Stream Profile Analysis of the Le Sueur River Stream Capture Event

The Le Sueur River—a major tributary of the Blue Earth River, which leads to the Minnesota River—is a principle source of sediment to Lake Pepin, which is filling in at an increasing rate. The Le Sueur River is unusual because it shows clear evidence of a recent stream capture event, which diverted the mouth of the river from downtown Mankato (along Stoltzman Road) to the Blue Earth River. With the use of Geographic Information Systems (GIS), I determined longitudinal profiles of the Le Sueur River and the Blue Earth River. The longitudinal profile shows the change in elevation of the riverbed over distance from the mouth. With these profiles, I identified the location of knickzones, which are sections of the profile with steeper slope that produce high amounts of erosion. I also used GIS to locate terraces, which mark where the river existed before it started cutting a new channel. I have obtained samples of material within the abandoned channel for absolute age dating using optically stimulated luminescence, which determines the time of last exposure of quartz to sunlight. By comparing the position of the Le Sueur River knickzone with the Blue Earth River knickzone, I determined the relative significance of the stream capture event in the evolution of the stream profile. Absolute age dating was used to quantify the results of the GIS analysis and determine the relationship between the stream capture event and the excess sediment load generated by the Le Sueur River