Journal of Paleolimnology 17: 331–346, 1997. 331c 1997 Kluwer Academic Publishers. Printed in Belgium.

The glacilacustrine sedimentary environment of Bowser Lake inthe northern Coast Mountains of British Columbia, Canada �

Robert Gilbert1, Joseph R. Desloges2 & John J. Clague3

1Department of Geography, Queen’s University, Kingston ON K7L 3N6 Canada (email:GILBERTR@QUCDN.QUEENSU.CA);2Department of Geography, University of Toronto, Toronto ON M5S 1A1 Canada (email:DESLOGES@GEOG.UTORONTO.CA);3Geological Survey of Canada, 100 West Pender Street, Vancouver BC V6B 1R8 Canada (email:JCLAGUE@GSC.NRCA.GC.CA)

Received 29 November 1995; accepted: 7 March 1995

Key words: glacilacustrine sediments, acoustic records, varves, jokulhlaups

Abstract

Bowser Lake, a fiord lake in the northern Coast Mountains of British Columbia, contains a thick Holocene fillconsisting mainly of silt and clay varves. These sediments were carried into the lake by proglacial Bowser Riverwhich drains a high-energy, heavily glacierized basin. Sedimentation in the lake is controlled by seasonal snow andice melt, by autumn rainstorms, and by rare, but very large jokulhlaups from glacier-dammed lakes in the upperBowser River basin which complicate environmental inferences from the sedimentary record. Sediment is dispersedthrough the deep western part of the lake by energetic turbidity currents. The turbidity currents apparently do notovertop a sill that separates the western basin from much shallower areas to the east. Large amounts of silt andclay are deposited from suspension in the eastern part of the lake, but sediment accumulation rates there are muchlower than to the west. Several strong acoustic reflectors punctuate the varved fill in the western basin; these maybe thick or relatively coarse beds deposited during jokulhlaups or exceptionally large storms. The contemporarysediment yield to Bowser Lake, estimated from sediments in the lake, is about 360 t km�2 a�1. This is a relativelyhigh value, but it is less than yields in some other, similar montane basins with extensive snow and ice cover. Themost likely explanation for the difference is that large amounts of sediment have been, and continue to be, storedon the Bowser delta and in small proglacial lakes.

Introduction

Studies of lakes in the Cordillera of western Cana-da provide understanding of distal glacilacustrine sed-imentation in mountainous terrain (Gilbert, 1975;Leonard, 1986a, b; Eyles et al., 1990, 1991; Mullinset al., 1990; Desloges & Gilbert, 1991, 1994a,1995; Gilbert & Desloges, 1992; Chikita et al., in

� This is the fourth of a series of four papers published in thisjournal, which is a contribution to IGCP 374, entitled: ‘Palaeoclima-tology and Palaeoceanography from Late Quaternary and HoloceneLaminated Sediments: A Global Joint Approach Using Marine andLacustrine Sediments’. This series was guest edited by Drs R. Gilbertand D. Lemmen.

press). Deposition in these lakes is strongly season-al and varves are common. Because the sedimentarysequences offer high temporal resolution and common-ly span thousands of years, in some cases the entireHolocene, they are excellent archives of past climatic,hydrologic, glacial, and geomorphic changes. Varia-tions in the texture and thickness of varves, for exam-ple, reflect changes in discharge and sediment deliverycaused by Holocene glacier fluctuations (e.g. Leonard,1986a, b).

In this paper, we describe the sedimentary environ-ment of Bowser Lake in northwestern British Columbia(Figure 1) based on a low-frequency acoustic survey,coring of sediments, and conductivity, temperature,

Article: jopl 378-IGCP Pips nr 115502 BIO2KAP

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Figure 1. Bowser River drainage basin at Bowser Lake. Abbreviations on index map: A - Ape Lake; G - Garibaldi Lake; L - Lillooet Lake; M -Moose Lake; S - Stave Lake.

and transmissivity measurements in the water column.This is the first such investigation of a fiord lake innorthern British Columbia and continues our stud-ies of glacilacustrine environments in the CanadianCordillera. The drainage basin of Bowser Lake hasexperienced significant environmental change duringthe Holocene, especially during the Neoglacial inter-val (Clague & Mathews, 1992; Clague & Mathewes,1996), as large dynamic glaciers waxed and wanedin response to changes in climate (Eyles & Roger-son, 1977). The upper section of Bowser River wasdammed several times during the Holocene by FrankMackie Glacier, forming Tide Lake (Figure 1) witha maximum volume of 1 km3 (Clague & Mathews,1992). Catastrophic drainings of Tide Lake and otherglacier-dammed lakes in the basin have had a signifi-cant impact on both Bowser River and Bowser Lake.One of the objectives of our study is to identify anom-alous sediment inputs related to these floods.

