ABSTRACT: High-energy sedimentary processes in Kluane Lake, Yukon Territory.

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Modern sedimentary processes were studied in Kluane Lake, Yukon Territory to determine the spatial and temporal patterns of sediment distribution in a large, dynamic glacial lake. In order to accurately interpret past environmental conditions from lacustrine sedimentary records, the processes that control both the delivery of sediment to a lake and the distribution of sediment within a basin must be properly understood (Bradley et al., 1996; Hodder et al., 2007). Kluane Lake provides an excellent opportunity to examine high-energy sediment distribution mechanisms and patterns, particularly with respect to turbidity currents dynamics. Kluane Lake’s primary source of water and sediment is Slims River, which drains Kaskawulsh Glacier, a major valley glacier that flows out of the Icefield Ranges in the St. Elias Mountains. In the twentieth century the delta has advanced 3.6 km, which is the highest rate of any glaciolacustrine delta in the Cordillera, decreasing from 74 m/a (1899-1914) to 18 m/a (1970-2006). Data from moored instruments, sediment traps, water column profiling, and high-resolution sub-bottom acoustic surveys were used to document the lacustrine sedimentary processes. Tilting current meters were constructed after Hendricks (1985), albeit with significant modifications, and deployed at 11 sites near the lake bottom. A river monitoring station near the Slims River delta was established to continuously record variations in discharge and turbidity. Surface sediment samples were collected from the lake floor with an Ekman box corer; however, the extremely underconsolidated sediment near the delta (water content 30-50% of the weight of the sediment) prevented the preservation of short cores from the Ekman corer or the recovery of long cores by conventional gravity coring or vibra coring. During the peak melt season, the suspended sediment concentration of Slims River is weakly dependent on river discharge and typically varies diurnally between 1 and 2 g/l, but can reach up to 5 g/l. While multi-day clockwise hysteresis events suggest seasonal sediment exhaustion is occurring in the proglacial system, diurnal hysteresis in Slims River is either negligible or follows a minor counterclockwise pattern. The high suspended sediment load in the river generates continuous, diurnally fluctuating turbidity currents in Kluane Lake with maximum velocities up to 0.6 m/s. During times of peak flow, variations in velocity can be traced to beyond 4.5 km from the delta. The vertical concentration and velocity profiles of the turbidity currents appear to be consistent with modeled and experimental results and show distinctive longitudinal variations. Turbidity currents dominate the sedimentary environment in Kluane Lake; sedimentation from surface settling accounts for less than 2% of total sedimentation in the prodelta region during the peak melt season. Diurnal laminations are apparent in several sediment traps close to the point of inflow and can be directly linked to variations in velocity (Figure 1). The particle size of the sediment in Kluane Lake is homogenous and typically ranges from 20 to 40% clay and 60 to 80% silt depending on the distance from the delta. Despite the high-energy nature of the environment, there is virtually no sand except in small amounts very close to the delta. Accumulation in the prodelta area of Kluane Lake has averaged 0.4 m/a between 1970 and 2006 based on acoustic surveys, and was 0.3 m in 2006 based on sediment trap results. Turbidity currents in Kluane Lake have created a 4.7 km2 field of large sediment waves in fine-grained sediment on the prodelta slope that are analogous to those of marine systems (Lee et al., 2002), but previously not reported from lakes. These bedforms have mean wavelength of 130 m (range 16 – 440 m) and mean amplitude of 2.3 m (range 0.1 – 9.0 m). Their internal architecture indicates that they are migrating up-slope; sedimentation on the stoss portions of the waves averages 2.7 times that on the lee portion. Slopes on the waves vary from –0.067 (i.e. sloping in the opposite direction to the regional slope) to 0.135. The sediment waves were formed on the flat surface of the lake floor in the absence of preexisting forms and they are altered and destroyed as the wave field advances or the characteristics of the turbidity currents change.

Bibliography of Canadian Geomorphology