red cedars are also present south of the buried that was blown down in a storm in 1991. The old- forest in the same area of the three surviving old-

Fig. 9. Stratigraphy of sediments exposed in trench 5. 326 J.J. Clague et al. / Geomorphology 53 (2003) 317–332 growth trees (Fig. 3). These trees bear distinctive This supposition was proved correct by radiocarbon springboard notches used by the first loggers in dating (see below). southwestern British Columbia to anchor boards on which they stood to cut down large trees. Although 4.2.1. Stratigraphy the exact date when these trees were cut is unknown, Of the seven trenches dug on the Cheakamus River it was probably about the time of first settlement of floodplain at NVOS, four are located in the area of the the area in the early twentieth century. Some of the buried forest (Fig. 3). At each of the four sites, a dark stumps are about the same size as the base of the red brown soil in which the tree stems are rooted was cedar that was blown down in 1991, suggesting that encountered at depths of 0.3–1.3 m (Figs. 9–11). The they were as much as 400–500 years old when cut soil is absent at the three trenches dug in the area (Fig. 8). populated by old-growth survivor trees and spring- The absence of buried trees within the area of old- board-notched stumps. growth trees (both survivors and springboard-notched The buried soil overlies, in succession: (1) 0.1–1 stumps) and, conversely, the absence of old-growth m of oxidized and mottled, yellow brown to olive trees within the area of the buried forest (Fig. 3) gray, very fine to medium sand; (2) a second thin, suggest that the two are contemporaneous. That is, brown, rooty soil containing charcoal fragments; (3) the buried forest is the same age as the old-growth up to 0.7 m of oxidized and mottled, yellow brown to forest; sediments buried the latter but not the former. olive gray, very fine to medium sand; and (4) pebble-

Fig. 10. Stratigraphy of sediments exposed in trench 7. J.J. Clague et al. / Geomorphology 53 (2003) 317–332 327

Fig. 11. Summary stratigraphy of trenches 5, 6, 7, and 8. cobble gravel. Based on water-well data from NVOS finest sediment of the sequence. The mud, in turn is and the Tenderfoot salmon hatchery (Jordan-Knox et conformably overlain by up to 0.8 m of olive gray, al., 2001), the gravel at the base of the trench horizontally stratified, sandy silt and silty very fine sequence is several tens of metres thick. sand with rare laminae of mud up to a few millimetres Sediments exposed in the three trenches dug in thick. Some of the mud laminae occur at about the the area south of the buried forest are oxidized, same stratigraphic level at all four trenches and thus very fine to fine sand similar to, and probably may correlate. The sediments are locally folded, correlative with, the two oxidized sand units (1 probably due to dewatering or syndepositional mass and 3 above) at the trenches to the north. The movement. They lack current structures, such as surface soil developed on these sediments is mot- ripple marks. Rare, circular, sand-filled burrows or tled and grades downward from dark brown to root casts are present in the uppermost 0.05–0.10 m brown at depths of 0.05–0.15 m. This surface soil of the unit, suggesting that there was at least a minor is much better developed than the surface soil hiatus between deposition of this unit and the sedi- within the area of the buried forest, consistent with ments overlying it. The next unit in the sequence is the inference that it formed, in part, during the 0.2–0.55 m of olive gray, rippled, very fine to fine interval spanned by the buried soil and the sedi- sand. In places, it truncates the uppermost strata of the ments that overlie it at the northern trenches. underlying unit. The uppermost unit in the sequence is The buried soil is overlain by the same sequence of 0.2–0.6 m of fine to coarse sand containing granules sediments at each of the four northern trenches (Figs. and small pebbles, dominantly of Garibaldi volcanic 9–11). Lying directly on the soil is up to 0.1 m of provenance. This sediment is the coarsest in the light gray to olive gray, massive mud, which is the sequence. It has a characteristic purplish hue, stem- 328 J.J. Clague et al. / Geomorphology 53 (2003) 317–332 ming from its Garibaldi volcanic provenance and its ended with deposition of rippled sand by flowing coarseness. The surface soil developed on these sedi- water. Stronger flows then deposited the coarser sand ments is typically immature, comprising 0.05–0.15 m that underlies the present forest floor. of dark brown humus and organic litter. Similar, but older, aggradational events are recorded The two upper units in the sequence dip down, and by the oxidized sand units underlying the buried forest thicken, into elliptical depressions that surround the bed. At least two periods of aggradation were separated buried trees. The flows responsible for deposition of by an interval during which the floodplain was colon- these sediments apparently scoured depressions around ized by trees and a soil formed. the bases of the trees. 4.2.3. Chronology 4.2.2. Interpretation of stratigraphy Three radiocarbon ages on the outermost rings of The sediment sequence overlying the buried forest roots of buried stumps at three sites range from 30 F 60 horizon records rapid aggradation of the Cheakamus to 110 F 60 14C year BP (Table 1). The outer preserved River floodplain by inundation and then successively rings on the stem of a buried tree yielded an age of more energetic flows. The first sediment deposited on 100 F 50 14C year BP. Calibration of these ages the forested floodplain is fine mud, deposited in still (Stuiver et al., 1998) indicates that the trees died no water. This phase of aggradation was soon followed by earlier than AD 1682 (Table 1). the rapid accumulation of very fine to fine sand, also in The time of tree death was further constrained by still water. Brief periods of mud deposition occurred ‘‘wiggle matching’’ two radiocarbon ages obtained on during this phase, apparently over a large area of the different sets of rings from the same buried tree: the floodplain at NVOS. A brief period of quiescence first 2 rings from the outside of the tree (100 F 50 14C occurred between the deposition of this unit and year BP); and 123 and 124 rings from the outside younger sediments. At that time, plants and animals (120 F 50 14C year BP) (Table 1). The probability colonized the plain. The period of quiescence distribution of the older calibrated age was shifted 123

