ABSTRACT
The Minnesota River carries the largest load of sediment to the Mississippi River in MM, most of which comes from channel sources of sediment. This study investigates bank retreat in the lower Minnesota River from 1938 to 2009. Specifically, we asked, How have changes to hydrology affected river form, sediment transport, and deposition in the lower Minnesota River and how did that affect ecohydrological processes? It was hypothesized that channel straightening, reduction in floodplain access, and streamflow increases contributed to increased channel-derived sediment load and decreased point bar deposition. Secondly, it was hypothesized that hydrologic changes have reduced woody riparian vegetation on sandbars, further promoting channel width increased by reducing the narrowing process that occurs on sandbars by plant growth. To quantify channel sediment and phosphorus loading rates in the lower Minnesota Rive: historic aerial photos were used to measure channel change; long-term erosion and deposition rates within the river corridor were assessed and calculations of channel sediment transport rates were made. Results from this study showed that the Minnesota River has widened by 52% between Mankato and St. Paul from 1938 to 2009, on average contributing 280,000 Mg/year with 153 tons of total phosphorus. Additionally, the river has shortened by 7% from 1938 to 2009, increasing bankfull shear stress and stream power. Sediment deposition rates in certain parts of the floodplain have increased since European settlement (circa 1840–1850) by an order of magnitude, approximately tracking the increased loads of sediment that occurred in that time. Ecohydrological studies showed that establishment of woody riparian plants has been inhibited on sandbars by prolonged summer flow duration and scour at high flow. This favored plant species that spread by clonal growth such as sandbar willow (Salix interior). Additional research done since the completion of the original study, published in 2013, was summarized, adding new insight to channel adjustment and ecohydrological processes, particularly on sandbars. Findings from this study provide an example of the geomorphic and ecohydrological consequences of flow increases from climate and land-use changes and inform management of large floodplain river systems.
