STREAMBANK EROSION AND FLOODPLAIN DEPOSITION PROCESSES IN THE LOWER MINNESOTA RIVER

Streambank erosion and floodplain deposition processes are key to understanding sediment dynamics in the Minnesota River. Historic investigations, field monitoring and modeling of channel evolution processes were conducted to quantify these processes, focusing on the lower Minnesota River from Mankato to St. Paul. Most streambanks were sand and silt by particle size, with low shear and cohesive strength. Long-term rates of bank migration ranged from 0.01 m/yr to 3m/year in tributaries and the main channel from 1938-2009 with the lower Minnesota River having the highest rates. Channel evolutionary trends varied by watershed position, with channel widening occurring in the lower main channel by about 50% over the 70 year period. Channel straightening and natural cutoffs resulted in the loss of 12 miles of river length during that same time period. Sediment loads are mitigated by a large rate of floodplain deposition although connectivity has been reduced by human structures in some areas. Floodplain soil boring data provided information on the depth and rate of post-European alluvium (PSA) deposition on the floodplain and channel entrenchment. Increased floodplain elevation from deposition may reduce connectivity over time thus increasing suspended sediment carried downstream. The depth of PSA on the floodplain was highly variable but average depths were greatest near the channel and tapered off toward the valley walls. The greatest quantities of recent deposits occurred on the sand levee near the channel and in back-channel cutoffs. Recent rates of sediment deposition on the floodplain, as estimated by the depth of PSA via physical and chemical characteristics was found to range from 0 to 3.8 cm/year. The higher rates were not maintained over the entire 150 year post-European period, but occurred after large floods. The median rate in the forested areas using the sediment excavation/tree ring method was 0.58 cm/year (mean 1.8 cm/yr). The median rate in backwater oxbows and organic flats located far from the main channel was lower at 0.20 cm/year (mean 0.19 cm/year). Modeling of floodplain inundation indicated that the river valley below Jordan was nearly 100% inundated by large floods, while the floodprone area upstream was much smaller.