Setting

Bowser Lake (368 m a.s.l.) is located at the eastern mar-gin of the Coast Mountains (Holland, 1964) in north-western British Columbia. It averages 1.5 km wide andextends 23 km from the delta of Bowser River on thewest to the outlet near Hickman Bay (Figure 2). Westof Cache Point, the lake transects the Longview Rangeand is bordered by steep bedrock slopes mantled bypatchy colluvium. At the maximum of the Little IceAge, two glaciers in the Longview Range terminatedat 1220 m a.s.l. at the heads of steep streams that emptydirectly into Bowser Lake. East of Cache Point, Bows-er Lake parallels the regional structural grain. In thisarea streams flowing from the Longview Range havebuilt large fan-deltas into the lake.

Bowser River drains 1196 km2 upstream of Bows-er Lake, while smaller streams flowing directly intothe lake drain an additional 190 km2. Elevations in therugged mountains of the basin reach 2700 m a.s.l.,and 46% of the basin is glacier-covered. Prior to1961, glacier-dammed Summit Lake (Figure 1) over-flowed into the Bowser River drainage, augmenting

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Figure 2. Bathymetry of Bowser Lake, core sites, and the acoustic survey lines shown in Figure 5. Isobaths and spot depths are in metres.

the basin area by about 63 km2. After the first recordedjokulhlaup from Summit Lake in 1961, overflow intoBowser River occurred intermittently until 1967, butsince then, the lake has not completely refilled and alloutflow has passed south through Salmon Glacier intoSalmon River (Mathews & Clague, 1993).

The alpine climate of the Bowser River basin isdominated by heavy orographic snowfall. Average pre-cipitation during winter (September–April) at Stewart,located at sea level 60 km south of Bowser Lake, is1.53 m water equivalent. Values at Bowser Lake areprobably similar based on snow course data from otherlow-elevation sites in the area (Figure 3). It is likelythat precipitation at higher elevations is significantlygreater and falls mainly as snow.

Late Quaternary environment

The outlet of Bowser Lake is controlled by the large ter-raced alluvial fan of Surveyors Creek (Figure 4). Thisfan is paraglacial in origin (sensu Church & Ryder,1972), the result of aggradation during or shortly afterdeglaciation, possibly while a tongue of late Pleis-tocene ice occupied at least part of what is now Bows-er Lake. It is inset into a higher (about 420 m a.s.l.)gravel, kame terrace (Figure 4). High alluvial terracesin adjacent Bell-Irving Valley which are probably cor-

Figure 3. Composite snow course data for the period 1978–1985 fortwo sites near Bowser Lake (see Figure 1 for locations). (Source:Environment Canada, Atmospheric Environment Service, SnowCourse Data 1978–1986).

relative with the Surveyors Creek fan record coevalaggradation by Bell-Irving River.

The level of Bowser Lake rose as the SurveyorsCreek fan grew. A pair of raised beaches or spits on thefan just beyond the east end of Bowser Lake indicate ahigher level of Bowser Lake than at present when thefan was fully developed (Figure 4). Surveyors Creekfan and the paraglacial fill in Bell-Irving valley wereincised during the Holocene, leaving a series of alluvialterraces adjacent to Surveyors Creek, Bowser River,and Bell-Irving River (Figure 4). This incision loweredthe level of Bowser Lake.

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Figure 4. Map of the area east of Bowser Lake showing impoundment by the Surveyors Creek fan. Contour interval is 200 m.

The modern floodplain of Surveyors Creek is braid-ed, and large amounts of sediment are carried intoBowser River by this stream. Bowser River upstreamof the mouth of Surveyors Creek is slack and essential-ly an extension of Bowser Lake. In contrast, the riverjust downstream of Surveyors Creek has a much steep-er gradient and a braided to anastomosing planform.

Bowser River has advanced its floodplain and deltaeastward into Bowser Lake during the Holocene. Thesupply of sediment to the lake probably has changedduring this time due to the episodic advance andretreat of glaciers in the watershed (Clague & Math-ews, 1992). Similar changes in the sediment load ofSurveyors Creek may have induced coincident minorchanges in the level of Bowser Lake.

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Methods

In July 1994 we conducted a subbottom acoustic sur-vey from a 5 m long inflatable boat using DatasonicsSBP220 3.5 kHz profiling equipment and an EPC 4800graphic recorder (cf. Desloges & Gilbert, 1991, 1994a,1995; Gilbert & Desloges, 1992). Positions accurateto about � 50 m were determined with a single glob-al positioning system (GPS) receiver, and were tran-scribed to the record at two minute intervals exceptwhere the signal was blocked at the beginning or endof some runs by adjacent high mountains. A total of54 km of record was used to map the lake floor, espe-cially of the western portion of the lake (Figure 2).

