SEDIMENT LOADING IN THE LE SUEUR RIVER, MINNESOTA RIVER WATERSHED

Sediment source contributions to the turbidity-impaired Le Sueur River in central Minnesota are being studied through a combination of field work and analyses of DEMs and historical aerial photographs. High sediment loads in the Minnesota River have led to declining water quality in Lake Pepin, leading to total maximum daily load (TMDL) projects addressing water quality issues. In response to turbidity TMDL projects there is an increased desire to understand sediment sources in the Minnesota River, of which the Le Sueur River is a primary contributor.

The Minnesota River is deeply incised, with tributaries cutting down through glacial and alluvial sediments. Potential sources of sediment include high bluffs along the lower Le Sueur, growing ravines and gullies, floodplains and terraces, and upland agricultural fields. To better understand source contributions, field work is being conducted starting summer 2007. We are using ground-based LiDAR scanning to measure bank, bluff, and ravine retreat. These field measurements are being extended through comparisons with historical aerial photographs dating back to the 1930s. Areas of exposed bluffs, banks, and ravines are being determined from high-resolution airborne LiDAR data. The airborne LiDAR are also being used to determine floodplain and terrace areas and volumes along three main branches of the Le Sueur River for use in a floodplain exchange model. Preliminary investigations show long-term migration rates as high as 3.23 m/yr for isolated meander bends and bluff retreat rates up to 0.43 m/yr.

A sediment gaging network has been established by the MPCA and Water Resources Center at MN State University to gage eight sites within the Le Sueur River basin. Sediment gaging eventually will be combined with source contribution data to develop a sediment routing model. This work will contribute to a broader understanding of the basin and how sediment moves throughout the river network.