The Offshore Sediments of Lake Winnipeg
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Early explorers passing through the Lake Winnipeg watershed occasionally commented on the muddy waters of the lake and the Red River (Ross 1856, p. 9; Palliser's Journals of 1857- 1860 in Spry 1968, p. 11,88,.94; Upham 1890, p. 18E). The deposition and resuspension of this Red River sediment load in Lake Winnipeg were observed by Upham (1890), who was also the first to study rigorously the geomorphology of the glacial lake deposits around the south basin of Lake Winnipeg. Upham (1890) gave the name Lake Agassiz to this Late Wisconsinan and Holocene body of water, the sediments of which are now found over some 518,000 km 2 of what is now Manitoba, Ontario, Saskatchewan, Minnesota, North and South Dakota (Elson 1967). Tamplin (1967) reviewed the history of Lake Agassiz research. Lake Agassiz clays and silts are now agricultural soils, and likely constitute the majority of the sediments carried to Lake Winnipeg by the Red River (and to a lesser extent, by the Winnipeg and Saskatchewan Rivers). These Lake Agassiz clays and silts appear to have been derived from Cretaceous shales and Quaternary glacial sediments to the south and west of the present Lake Winnipeg (Kushnir 1971) via rivers draining the Dakotas and as far west as the Rocky Mountains in Alberta (Teller 1976) during the Pleistocene. Sediment supply to the north basin of Lake Winnipeg in recent time via the Saskatchewan River appears to have been relativelY less than the Red River sediment supply to the south basin of the lake. Upham.(1890) noted that the north basin of Lake Winnipeg was relatively clear, compared to the south basin of the lake. Many early explorers commented on the trapping of Saskatchewan River sediment in Moose and Cedar Lakes (Mackenzie 1801, p. ll5; Hopwood 1971, p. 85). Denis (1916, p. 123) indicated that the Saskatchewan River deposited much of its seqiment load into Cedar Lake (about 80 mm/yr in some locations). Sediment supply to the north basin of Lake Winnipeg has been further decreased by the construction of numerous reservoirs on the Saskatchewan River from 1913 to the present (Report of the Saskatchewan-Nelson Basin Board, 1972). Lake Diefenbaker collects about 23 x 106 tonnes sediments yr- 1 • and Tobin Reservoir collects about 15 x 106 tonnes yr- 1 ). Frankl in (1823, p. 43) found the waters of the north shore of Lake Winnipeg rather turbid. however. and told a Cree story which accounted for the muddiness of the 1 ake and its name (Winnipeg = muddy dirty water in Cree). The literature on Lake Agassiz sediments is concerned largely with beaches, outlet structures. clay stability for road and building construction, and general studies concerned with the history of that expansive and oscillating glacial lake. Much of this literature is reviewed in MayerOakes (1967). We have been unable to find any references to the thickness or characteristics of Lake Agassiz sediments in the present Lake Winnipeg basin. Teller et al. (1976) have mapped the thickness of glacial sediments and bedrock topography in southern Manitoba. Kushnir (1971) speculates that some laminated clay clasts 1 from dredged sediments from the south basin of Lake Winnipeg are subaqueous outcrops of glacial Lake Agassiz sediments, based partly on sonar reflections and clay mineralogy. The sediments carried by the Red and Assiniboine Rivers to the south basin of Lake Winnipeg are, of course. largely Lake Agassiz sediments. Other studie$ on Lake Winnipeg sediments include Solohub and Klovan (1970), Solohub (1967), and Veldman (1969), who were concerned largely with nearshore and beach sediment movement, deposition. and particle size sorting. Wallace and Maynard (1924), Elson (1961), Ehrlich et a1. (1955), Wicks (1965), Last (1974), and Teller (1976) gave chemical, mineralogical, and particle size data for Lake Agassiz sediments, which may be useful in studies of Lake Winnipeg. sediments. Ward (1926). contributed an estimate of Red River sediment transport rates, particle size and chemical analyses of Red River suspended sediments, Red River bottom sediments, and Lake Winnipeg sediments near the mouth of the Red River. Bajkov (1930) briefly described and categorized surface sediments of Lake Winnipeg. There have been no synoptic chemical studies on Lake Winnipeg sediments, except for Allan and Brunskill (1977) which is part of this work. We now give descriptive data for Lake Wiinnipeg sediment samples, interstitial water, and riverborn sediments. Our objectives wete to provide a whole lake view of Lake Winnipeg off-shore sediment texture, chemistry, and sediment supply rates, as a background for future detailed studies on selected problems.