Response of the St. Joseph River to lake level changes during the last 12,000 years in the Lake Michigan basin |
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Authors: | Kevin A Kincare |
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Institution: | (1) Michigan Geological Survey, P.O. Box 30256, Lansing, MI 48909-7756, USA |
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Abstract: | The water level of the Lake Michigan basin is currently 177 m above sea level. Around 9,800 14C years B.P., the lake level in the Lake Michigan basin had dropped to its lowest level in prehistory, about 70 m above sea
level. This low level (Lake Chippewa) had profound effects on the rivers flowing directly into the basin. Recent studies of
the St. Joseph River indicate that the extreme low lake level rejuvenated the river, causing massive incision of up to 43 m
in a valley no more than 1.6 km wide. The incision is seen 25 km upstream of the present shoreline.
As lake level rose from the Chippewa low, the St. Joseph River lost competence and its estuary migrated back upstream. Floodplain
and channel sediments partially refilled the recently excavated valley leaving a distinctly non-classical morphology of steep
sides with a broad, flat bottom. The valley walls of the lower St. Joseph River are 12–18 m tall and borings reveal up to
30 m of infill sediment below the modern floodplain. About 3 × 108 m3 of sediment was removed from the St. Joseph River valley during the Chippewa phase lowstand, a massive volume, some of which
likely resides in a lowstand delta approximately 30 km off-shore in Lake Michigan.
The active floodplain below Niles, Michigan, is inset into an upper terrace and delta graded to the Calumet level (189 m)
of Lake Chicago. In the lower portion of the terrace stratigraphy a 1.5–2.0 m thick section of clast-supported gravel marks
the entry of the main St. Joseph River drainage above South Bend, Indiana, into the Lake Michigan basin. This gravel layer
represents the consolidation of drainage that probably occurred during final melting out of ice-marginal kettle chains allowing
stream piracy to proceed between Niles and South Bend.
It is unlikely that the St. Joseph River is palimpsest upon a bedrock valley. The landform it cuts across is a glaciofluvial-deltaic
feature rather than a classic unsorted moraine that would drape over pre-glacial topography.
This is the fifth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were
presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University
of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M. Lewis were guest editors of this special issue. |
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Keywords: | Base-level Lake Michigan Chippewa Algonquin Incision Terrace |
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