CHANNEL-FLOODPLAIN SEDIMENT EXCHANGE IN A MEANDERING AND ACTIVELY INCISING RIVER

We have developed a sediment budget and routing model for the Le Sueur River, southern Minnesota. An integral component of the model considers the sediment storage and erosion dynamics of the coupled channel-floodplain system. The Le Sueur River system can be broken down into two reaches that are separated by a knickpoint that has been propagating upstream over the past 11,500 years. The upper reach is approximately 120 km long and characterized as low-gradient (slope = 0.0004 m/m) and gently meandering. The lower reach includes the last 40 km of the river upstream from the mouth and is characterized as a relatively high gradient (slope = 0.002 m/m) and is actively incising and meandering. In this study we apply the geometric model developed by Lauer and Parker (Geomorphology, 2008) to quantify the mass balance of sediment stored in the floodplain and inside bends of meanders versus sediment eroded on the outside bends of meanders to compute the net contribution of sediment from the coupled channel-floodplain system. Availability of air photos from 1938 and 2005 permitted calculation of decadal-scale meander migration rates and high-resolution LiDAR topographic data allowed extraction of the difference in bank heights. These two data sources serve as the primary inputs to our model. We report that the channel migration is responsible for a small, but discernable local net contribution of sediment to the Le Sueur River, accounting for 5-20% of the measured suspended sediment loads. Ongoing work will answer the question as to whether or not this local net influx is balanced by floodplain storage within the system.