Bathymetric and seismic-reflection data show a northern ridge in the study area composed largely of glaciolacustrine sediments. A northeast-southwest oriented channel crossing the center of the study area, which was cut by fluvial processes, is now partly filled by Holocene marine and transitional deposits. The sidescan-sonar imagery indicates seven backscatter patterns consisting of sand waves, boulders, benches, low backscatter, moderate backscatter, high backscatter, and trawl marks. Sand waves cover much of the northern ridge and bathymetric high in the southeast. Boulders are scattered throughout the study area, but are mostly concentrated in the southeast. Erosional benches occur on the northern banks of the central channel and southeastern bathymetric low and are interpreted to represent paleoshorelines from 9-10 ka. Areas of low backscatter in some of the deepest parts of the study area are interpreted to be finer grained sediments; areas of high backscatter are interpreted to reflect coarser grained sediments. Trawl marks are plentiful in the central channel. Four sedimentary environments are found in the study area including those characterized by processes of erosion or nondeposition, coarse-grained bedload transport, sorting and reworking, and fine-grained deposition.