Sediment was recovered from the lake floor withthree samplers: an Ekman grab for bulk samples, a30 or 40 mm diameter gravity corer, and a percussioncorer with 72 mm diameter core tube (Gilbert & Glew,1985; Reasoner, 1993) for relatively undisturbed sam-ples. Gravity cores ranged from 0.46 to 0.67 m in lengthand percussion cores from 0.7 to 2.7 m in length. In thelaboratory, the cores were split, sampled for water con-tent, dried, and photographed. Detailed stratigraphywas recorded using a binocular microscope coupledto a video monitor. Digital readout permitted precisemeasurement of the thickness of laminae and compar-ison of the two halves of each core. The thickness datapresented below are corrected to the wet length of thecores to account for shrinkage during drying. Mea-surements of organic content (following Dean, 1974)and grain size were made on samples from selectedlithostratigraphic units. Grain-size data were obtainedby wet sieving of the sand-size fraction and SediGraphanalysis of the mud.

Profiles through depth of conductivity (dissolvedsediment concentration), temperature, and transmis-sivity (hereafter called CTD profiles) were obtainedwith a Hydrolab Datasonde 3 instrument. Transmis-sivity was calibrated with 0.5 l water samples filteredat 0.47 �m to determine suspended sediment concen-tration.

Acoustic survey

Bathymetry

West of Graveyard Point, Bowser Lake is a simplebasin with steep, relatively straight side walls and anearly featureless flat floor that extends to a maximumdepth of 119 m (Figures 2, 5a). There is no evidence of

greater deposition on the south side of the lake in thisarea due to the Coriolis effect, as has been reported insome glacial lakes (Smith et al., 1982; Pickrill & Irwin,1983). This suggests that most of the deposition is fromturbidity currents that travel with sufficient velocity tominimize the Coriolis effect (Gilbert, 1983), as com-pared with slower moving overflows and interflows.

Except on the transect nearest the Bowser delta,there are no slump mounds on the lake floor or buriedin the sediments (Figure 5a) as reported, for example,from Stave Lake in the southern Coast Mountains byGilbert & Desloges (1992). This indicates that littlesediment is deposited on the steep side walls, or, alter-natively, that it is continually removed before thickdeposits can accumulate and fail.

The foreset slope of the Bowser delta is similar tothose of other proglacial lakes. From the surface to50 m depth, the slope is smooth and declines fromabout 24 � to less than 10 �. Below 50 m, the slopedecreases to less than 5 � and small mounds of slumpedsediment cover much of the lake floor. These moundspersist to about 105 m depth where the slope is lessthan 0.3 �, but they are very subdued features less than0.5–1 m high (cf. Gilbert, 1975).

The western basin ends abruptly at a sill about0.9 km southeast of Graveyard Point (Figures 2, 5b).The sill is located about where the lake becomes lessconfined by the steep high slopes of the LongviewRange. It is either a late Pleistocene moraine or a stepin the bedrock surface resulting from decreased erosionwhere the Pleistocene glacier flowing down Bowservalley entered the broader Bell-Irving valley.

Our survey is insufficient to characterize thebathymetry of the eastern part of the lake in detail,but it does show that the lake is much shallower andmore irregular here that to the west. Southeast of thesill, water depths increase to a maximum of 63 m in asmall, flat-floored basin (Figure 2). Farther southeast,depths are shallower, nowhere exceeding 30 m.

Subbottom record

Maximum depths of subbottom penetration exceed120 m (uncorrected for sound velocity) and are every-where greater than 60 m in the western basin, exceptwithin 2 km of the Bowser delta. These values aresignificantly greater than normally obtained with therelatively low energy, high frequency equipment usedin this survey and indicate that fine-grained, undercon-solidated sediment underlies most of the lake floor (but

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Figure 5. Acoustic records (a) across the western basin, (b) over the sill near Graveyard Point, and (c) in the eastern part of the lake (see Figure 2for locations). The depth scales are based on a sound velocity of 1460 m s�1.

see below), with coarser sediment (silty sand) restrict-ed to the delta slope.

Only one acoustic facies, consisting of well-layeredglacilacustrine mud, occurs in the subbottom records.There is no evidence of colluvial sediment of subaeri-al or subaqueous origin, nor of ice-proximal sedimentbeneath glacilacustrine mud where the latter is thin(Figure 5b, c). However, acoustic reflectors in the upperpart of the sequence are significantly stronger thanthose at depth (Figure 5a), an effect greater than can beaccounted for by attenuation of sound with depth. AtCache Point, strong reflectors occur at depths of 4.2 m,6.7 m, 13.3 m, 16.5 m, and 18.7 m, the deepest beingthe strongest. These reflectors mark changes in materi-al properties that may relate to a change in depositionalregime, sediment source, or both.