Table 1 Radiocarbon ages, North Vancouver Outdoor School Radiocarbon Calibrated Laboratory Trench Location Dated material Sample age (year BP)a age (AD)b numberc number depth (m)d Latitude (N) Longitude (W) 30 F 60 1682–1734, GSC-6397 6 49j49.0V 123j09.3V Thuja plicata root, 1.25 1806–1931 rings 3–10 100 F 50 1668–1882, GSC-6395 3 49j49.0V 123j09.3V Thuja plicata root, 1.15 1795–1950 rings 3–8 100 F 50 1668–1882, TO-8870 – 49j49.0V 123j09.3V Thuja plicata stem, – 1795–1950 rings 1–2 110 F 60 1659–1955 GSC-6393 5 49j49.3V 123j09.3V Thuja plicata root, 1.25 rings 2–6 120 F 50 1682–1734, TO-8871 – 49j49.0V 123j09.3V Thuja plicata stem, – 1806–1950 rings 124–125 410 F 90 1269–1952 TO-8282 6 49j48.8V 123j09.3V Charcoal 0.9 490 F 70 1268–1665, TO-8293 6 49j48.8V 123j09.3V Charcoal 1.4 1784–1790 630 F 60 1165–1167, TO-8291 5 49j49.3V 123j09.3V Charcoal 1.3 1187–1477 650 F 60 1266–1413 GSC-6562 8 49j49.0V 123j09.3V Bark 1.7–2.2 a Laboratory-reported uncertainties are 1r for TO ages and 2r for GSC ages. Ages are normalized to d13C=À 25.0xPDB. b Determined from atmospheric decadal dataset of Stuiver et al. (1998) using the program CALIB 4.0. The range represents the 95% confidence limit calculated with an error multiplier of two. c GSC—Geological Survey of Canada, TO—IsoTrace Laboratory (University of Toronto). d Depth below ground surface. J.J. Clague et al. / Geomorphology 53 (2003) 317–332 329 years, corresponding to the known difference in age of words not more than 734 years old, which is within the two dated samples, and then compared to the the range estimated from the circumferences of the probability distribution of the younger calibrated age. trees (600–1000 years). The overlap between the two distributions limits the We attempted to date buried trees at NVOS by time of tree death to AD 1800–1904 or 1914– comparing their ring widths with those of (1) an old- 1960. The age of living trees at the site further growth red cedar that died in 1991, when it was restricts the time of tree death to AD 1800–1868. blown over during a storm, and (2) trees that were Four additional radiocarbon ages were obtained killed during the 1855–1856 Rubble Creek landslide on samples collected from sediments below the (Fig. 12). Correlations between the NVOS-Rubble buried forest horizon. Charcoal recovered from sand Creek and master ring width chronologies in each between the two buried soils at trench 5 yielded an segment were significant ( p < 0.01) for correlation age of 630 F 60 14C year BP, corresponding to a coefficients larger than 0.328. calibrated age range of AD 1187–1477 (Table 1). Trees at NVOS and Rubble Creek have complacent, Two samples of charcoal collected from oxidized or low variability, ring-width series because they grow sand at trench 6 gave ages of 410 F 90 and in a setting where climate rarely limits their growth. 490 F 70 14C year BP; the calibrated age ranges Cross-dating of such trees is thus difficult. However, are AD 1269–1952 and AD 1268–1665, respec- we were able to successfully cross-date one of the tively. A sample of detrital bark collected from Rubble Creek trees and one of the buried NVOS trees clayey silt at trench 8, close to a cut old-growth with the dated old-growth red cedar. The outermost red cedar, yielded an age of 650 F 60 14C year BP; rings of the cross-dated Rubble Creek and NVOS trees its calibrated age range is AD 1266–1413. The three date to 1836 and 1841, respectively (Table 2). Neither old survivor trees grow on the surface underlain by the buried trees at NVOS nor those at Rubble Creek these sediments and thus must be younger than the have preserved bark; thus their outermost rings are dated charcoal and bark at the tree sites. The trees older than the time of burial. The tree-ring data, in thus must be younger than AD 1268, or in other combination with dates on second-growth living trees

