ABSTRACT
The Minnesota River and major tributaries have experienced large increases in discharge over the past century. Aerial photograph-based measurements of channel width were made for the 1938–2015 period at 16 multibend subreaches by digitizing the area between vegetation lines and dividing by centerline length. Results show considerable increases in width for the mainstem (0.62 ± 0.10%/year) and major tributaries (0.31 ± 0.08%/year) but are inconclusive for smaller channels (width < 25 m). Width changes for a 146.5 km reach of the lower Minnesota River between 1938 and 2008 are similar to that from the subreach-scale analysis. Widening was associated with lateral centerline movement and temporal change in at-a-station hydraulic geometry for water surface width, indicating that widening is associated with cross-sectional change and not simply upward movement of the vegetation line. Digital elevation model analysis and regional hydraulic geometry show that the mainstem and larger tributaries account for the vast majority (~85%) of bankfull channel volume. High-order channels are thus disproportionately responsible for sediment production through cross-section enlargement, although floodplains or off-channel water bodies adjacent to these channels likely represent important sediment sinks. Because channel enlargement can play an important role in sediment production, it should be considered in sediment reduction strategies and carefully evaluated in watersheds undergoing long-term increases in discharge.