Continuity of strata in the western basin is remark-able (Figure 5a); many reflectors can be traced theentire length of the basin. Turbidity currents originat-ing at the Bowser delta are the only agents capable ofdistributing sediment widely and uniformly over thelake floor. No direct observations of turbidity currentswere made, but a well-defined debris line and plungepoint was observed at the delta during the survey inJuly 1995.

The sediment fill in the eastern part of the lake(Figure 5b, c) is similar to that in the western basinexcept that (1) reflectors are weaker in the upper part

of the sequence, (2) the sediments conform more tothe underlying irregular surface, suggesting that depo-sition is more from suspension than underflow, and(3) the total amount of sediment is much less.

Sediment thickness and volume

The 3.5 kHz profiler did not penetrate all of the thicksediment in the western basin, but we could estimatethe total thickness of the fill from the available records.The sidewalls of the lake were assumed to represent thebedrock surface, as there are no subbottom reflectorsbeneath them (Figure 5a). These were digitized oneach transect from the surface to the greatest depththey could be recognised. No correction was madefor errors due to sound return from a sloping surface(Sylwester, 1983) or for greater sound velocity in thesediment. A sixth-order polynomial curve was fittedto the subaqueous data and to points above the lakesurface transcribed from a 1:50 000 scale topographicmap in order to estimate the shape of the cross-sectionof the valley beneath the lake. A similar procedure wasused for the Bowser River sandur, although here thereare no data below the sandur surface. Depth to bedrockin the eastern basin was measured on the long profile(Figure 5b, c).

Figure 6 summarizes inferred maximum sedimentthickness data along the valley. The average maximum

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Figure 6. Long profile of the lower Bowser River sandur and Bowser Lake showing the surface, lake bottom, bedrock surface, and the estimatedthickness of lacustrine and sandur sediment (dashed line).

thickness of the sediment fill beneath the Bowser Riv-er sandur and the western basin is 234 m. East ofGraveyard Point, there is much less sediment (aver-age 40 m), reflecting the distal environment and thetrapping of turbidity currents in the western basin. Thetotal volume of sediment in the lower sandur and inBowser Lake is estimated to be 4.1 km3, 50% beneaththe sandur, 45% in the western basin, and 5% in theeastern area.

Although large amounts of sediment have reachedBowser Lake throughout the Holocene, sediment inputat the close of the last glaciation was not exceptional.In this respect, Bowser Lake is similar to other largelakes in the Coast Mountains (Gilbert, 1975; Desloges& Gilbert, 1991, 1994a, b; Gilbert & Desloges, 1992).

CTD profiles

Four casts were made, one on 18 July 1995 (C3, Fig-ure 2) and three on 20 July 1995 (C1, C2, C4). There isa consistent pattern of temperature variation through-out the lake. The epilimnion is very thin: the metal-imnion occurs at 5–7 m depth in the western part ofthe lake, 12 m in the mid-section, and about 20 min the eastern region (Figure 7a). Maximum tempera-tures are at the surface, with values ranging from 7 �Cin the western basin to 11 �C in the east. This reflectscalm conditions, warm air temperatures, and very lim-ited penetration of solar radiation due to the high sed-iment concentration. The epilimnion, although thin, isprobably a relatively stable feature of the lake duringsummer because the temperature gradient is large. Thehypolimnion is uniformly cold at 4.5–4.8 �C.

Dissolved sediment concentration shows a similarpattern to temperature, with the highest values, up to78 mg l�1, in the epilimnion (Figure 7b). However,there is a clear indication of a different water mass atthe floor of the lake at at site C1 where concentrationsare significantly lower in the lowest 20 m of the watercolumn and at C2 where somewhat lower values occurin the lowest 30 m. This may reflect passage of a turbid-ity current containing relatively low concentrations ofdissolved sediment. These low concentrations wouldbe expected in water derived directly from glaciers.In contrast, during periods of lower melt, waters thatspread through the lake had been in contact with therock and sediment of the drainage basin for a longerperiod and had picked up a greater load of dissolvedsediment.

Dissolved sediment concentrations are also lowerat the lake floor at the east end of the lake (Figure 7b).This may represent spilling of turbidity currents overthe sill at Graveyard Point (turbidity currents were firstrecognised in Lake Mead by the presence of turbidwater at the water surface at Hoover Dam, where thecurrents had welled up after striking the base of the dam– Gould, 1951). But it is unlikely that they are impor-tant agents of sediment transport in eastern BowserLake because they would have to rise from the lakefloor at 119 m to less than 40 m over the sill.

