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Palaeohydrographic reconstructions from strandplains of beach ridges in the Laurentian Great Lakes
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2014
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The current temporal and spatial context of water-level change, drivers of change, and
possible future scenarios of the Laurentian Great Lakes is controversial. Palaeohydrographs are
being constructed from measured subsurface elevations of palaeo-swash zones and modelled
ages in strandplains of beach ridges that are preserved in embayments along the lakes’ edge.
More than 800 elevations and 200 ages have been collected from 15 strandplains to construct
site strandplain palaeohydrographs. Palaeo-beach elevations from whole strandplains or sets of
correlative palaeo-beaches within strandplains are then used to establish an outlet palaeohydrograph
for each lake. Adjusting strandplain palaeohydrograph elevations to account for glacial
isostatic adjustment and refining age models help define the outlet palaeohydrograph. Common
basin-wide water-level patterns and changes in outlet location or conveyance can then be interpreted.
Systematic patterns of elevation and geomorphic/sedimentologic properties in individual,
groups and sets of beach ridges in strandplains suggest that long-termpatterns of water-level change
and sediment supply occurred on decadal, centennial and millennial scales. Outlet palaeohydrograph
construction for Lake Superior revealed discrepancies between geological and historical
rates of glacial isostatic adjustment. These differences are currently being investigated using new
data from Lake Huron.
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JOHN W. JOHNSTON1*,
1Department of Geography and Environmental Studies, Wilfrid Laurier University,
75 University Ave. W, Waterloo, Ontario, Canada N2L 3C5
TODD A. THOMPSON2
2Indiana Geological Survey, Indiana University, 611 North Walnut Grove Avenue,
Bloomington, IN 47405-2208, USA