CONTROLS ON VALLEY WIDTH OF POST-GLACIAL STREAMS IN THE LAKE SUPERIOR BASIN

Streams show variability in valley width associated with river incision and knickpoint migration through mixed lithologies. This research studies the controls on valley width of post-glacial streams along the north shore of Lake Superior in Duluth, Minnesota. These streams are incising through a mix of lithologies encompassing crystalline and sedimentary bedrock and Quaternary deposits following glacial lake level fall at the end of the last glaciation. To investigate the controls on lateral migration of post-glacial streams, we measured variations in valley width upstream of knickpoints through different parent material. We combined field measurements of bedrock compressive strength and channel width, field mapping of knickpoint and terrace locations, and lidar analyses to determine longitudinal variations of stream power and valley width in Lester River, Amity Creek, and Mission Creek. These three rivers have different lithologic complexity and competency that could impact river incision and lateral migration of the streams. The goal is to investigate whether parent material impacts valley widening, and whether knickpoints act as base level controls for widening to occur upstream. Results in Amity and Lester show wider valley widths where lithology changes from bedrock into glacial till. Mission Creek is composed of mainly glacial till and less competent Precambrian sandstones and thus follows a typical pattern of downstream valley widening. Where knickpoints are bounded by bedrock valley walls there are greater valley widths immediately upstream of the knickpoints. This implies that knickpoints act as local base level controls allowing for more lateral migration upstream while incision is occuring downstream. The evolution of river valley width in these incisional channels are controlled by knickpoint locations and lithologic competency.