MODERN SEDIMENTATION OF MENDENHALL LAKE, JUNEAU, ALASKA, USA

By analyzing the geological features of the lake, the filtered suspended sediment samples, and lake bottom samples, the glaciolacustrine processes taking place in this system can be understood and compared with similar modern glacial systems. Mendenhall Glacier, located just outside of Juneau, Alaska, is an outlet glacier from the Juneau Icefields. Mendenhall Lake, which is located at the terminus of the glacier, is an ice-contact proglacial lake that has formed over the last 50 years as the glacier has retreated up valley. Sediment distribution correlates with several factors such as suspended sediment load present at different localities throughout the lake, including the inlets to the lake. Mineralogy of the glacial debris and suspended sediment load controls density variations which, in turn, affects sedimentation rates. Samples were gathered (bottom grab-sampler) from the floor of the lake at locations that represent different micro-depositional environments. Suspended sediment samples also were gathered and filtered from stations located throughout the lake and water column. The bottom samples were analyzed with a Laser Particle Analyser for grain size and both the bottom and filtered samples were analyzed with an X-Ray Diffraction Unit (XRD) for mineralogy. The samples suggest that debris going in to lake from the glacier is not only from the immediate igneous bedrock, but also from metamorphic bedrock located up valley. Mineralogy and chemistry of the water is also important to understand the influence of flocculation on sedimentation. With high amounts of flocculation, the settling rates increase significantly, causing the lake to act as a "sediment trap." Geological features surrounding the lake strongly influence glaciolacustrine processes. Grain size is a reflection of the physical properties of the lake such as depth-to-bottom and distance from the ice-front. Grain size of the sediments from the bottom samples varies dramatically from medium to coarse sand in the ice-proximal portions to silt and clay in the ice-distal portions of the lake.