HOLOCENE STREAM CAPTURE OF THE LE SUEUR RIVER, MINNESOTA: IMPLICATIONS FOR MODERN SEDIMENT LOADING

Recent monitoring has documented elevated sediment loads from the Le Sueur River, a tributary of the Minnesota River, which is a principle source of sediment to the Upper Mississippi River system. The Le Sueur River also experienced a significant Holocene (post-Glacial) stream capture event, evidenced by a 9.4-km reach of abandoned channel that cross-cuts Pleistocene terraces of the Minnesota River. This capture event diverted the mouth of the Le Sueur River from a direct confluence with the Minnesota River to a confluence with the Blue Earth River, a nearby tributary of the Minnesota River. Longitudinal profiles of the Le Sueur River, Blue Earth River and their tributaries created in Geographic Information Systems (GIS) were used to identify knickzones and terraces and infer the long-term incision histories of the rivers. Terraces within the lower knickzone of the Le Sueur River form a continuous surface between the head of the longitudinal profile of the abandoned channel and the middle portion of the Le Sueur River profile, suggesting that this knickzone was developed subsequent to the stream capture event. Modern sediment flux from the watershed may be impacted by the presence of this relatively young, 28-km long knickzone with 52-m of relief near the river mouth. Elevated rates of erosion in the knickzone may also be influenced by the confluence of two major tributaries within it, and a lesser degree of bedrock armoring in the channel relative to the pre-capture configuration.

Post-glacial fluvial sands from the bed of the abandoned channel were sampled for optically stimulated luminescence (OSL) age dating. Preliminary dates will be presented to further constrain the Holocene age of the stream capture event, and evaluate its potential to influence long-term erosion rates and the modern sediment flux from the Le Sueur River watershed.