Fig. 12. Remnants of forest buried by debris flow deposits of the Barrier landslide, which occurred in 1855 or 1856. The trees were exhumed when Cheakamus River incised the debris flow deposits. 330 J.J. Clague et al. / Geomorphology 53 (2003) 317–332

Table 2 River than they do today, inducing channel instability COFECHA cross-dating statistics and aggradation along the non-canyonized reaches of Laboratory Cross-dated Mean Outer Sample the river. Other researchers have noted that rivers a number interval segment ring year description draining glacierized basins in the correlationb had different channel planforms and higher base 01YC03 1649–1991 0.51 1991 Old-growth levels than today (Desloges, 1987; Desloges and NVOS Thuja plicata Church, 1987; Gottesfeld and Johnson-Gottesfeld, 01YC02b 1746–1839 0.54 1841 Buried NVOS 1990). Morice River in the central Coast Mountains, Thuja plicata for example, is flanked by low terraces that define a 01YR02 1720–1834 0.39 1836 Buried Rubble broad floodplain dating to the Little Ice Age (Gottes- Creek stump feld and Johnson-Gottesfeld, 1990). Gottesfeld and a University of Victoria Tree-Ring Laboratory sample number. b Johnson-Gottesfeld (1990) showed that the Morice Calculated for 50-year segments common to all three dated River during the Little Ice Age was braided and samples. Correlations above 0.328 are significant at p < 0.01. occupied a much greater width of the valley bottom than it does today. A reduction in sediment supply mentioned above, limit the time of tree death at NVOS during the twentieth century coincided with floodplain to the period 1841–1868. incision, a change in planform from braided to anab- ranching, and narrowing of the active floodplain. Similar changes are inferred for Cheakamus River at 5. Discussion NVOS. Although this second explanation cannot be ruled out, we favour the first because of the apparent Data presented in this paper indicate that Cheaka- coincidence of the landslide (1855–1856) and aggra- mus River aggraded its floodplain by 1–2 m at North dation and tree burial at NVOS (1841–1868). Vancouver Outdoor School in the middle of the The effects of the disturbance persisted well into the nineteenth century. The most likely cause of the twentieth century. Airphoto examination indicates that aggradation is the large (15–25 Â 106 m3) landslide Cheakamus River was unstable and still carrying a from The Barrier in 1855 or 1856. The landslide large sediment load in 1947. Only dyking that began in swept down Rubble Creek and came to rest in the late 1950s fixed the river in its present course. Our Cheakamus Valley 15 km north of NVOS, where it observations thus suggest that the effects of a pertur- dammed Cheakamus River with many millions of bation in sediment supply to a river may persist for cubic metres of debris. Since 1856, Cheakamus River decades, perhaps even centuries after the triggering has incised the landslide debris and reestablished its event. pre-slide grade (Fig. 12). Much of the incision prob- Other rivers with highly variable sediment loads ably occurred in the years immediately following the have close coupling between sediment supply and landslide. The eroded sediment was quickly trans- base level response (Harvey, 2002). Floodplain aggra- ported south through Cheakamus Canyon to the dation in response to sudden increases in sediment lower, broader reach of Cheakamus Valley where it supply is well documented, particularly where the was redeposited. After the river reached its pre-slide disturbance is caused by humans, for example through level, its debris load decreased and it incised the forestry, grazing, and mining (Knighton, 1989; Madej sediment just deposited at NVOS. and Ozaki, 1996; Marcus et al., 2001; Kondolf et al., We considered a second possible explanation for 2002). The time it takes for fluvial systems to respond the aggradation at NVOS—an increase in sediment to rapid inputs of sediment differs because of differ- delivery to streams during the last phase of the Little ences in basin size, degree of hillslope-channel cou- Ice Age. During the nineteenth century, glaciers on the pling, and the magnitude of the disturbance event Mount Garibaldi massif and elsewhere in the head- (Harvey, 2002). The time required for fluvial systems waters of Cheakamus River were much more exten- to stabilize and remove excess sediment following sive than at present (Mathews, 1951). The glaciers disturbance ranges from decades (Madej and Ozaki, may have delivered more sediment to Cheakamus 1996) to centuries (Knighton, 1989). J.J. Clague et al. / Geomorphology 53 (2003) 317–332 331