Suspended sediment data (Figure 7c) do not showthe metalimnion as clearly as temperature or dissolvedsediment concentrations. The greatest concentrationsof suspended sediment are near the lake floor, probablydue to the passage of turbidity currents, although not atthe moment of the survey.Records from the eastern partof the lake also show higher concentrations at depth,

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Figure 7. CTD results from Bowser Lake, July 1994. Transmissivity, T, in NTU units was converted to suspended sediment concentration, C,in mg l�1 using the relation C =�1.825 + 0.7557 T (r2 = 0.983). The locations of the profiles are shown in Figure 4.

again suggesting that at least parts of turbidity currentsspill into this region or, less likely, that local streamsare sources of turbid water. The values for BowserLake are high compared to those of other glacial lakes(Gilbert & Desloges, 1987), with concentrations threeto four times those of Garibaldi Lake and of ice-contactApe Lake before it drained, and up to double those ofLillooet Lake.

Bowser Lake sediments

Western (deep) basin

Sediments in the western basin (cores G1, G2, G3,G4, and P1; Figure 2) consist primarily of silt and clayrhythmites (Figure 8a). The texture, thickness, andrhythmic nature of the couplets, and their similarityin form to annual deposits elsewhere in the Cordillera(Desloges & Gilbert, 1994b), suggest that they arevarves (see 137Cs results below). Core G1, 3.4 kmfrom the Bowser delta, contains some laminae with upto 30% fine sand, three of which are up to 1 cm thick.However, none of the other analysed samples containsmore than about 1% sand (most have less than 0.2%).Clay (<4 �m) contents range from 50 to 93%.

Clay layers make up 25–50% of the total thicknessof the couplets. Such thick clay layers are explainedpartly by the distal environment of deposition (Smith &Ashley, 1985). Another factor is the fineness of the sed-

iment that reaches Bowser Lake; much of the coarse-grained fraction of the sediment load is trapped in aseries of small lakes along Bowser River downstreamfrom glacier sources. Gilbert (1975) attributed thickwinter layers in Lillooet Lake to autumn inflow events,when large amounts of fine sediment are introducedinto the lake but are not flushed out because flow sub-sequently decreases with the onset of winter. Similarautumn storms are common in the Bowser River basin(Figure 9), occasionally producing peak discharges aslarge as those of summer warm periods.

Laminae of coarser sediment are present in some ofthe clay layers. The clayey parts of the 1981 and 1982couplets in core G4, for example, contain coarse siltlaminae (Figure 8a) which may record high autumndischarges (Figure 9). However, coarser laminae areless common in Bowser Lake sediments than in manyother varved sequences where they have been attributedto increased inflow during mild, mid- to late-wintercyclonic storms (Gilbert, 1975; Shaw et al., 1978).In the Bowser River basin, once the Arctic Front isestablished well to the south during winter, weatherwarm enough to cause significant melting is rare untilnival melt begins in April or May. This conclusionis supported by runoff records from nearby Bear Riverwhich drains alpine terrain similar to that in the Bowserbasin. Between 1967 and 1993, winter floods exceededthe bankfull value of 120 m3 s�1 eight times, whichis about two-thirds the frequency of bankfull winterdischarges of rivers farther south (Desloges, 1987).

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Figure 8. a. Varves denoted by horizontal lines in core G4 from thewestern basin. Calendar dates of two varves are marked. Note gradedsilt above clay (open arrow) and thin coarser laminae (closed arrow).b. laminae in core G5 near the east end of the lake. c. laminae in coreP2 near the east end of the lake. Scale is in centimetres.

Summer portions of couplets are remarkably finegrained (nearly 100% mud) for an environment withsuch high sediment input. However, the bases of somesummer layers in both proximal and distal cores gradeupward from very fine sand to silt (visible as distinctpartings in dry cores). These, and other microlaminaeof fine sand, represent the passage of individual turbid-ity currents (cf. Gilbert, 1975) during summer, but theyare difficult to discern because of the uniformly highclay content. Thin laminae of darker finer sediments(Figure 8a) are probably associated with relatively briefintervals of lower inflow in the period between nivalmelt and glacier melt later in the summer (Figure 9).

