STATUS OF THE LAKE WINNIPEG PROJECT (NTS 62I, 62P, 63A, 63B, 63G AND 63H)
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Date
1998
Authors
Thorleifson, H.
Anderson, T.
Betcher, R.
Bezys, R.
Buhay, W.
Burbidge, S.
Cobb, D.
Courtier, N.
Doering, J.
Fisher-Smith, G.
Journal Title
Journal ISSN
Volume Title
Publisher
Government of Manitoba
Abstract
The Lake Winnipeg Project was launched by the Geological Survey of Canada (GSC) and Manitoba Geological Services Branch (MGSB) in 1994, with the support of Manitoba Hydro and the Manitoba Sustainable Development Innovations Fund, in order to support management of issues such as shoreline erosion and water quality. A
four-week cruise of the Canadian Coast Guard Ship (CCGS) Namao in 1994 was followed by a similar effort in 1996. Low frequency air gun seismic, high frequency seismic, side-scan sonar, and coring operations, guided by real-time differential global positioning system (GPS) navigation, were supplemented by limnological and biological sampling carried out in cooperation with the Freshwater Institute. A five-year program of absolute gravity measurements and GPS data collection is monitoring
uplift, a key factor in shoreline erosion, along a transect from Iowa to Churchill. Accompanying research on shoreline processes has included reconnaissance surveys in 1994, targeted investigations in 1996, and a month-long intensive effort in September 1997. Previously acquired wave data have been supplemented by data from three waverider buoys deployed in 1996. Although targeted follow-up research is anticipated, final outputs from the Lake Winnipeg Project are in preparation for release in 1999. Two major questions have been addressed by this work: What is the structure of the Lake Winnipeg basin? Results have demonstrated that the structure of sediment and rock below Lake Winnipeg dramatically differs from expectations. Prior to the 1994 cruise, it was thought that sedimentary rocks extended close to the
eastern shore, and that these rocks were buried by at most 15 m of sediment. In fact, sedimentary rocks only extend 10 km east from the end of Long Point, and terminate at a buried escarpment south of Hecla Island. Beyond these Paleozoic rocks, sediments consisting almost entirely of Lake Agassiz clay reach unanticipated thicknesses of over 50 m in the South Basin and over 100 m in the North Basin. Till and other gravel-bearing glacial sediments are not extensive, but are present as formerly unrecognized major moraines at George Island and Pearson Reef. Sediments deposited by postglacial Lake Winnipeg, which rarely exceed 10 m in thickness, rest on a regionally pervasive, low-relief angular unconformity, and are ornamented by a complex array of furrows formed by the action of lake ice. Vigourous currents have stripped sediments from The Narrows and east of Black Island, producing the greatest water depths in the lake, over 60 m. Are present-day environmental changes superimposed on long-term evolutionary trends? Without knowledge of the history of a lake, as recorded in its sediments, it is difficult to determine whether a
basin was in a state of equilibrium prior to human intervention, or whether recent perturbation is only an addendum to more profound natural changes. The surveys have shown that Lake Winnipeg has, for centuries and millenia, been undergoing a steady expansion. Sediment cores from the centre of the South Basin have revealed that Lake Winnipeg offshore sediments have buried fossiliferous material that could only have been deposited at a pre-existing shoreline. Radiocarbon and paleomagnetic
analysis of this material indicate that most of the South Basin was dry land at 4 ka BP (4000 radiocarbon years ago). The dominant control that has caused southward transgression is tilting, as a result of the uplift of the Hudson Bay region that resulted from melting and breakup of the continental ice sheet around 10 ka BP. The rise of the lake has been punctuated by climate change, especially the shift to cooler, moister conditions around 4 ka BP, diversion of the Saskatchewan River into the North Basin at 4.7 ka BP, switching of the Assiniboine River from a path through Lake Manitoba to the North Basin over to the Red River and the South Basin around 4 ka BP, as well as progressive merging of several sub-basins from Playgreen Lake to the South Basin into one Lake Winnipeg. Unlike other lakes, outlet downcutting has not been a significant factor on Lake Winnipeg, due to the low and easily erodable barriers that overlie resistant substrates.
Description
Keywords
Lake Winnipeg, Geological Survey, Lake Winnipeg Basin
Citation
Thorleifson, H., Anderson, T., Betcher, R., Bezys, R., Buhay, W., Burbidge, S., Cobb, D., Courtier, N., Doering, J., Fisher-Smith, G. Forbes, D., Franzin, W., Friesen, K., Frobel, D., Fuchs, D., Gibson, C., Henderson, P., Jarrett, K., James, T., King, J., Kling, H., Lambert, A., Last, W., Lewis, M., Lockhart, L., Matile, G., McKinnon, T., Moran, K., Nielsen, E., Pullan, S., Rack, F., Risberg, J., Rodrigues, C., Salki, A., Schröder-Adams, C., Stainton, M., Telka, A., Todd, B., Vance, R. and Weber, W. 1998: Status of the Lake Winnipeg Project (NTS 62I, 62P, 63A, 63B, 63G and 63H); in Manitoba Energy and Mines, Geological Services, Report of Activities, 1998, p. 196-209.