Holocene evolution of the Assiniboine River paleochannels and Portage la Prairie alluvial fan

The Portage la Prairie alluvial fan was constructed by numerous successive paleochannels of the Assiniboine River along the western side of the Lake Agassiz basin as the level of the lake rapidly declined beginning 9550 yr ago. Paleochannel morphologies and cross-cutting relations, soil maturity, and radiocarbon dates indicate that by 6000-7000 yr ago flow was northward into Lake Manitoba. This direction was maintained until about 3000 yr ago, when avulsion redirected the Assiniboine eastward to the Red River near Winnipeg. The morphologies of the paleochannels suggest that channel-forming discharges and sediment loads of the ancestral rivers have not differed significantly from the modern values despite palynological evidence that the climate was warmer and drier during much of the Holocene. -from Authors