Figure 10 shows varve thickness as a time seriesfor cores from the western basin, and Figure 11 showsmean thickness and standard deviation in relation tolocation within the lake. Mean varve thickness dou-

bles from 12.8� 5.4 mm at 3.4 km to 25.7� 12.2 mmat the distal end of the western basin, 11 km fromthe river mouth. The maximum thickness of varvesat the two sites is 32 and 48 mm, respectively. Themean values are comparable to those from LillooetLake (Gilbert, 1975), but the standard deviation ismuch higher, reflecting a greater temporal variabili-ty in accumulation rates. The greater thicknesses atthe distal end of the western basin are probably dueto the sill at Graveyard Point impeding turbidity cur-rents. As well, the large area of featureless lake floor atdepths greater than 110 m (Figure 2) allows turbid sed-iment plumes to pool and deposit their loads. Reflectedturbidity currents have been reported from the marineenvironment (Pantin & Leeder, 1987), and pooling hasbeen inferred from fiord and lake sediments (Gilbertet al., 1993; Chitka et al., in press). No direct evidenceof these processes has been found in British Columbialakes, although Gilbert & Desloges (1987: 1741, Fig-ure 12) report acoustic evidence that sediment in thedistal part of Ape Lake is more than twice as thick asthat closer to the source.

Distal (shallow) region

Although there are varves in the uppermost 0.86 m ofcores P2 and G5 near the east end of Bowser Lake,they are difficult to distinguish from more massivematerial (Figure 8b). Below 0.86 m, the layering ismore rhythmic and silt-clay varves are easily recog-nised (Figure 8c). It is from this section of the core thatthe average accumulation rate of 8.2 mm a�1 shownin Figure 11 was derived. Sediments in the distal shal-low parts of Ape Lake (Gilbert & Desloges, 1987) andMoose Lake (Desloges & Gilbert, 1995) changed fromvarved to diffusely laminated when turbidity currentsno longer affected these areas. While turbidity currentsare not necessary for the formation of varves (cf. Kue-nen, 1951), they do provide pulses of coarser sedimentthat make varves more recognisable.

137Cs activity was measured on selected samplesfrom core G5 from the east end of the lake to betterdocument sediment accumulation rates. Samples wereselected assuming that the 8.2 mm a�1 average accu-mulation rate noted above is close to the actual accu-mulation rate. The small amount of sediment availablefor analysis and the low activities required that severalcouplets be combined into a single sample.

There is a statistically significant peak in 137Csactivity at 29–33 cm depth in core G5 (Figure 12inset). This peak most likely corresponds to the 1957–

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Figure 9. Mean daily discharge of Bowser River at Berendon Glacier (Figure 1) for the period 1968–1972. The drainage basin area is 83 km2, 7%of the drainage area of Bowser River at Bowser Lake. (Source: Environment Canada, Water Survey of Canada, Surface Water Data 1967–1973).

Figure 10. Varve time series for cores from the western basin (see Figure 2 for locations). Distances from the mouth of Bowser River are shownat the top. Dashed lines indicate couplets that were used to match the cores.

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Figure 11. Mean annual accumulation rates in Bowser Lake derived from the data in Figure 10. Error bars are� 1�.

1962 maximum in atmospheric inputs of 137Cs to NorthAmerica (Robbins & Eddington, 1975) and indicatesthat accumulation rates are in the range 7 to 10 mma�1.

Figure 12 shows the thickness of laminae mea-sured in the distal cores. The uppermost 0.35 m ofthe sequence is from core G5 and the remainder isfrom core P2. Of note are two thick massive silty lay-ers at depths of 42–52 cm and 78–86 cm. The twolayers are separated by sequences of thin couplets. Ourinterpretation is that the massive layers are products ofrapid influxes of fine sediment. If one assumes a con-stant accumulation rate of 8.2 mm a�1, the two thickestlayers would be about 50 and 85 years old. Coupletsin the upper part of core P2, however, are relativelythin, suggesting that a lower accumulation rate shouldbe used in calculating the ages of the two silty layers.Clague & Mathews (1992) report that the last drainingof Tide Lake occurred in AD 1930, 65 years prior toour sampling. This produced a large flood in the Bows-er valley and may be responsible for the upper thickmassive layer in the distal region of the lake. Such aflood would leave a much thicker layer in the westernbasin but we were unable to resolve individual beds inthe uppermost few metres of the Bowser Lake fill withour acoustic equipment.

Below 0.86 m in core P2, laminae are better definedand more regular, and there is little evidence for theunusual events inferred from the upper part of the sedi-ment sequence. However, between 1.20 and 2.25 mseveral laminae of fine sand occur. Five of thesebetween 1.90 and 2.20 m are a distinctive dark brown toblack colour and are as thick is 5 mm. The mineralogyof the sand suggests derivation from black shale andsiltstone outcropping on nearby hillslopes. The lay-

ers thus record localized inputs of sediment, perhapsduring storms or landslides.

Discussion

Physical processes in the lake and varve formation

Sediment is delivered to Bowser Lake by a proglacialriver, with strong seasonal contributions of water andsediment from melting glaciers. During summer, tur-bid water enters the lake in one major and two ormore minor distributaries at the delta front. The impor-tance of turbidity currents in dispersing sediment thatis carried into the lake is indicated by (1) high concen-trations of dissolved and suspended sediment in thehypolimnion, (2) the presence of sand in the proximal,but deep, part of the western basin, (3) distal thicken-ing of the sedimentary fill in the western basin, and(4) acoustic reflectors that are continuous throughoutthe western basin. Irregular draping of acoustic reflec-tors in the eastern part of the lake indicates depositionfrom suspension in a lower energy environment.

