Late-Summer Peak in Sediment Accumulation in Two Lakes with Contrasting Watersheds, Alaska

Abstract:

Understanding the hydro-climatic and geomorphic controls on sediment transport and limnologic controls on deposition in glacier-fed lakes is important to improving paleoclimatic inferences from lake sediments. The timing of clastic sedimentation in two glacial lakes with contrasting watersheds was monitored using sequencing sediment traps for two consecutive years at Allison Lake (Chugach Range, Alaska) and four months at Shainin Lake (Brooks Range, Alaska). Shainin Lake is a weakly stratified lake fed by distant glaciers, whereas Allison Lake is strongly stratified and fed predominantly by proximal glaciers. At Shainin Lake, sediment accumulation started in late June and reached its maximum in mid-August, just before lake mixing and during a period of low river discharge. The median grain size of the sediment reaching the bottom of Shainin Lake was constant throughout the summer, based on analysis of deflocculated samples. Sedimentation appears to be controlled by flocculation rather than by settling of individual grains. At Allison Lake, rapid sedimentation events of coarse particles during late summer and early fall storms were superimposed on the fine-grained sedimentation pattern similar to that observed at Shainin Lake. These storms triggered underflows that were noticeable in the thermal structure of the lake and deposited large volumes of sediment. The sequencing sediment traps reveal a lag between fluvial sediment transport and the deposition of fine-grained sediments at the bottom of both lakes, implying limitations to interpreting intra-annual sedimentary features within the sedimentary sequence in terms inflow discharge.