PALEOCHANNEL HISTORY IN THE LAKE SUPERIOR BASIN: AMITY CREEK AND LESTER RIVER

Stream capture and river migration are crucial aspects of the evolution of landscapes, particularly in the postglacial Lake Superior Basin. These processes can drastically alter the landscape and leave behind depositional features that can be analyzed to interpret the history of ancient channels. In this study, we examined the bedrock rivers of Lester River and Amity Creek in Duluth, Minnesota, which have a complicated history of river migration, stream capture, and deposition following glacial retreat at the end of the Wisconsinan glaciation. We studied the geologic history of river migration and evolution in these two rivers at a site where the much larger Lester River now flows past a paleochannel entering from the direction of the smaller Amity Creek. We sampled the modern channel bed of the Lester River and the left bank wall containing an ancient fluvial deposit believed to be sourced from the paleochannel, to compare the processes that formed them, as well as identify which paleobasin likely put the ancient deposit in place. We calculated the critical shear stress using pebble counts, calculated the upstream drainage area for multiple paleobasin options, and found slopes from long profiles. This data was used to create two ratios of Lester River versus Amity Creek to compare the critical depth needed to mobilize the grain size as well as the stream power from the upstream areas for different paleochannels. These ratios indicate that the grains in the ancient fluvial system were produced from a smaller system rather than the modern Lester River and were most likely deposited from a paleo-Amity channel prior to its capture to its modern position.