Varves form in both settings, but their physicalproperties differ. In the deep western basin, they arethick and commonly weakly graded just above claycontacts. Multiple, thin (1 mm or less), graded lami-nae occur within summer layers in about 20% of thevarves. The clayey parts of couplets may also containcoarse microlaminae. Varves in the eastern basin aremuch thinner, less distinct, and are not graded. Claylayers are thicker relative to the silt layers than in thewestern basin.

We propose that turbidity currents develop in springand early summer, as in other alpine glacial lakes(e.g. Gilbert, 1975), when sediment-charged melt

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Figure 12. Couplet thickness in cores from the eastern part of BowserLake. The series is a composite of core G5 (uppermost 35 cm) andcore P2. Inset shows 137Cs activity in the upper part of the sedimentsequence (see text for discussion).

water enters the lake. The first large flow deposits thefinely graded layer on top of the winter clay. Duringlate July and August, high discharges in Bowser River(Figure 9) and high sediment concentrations in BowserLake are maintained by the melt of snow and especial-ly sediment-laden glacial ice. Less energetic turbiditycurrents may develop at this time, given that the riv-er water may be warmed and its density thus reduced

in comparison to the hypolimnic water which is near4 �C (Figure 7). Silt settles from suspension in allareas of the lake during late summer and early autumn.Overturn probably occurs before freeze-up, promotingmixing of the fine sediment remaining in the water col-umn, which then slowly settles to the lake floor duringwinter. This is complicated by intense autumn rain-storms that occur as late as December (Figure 9).Often,the storms are accompanied by rapidly rising freezinglevels and associated snowmelt, producing floods thatcarry coarser sediment into the western part of the lake.One such storm in October 1982 lasted almost six daysand may be responsible for the anomalously thick 1982couplet in the western basin (Figure 10).

The two thick massive silty layers in core P2 at theeast end of the lake may have a different origin. Thesediment constituting the two layers is light colouredand thus is not derived from the local dark bedrock.Instead, it probably was transported from farther up thelake and deposited from suspension. These two thicklayers may be the result of high-magnitude sedimentinputs by Bowser River, following catastrophic emp-tying of Tide Lake or another glacier-dammed lake inthe basin. Exceptional rainstorm-generated floods areanother possibility, but Desloges & Gilbert (1994a)note that period-of-record rainstorm floods at LillooetLake produced varves that are only 2–3 standard devi-ations above the mean. The order-of-magnitude dif-ference in varve thickness, noted here, requires mobi-lization of unusually large quantities of fine sediment.Such sediment is available within and downstream ofTide Lake (Clague & Mathews, 1992). Outburst floodselsewhere in British Columbia have had similar effectson the sediment transport regime of the affected rivers(Desloges & Church, 1992).

Chronology

Establishing the precise ages of the thin dark layersand the coarser massive layers near the east end of thelake is not possible because accumulation rates havevaried through time. However, we have shown thatthe average accumulation rate for this part of the lakeis probably between 7 and 8 mm a�1, in which casecore P2 spans the last 280 to 320 years. The most sig-nificant sediment influx events during this period arerepresented by the two thick massive layers in core P2,likely dating to the late nineteenth or early twentiethcenturies. This period overlaps the last part of the Lit-tle Ice Age in this region (Clague & Mathews, 1992;Clague & Mathewes, 1996). In contrast, there is lit-

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tle evidence for extreme events in the rhythmicallylaminated sediment that dominates the lower part ofcore P2. This implies that glacier-dammed lakes inthe Bowser valley were either empty or, more proba-bly, stable and not subject to sudden draining duringthe period in which this sediment accumulated. It alsoimplies that the runoff regime had greater seasonalcontrasts (enhanced snow and glacier melt). Locallyderived fine sand layers in this part of the core areattributed to instability of local hillslopes, due mainlyto higher winter snowfall or cooler wetter summers.

The ages of the five prominent acoustic reflectorsin the western basin near Cache Point are also difficultto determine. Using the present accumulation rate of18 mm a�1 in this part of the lake, they are about 230,370, 740, 920 and 1040 years old. These values arenot corrected for an increase in sound velocity withdepth, which in any case introduces an error of lessthan 2% in these shallow underconsolidatedsediments.The youngest of these ages falls within the early partof the interval spanned by core P2, but we were unableto identify an obvious event layer in the lower partof the core that correlates with the uppermost acousticreflector. We thus are unable to confirm the significanceof the reflectors from our data.