The response of Cheakamus River to changes in of decades to centuries (Gedalof and Smith, 2001) may sediment delivery is similar, albeit on a much smaller change base level enough to play havoc with these scale, to that of most rivers in British Columbia to engineered works. terminal Pleistocene deglaciation. As the last Cordil- leran ice sheet melted and ice-dammed drained, the present-day drainage began to evolve. Large 6. Conclusions amounts of unvegetated glacial sediment were trans- ported to valley floors by streams and landslides, A mature coniferous forest growing on the flood- overloading the carrying capacity of rivers (Church plain of Cheakamus River at North Vancouver Out- and Ryder, 1972; Clague, 1986). Rivers responded to door School was buried by up to 2 m of fluvial silt and these large sediment inputs by aggrading their chan- sand in the mid 1800s. A large landslide from The nels and floodplains. Over a period of perhaps several Barrier near Mount Garibaldi probably triggered hundred years, local base level in many river valleys aggradation and channel instability at NVOS, 15 km rose many tens of metres to more than 100 m due to to the south. Channel instability persisted until Chea- colluvial and alluvial deposition. Soon, however, this kamus River was dyked in the late 1950s. pattern was reversed as drift supplies were reduced This study and those of Desloges and Church and became increasingly stabilized by vegetation. (1987) and Gottesfeld and Johnson-Gottesfeld Rivers began to incise their ‘‘paraglacial’’ (Church (1990) show that rivers in the and Ryder, 1972) fills, gradually lowering base level Mountains respond to modest changes in sediment and leaving terraces delineating former higher flood- delivery by aggradating or incising their channels and plain levels (Ryder and Church, 1986). These base- altering their planforms. During times of high sedi- level changes dwarf those that we have documented ment delivery, rivers may aggrade the alluvial reaches in Cheakamus Valley, but both examples highlight the of their channels and adopt a braided habit. Rivers sensitivity of streams to changes in sediment supply in may incise their channels and wander less across a source areas. In some cases, as at NVOS, aggradation narrower active floodplain when they receive and or degradation can occur far from the sources of the transport less sediment. Fisheries managers and others sediment. responsible for making decisions about land use on Finally, our study has implications for fisheries floodplains should consider the possibility of rapid management in British Columbia. The federal Depart- changes in base level and channel position induced by ment of Fisheries and Oceans has invested tens of climate change or large landslides. millions of dollars enhancing and protecting spawning habitat in British Columbia. At NVOS, for example, a network of artificial salmonid spawning channels has Acknowledgements been constructed and more are planned. The channels have greatly increased the spawning success of chum We thank Victor Elderton, Jim Wisnia, and Carl and coho salmon in Cheakamus River. Most fisheries Halvorson for allowing us to work on the property of managers, however, do not realize that rivers can North Vancouver Outdoor School. Carl Halvorson aggrade or degrade their channels, and can rapidly alter generously provided local maps and information, and, their form or position as sediment supply increases or along with Drew Leatham and Greg Dehn, assisted in decreases. Yet, many in-channel and side-channel sal- the field. Dan Smith allowed Reyes to use the monid enhancement works are vulnerable to damage or University of Victoria Tree-Ring Laboratory to ana- destruction with changes in sediment supply. Large lyze samples. Richard Franklin and Velibor Veljkovic landslides have occurred on volcanoes near our study drafted the figures. The Geological Survey of Canada area in historic and prehistoric times (Evans and and Natural Engineering and Research Council of Brooks, 1991; Bovis and Jakob, 2000). Similar events Canada provided financial support for the study. Phil in the future may raise base level and damage or destroy Ashworth, Lionel Jackson Jr., and an unidentified spawning channels along the rivers draining these journal referee provided critical reviews that greatly volcanoes. Even small changes in climate over periods improved the paper. 332 J.J. Clague et al. / Geomorphology 53 (2003) 317–332

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