The geologic framework of the northernmost St. Clair River presented in this report is based on the mapping of three seismic stratigraphic units and their associated unconformities. Bedrock is not a controlling factor in shaping the modern fluvial or lacustrine substrate, although bedrock topography likely had some influence on the deposition of glacial drift. A minimum 8 m of overburden is deposited above bedrock. Glacial-drift thickness is relatively uniform, except in the northernmost part of the river, where it was scoured out and partially filled with as much as 19 m of coarse-grained, southward-prograding deposits of sand and gravel. These coarse-grained deposits are derived from subaquatic glaciofluvial fans deposited near the riverhead. The internal seismic facies indicate that this deposit was transported a short distance downriver; however, we cannot determine how much of this deposition was the result of glaciofluvial processes and how much is derived from reworking by the post-glacial river. At a minimum, one of the linguoid bars in the upper part of the river appears to have formed some time between 1971 and 2000. The seismic stratigraphy shows that glacial-drift (mostly till) deposits are exposed or very close to the bed in deep areas in the northernmost part of the modern river channel and in a broad area near the mouth of the Black River.

We mapped the surficial geology of the lake and riverbed on the basis of acoustic-backscatter data, the distribution of stratigraphic units, and video, photographs, and sediment samples. The uppermost seismic stratigraphic unit dominates the surficial deposits. Exposures of glaciofluvial, glaciolacustrine, fluvial, and lacustrine deposits are differentiated on the basis of sediment texture. The substrate at Lake Huron near the head of the river is predominantly glaciofluvial sand and gravel, except nearshore, where lacustrine sand is being transported to the river by littoral drift. This sand enters the river and presumably is the primary source of sand deposited along the margins of the river. Much of the northernmost-river substrate is coarse gravel and cobbles. Substantial areas of clay till are exposed where the seismic stratigraphy indicates this unit is at the riverbed. Clay-till exposures near the Black River have scour features that indicate active erosion. The rate of erosion in these areas is indicated from bathymetric comparisons. Two downriver transects show similar stratigraphy and substrates to the northernmost part of the river. Both areas show thin, coarse-grained deposits covering the riverbed, with glacial drift exposed or near the surface.

Some questions remain for future studies. We do not know whether the till was scoured out in the northernmost part of the river during glacial time or when the outlet at Port Huron became active when postglacial flow started 5,000 yr b.p. We were unable to differentiate upper-river coarse-grained deposits into glacial and postglacial deposits; consequently, we do not know when most of this deposit filled the depression in the till surface or how much of the deposit is glaciofluvial and how much has been reworked from glaciofluvial deposits by the modern or pre-modern river. Comparisons of our data with soundings from 1971 indicate that some downstream transport and deposition likely has occurred since 1971.

Detailed geologic-framework surveys such as this one for the northernmost part of the river, extended for the full length of the St. Clair River, would provide a more complete assessment of a base in relation to changes in river morphology. Preliminary observation of video transects spaced every 2,000 m for the length of the river indicate that our downriver site surveys at Marysville, MI, and Port Lambton, Ontario, Canada, may be typical for the river south of the Black River. By obtaining acoustic-backscatter data over the entire length of the river, we would be able to verify or refute this conclusion.