Streambank erosion and channel evolution processes in the Minnesota River Basin

The Minnesota River Basin (MRB) is the largest contributor of sediment and nutrients to the Upper Mississippi River in Minnesota and is impaired for turbidity and nutrients over much of its length. Several recent studies indicate that the majority of sediment in the river originates from channel erosion of ravines, bluffs or streambanks. The objectives of the studies synthesized in this report were: to quantify sources of sediment to the Minnesota River; to determine channel evolutionary processes and stages; and to quantify the impacts of direct channel modification on sediment transport efficiency of the lower Minnesota River. Streambank properties that influence erosion rate were measured including shear strength, cohesive strength, particle size distribution and bank erosion hazard index (BEHI). Direct measurement of bank erosion rates were done at twenty streambank sites over three years. Historic bank migration rate, changes to channel width, sinuosity, and total river length were measured between 1938, 1991 and 2009. Preliminary modeling of changes to streambank erosion potential using bankfull shear stress as an indicator was performed to understand the counteracting effects of river channelization and river widening on the lower Minnesota River from St. Paul to Mankato. Data collected on the erodibility of channel materials in ravines and streambank showed that all sites on the main Minnesota River and tributaries had low shear strength (mean 2 Pa) but moderate cohesive strength (mean 19 kPa) making them susceptible to erosion from greater flow. Long-term rates of lateral channel erosion measured in aerial photos showed that the highest rates of bank erosion were found on the main channel of the Minnesota River between Mankato and St. Paul, ranging from 10 to 152 cm/yr on the outer bend. The steep tributaries monitored in this study, including High Island Creek, Rush River and the LeSueur River had the second highest lateral erosion rates followed by tributaries in the flatter glacial till plains (Elm, Mound and Frische Creeks). Meanwhile, monitoring of current bank erosion through re-surveys from 2007-2010 revealed that channels may widen quite rapidly, even from single large flood events. In contrast, the return to equilibrium requires aggradation and revegetation that may take many years. Along with high rates of lateral erosion, overall river width increased in the Minnesota River by about 50% since 1938, contributing an estimated 400,000-600,000 tons/year of sediment. Planview changes to sinuosity measured on the main Minnesota River showed the river's length was shortened by 12.5% since 1938 from channelization at road crossings and natural cutoffs. In preliminary modeling efforts, calculations of sediment transport capacity show increases since 1938 due to greater slope from channelization and river cutoffs with little re-meandering of straightened reaches. Bankfull shear stress was estimated to have increased in 38% of 72 stream segments. The findings have important implications for sediment management in the Minnesota and Upper Mississippi Rivers. Increased flow and anthropogenic changes to land cover and surface/subsurface drainage have purportedly led to higher rates of channel erosion of ravines, bluffs and streambanks. A strategy is needed for hydrologie management and stream restoration to reduce sediment loading in the basin to meet water quality standards. More research is needed to determine the best strategies for prioritizing the balance of these two approaches and the best locations for reducing channel-derived sediment.