The average thickness of the fill in the western basinis 215 m (range 170–260 m). If the contemporary sed-imentation rate of 22 mm a�1 (Figure 11) is equiva-lent to the long-term average rate, the fill representsaccumulation over the last 9800 years. This estimate,however, may be in error for several reasons. (1) Thecharacter of the basal part of the fill is unknown, but itmay contain late-glacial ice-proximal sediment. If so,the overlying finer sediment accumulated in less than9800 years. (2) The build-up of sediment in the valleyinitially would have been more rapid due to the narrow-er cross-section and the greater availability of sedimentduring late-glacial and early postglacial time. This fac-tor would also reduce the time required to accumulatethe fill. (3) Offsetting this are possibly greatly reducedrates of sediment delivery to Bowser Lake during theearly Holocene when climate was warmer and proba-bly drier than today (e.g. Anderson et al., 1989), andalso during parts of the Neoglacial when Tide Lakeand other glacier-dammed lakes in the upper Bowserbasin trapped large quantities of glacier-derived sedi-ment (Clague & Mathews, 1992).

Sediment yield and denudation

The average bulk density (dry weight/wet volume) ofthe sampled sediments in the western basin of Bows-er Lake is 1210 kg m�3. The lake floor accumulationarea is approximately 32 km2. Using area-weightedestimates of accumulation rates for different parts ofthe lake, the total sediment volume and mass of fine-grained sediment delivered in an average year are about4.16� 105 m3 and 5.03� 105 t, respectively. For adrainage area of 1400 km2, the specific yield of finesediment to Bowser Lake is about 360 t km2 a�1, cor-responding to about 0.13 mm a�1 denudation. Thisis near the high end of the range reported for largeglacier-fed lakes in the Canadian Cordillera (Desloges,1994), but is much lower than reported in other alpineenvironments where rock types are more erodible (seesummary in Harbor & Warburton, 1993).

The estimated 4.1 km3 of sediment in the lowerBowser River sandur and in Bowser Lake represents arate of denudation in the basin of approximately 3.0 msince deglaciation assuming a bulk density of the sed-iment of 1800 kg m�3 (i.e. about 0.3 mm a�1). Thisvalue should be considered a minimum as it does nottake into account sediment stored in the upper reachesof Bowser River or in Holocene moraines and otherglacial features. Nor does it account for the throughputof sediment from Bowser Lake, although this amountis relatively small in a large lake (less than about 4%;Brune, 1953).

If the denudation value of 0.13 mm a�1 accountsfor about 50% of the yield, when the amount storedin the sandur is considered, the resulting estimate ofsediment supply is of about the same order as that basedon the acoustic results.

Conclusions

1. Sedimentation in Bowser Lake is controlled by(i) seasonal snow and ice melt in the heavily glacierizedbasin, (ii) autumn rainstorms, the frequency and impactof which are smaller than farther south, and (iii) rarecatastrophic drainage of glacier-dammed lakes.

2. Acoustic profiling and coring reveal that largeamounts of sediment have been carried into BowserLake throughout the Holocene to the present.

3. Turbidity currents are the main agent of sedi-mentation in the deep western basin of Bowser Lake.A sill near Graveyard Point reduces or eliminates sed-imentation from turbidity currents originating on the

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Bowser River delta in the shallower eastern part of thelake. In the eastern area, sedimentation occurs more byslow rainout of suspended silt and clay, and sedimentaccumulation rates are considerably less than fartherwest.

4. With our short cores, it is not possible tolink prominent reflectors in the acoustic subbottomrecord with specific events. It is likely, however,that the reflectors record exceptionally large stormsor catastrophic drainings of glacier-dammed lakes inthe upper Bowser basin during Neoglacial time.

5. Contemporary sediment yield of fine sediment toBowser Lake, which we estimate to be 360 t km�2 a�1,is not as high as might be expected for such a heavi-ly glacierized, high-energy drainage basin. Significantstorage of sediment is occurring, or has occurred, inseveral places upstream of the lake including: (i) theBowser River delta, (ii) existing small proglacial lakes(for example, at Frank Mackie and Knipple glaciers),and (iii) the small deltas, fans, and braided floodplainsof former Tide Lake.

Acknowledgments

The work was supported through research and equip-ment grants from the Natural Sciences and Engineer-ing Research Council of Canada and operating fundsfrom the Geological Survey of Canada.Field assistancefrom Olav Lian and the logistical support of MichaelChurch (University of British Columbia) and CharlieGreig (Barrick Resources Ltd.) greatly facilitated thework. We thank Gail Ashley and David Liverman fortheir reviews of the paper. This is Geological Surveyof Canada contribution 44495.

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