ABSTRACT: Sedimentation patterns in Quesnel Lake, British Columbia: Implications for late-glacial and Holocene environments.

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Abstract

Quesnel Lake is one of the largest lakes (~270 km2) and the deepest lake (~512 m) in British Columbia. The Quesnel Lake drainage basin covers ~5850 km2 of the interior plateau and Columbia Mountains. Pleistocene glaciations have eroded and scoured the lake to its current form and left behind a moderate to thick mantle of glacial and glaciofluvial sediments throughout the catchment. Less than 1.4% of the total chatchment area today is glacier covered. Quesnel Lake is primarily oligotrophic due to the cool-cold climates that dominate the higher plateau regions of central British Columbia. Sedimentation patterns in the lake were investigated during 2004 and 2008 using CHIRP II acoustic sub-bottom surveys covering a total of 205 km’s. In 2008, six cores were recovered from the lake bottom using a Rossfelder submersible virbra corer. In most areas throughout Qusenel Lake, the basement of bedrock or glacial sediment (till or glaciofluvial deposits) can be distinguished from the overlying lacustrine sediment. In the west half of the lake, sediments are generally thin (< 2 m) with pockets of muds up 45 m thick. Sediment is generally absent from the side walls in these deeper sub-basin areas but has been focused to form nearly flat-lying fill in the depressions. The sediment varies from acoustically opaque to transparent and is well laminated in places. Some transects show a conformable veneer and others show no trasparent sediment suggesting bottom currents are actively re-working and redistributing sediment on the lake floor. In the deeper parts of the east half of the lake (including the south half of the North Arm) the irregular floor is infilled with as much as 100 m of acoustically laminated lacustrine sediment. Flat and parallel reflectors suggest some deposition by turbidity currents. In the narrow, trench-like, eastern arm sediment has ponded along the thalweg of the trench but with a remarkabley small thickness reaching only 60 m isolated pockets. Sediments in cores show variability both down-lake and within specific sub-basins of the lake. In the North Arm closer to some of the melt-water inputs, sediments are mainly clastic with massive to weakly laminated silts and clays interupted by coarse silt beds and fine sand laminae. The coarser units appear to be episodic in nature and are probably tied to distinct runoff events into the North Arm from glacier sources. Opposite the Horesfly River delta, sediments are much more orangic rich with thick beds of needle rich debris, large wood fragments and isolated thin laminae of coarse silt and find sand. In the West Arm approaching the outlet of the lake, sediments vary significantly between the deep pockets of accumulation. One core shows predominately clastic laminae of irregular frequency and thickness separated by massive silty-clay. Another shows mostly organic fragments of needles and wood mixed with silt and some fine sand. Accumulation rates throughout the lake are probably a faction of a mm per year. The relatively small accumulation of late glacial and Holocene sediment in Quesnel Lake is similar to other large lakes of the British Columbia Cordillera such as Chilko Lake (Desloges and Gilbert 1998) and Atlin Lake (Gilbert et al. 2005). These patterns reflect a particular style of deglaciation and Holocene sediment inputs.

Bibliography of Canadian Geomorphology