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 main stem (0.62 ± 0.10%/y) and major tributaries (0.31 ± 0.08%/y) but are inconclusive for smaller channels (width < 25 m). Width change for a 146.5-km reach of the lower Minnesota River between 1938 and 2008 is 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 main stem 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 in the Minnesota River basin and carefully evaluated in other watersheds undergoing long-term increases in discharge.
Bibliographical noteFunding Information:
We would like to thank a large number of colleagues and students who collected data or provided ideas in support of this study. The work benefited from feedback provided by Karen Gran, Peter Wilcock, Gary Parker, Enrica Viparelli, Carrie Jennings, Hervé Piégay (who suggested the use of a photographically based measure of bankfull width), Utah State University undergraduate researcher Tim Beach and graduate student Sara Kelley, and Seattle University undergraduate researchers Jenny Graves and Matt Tiedeman. We also appreciate the thoughtful review comments provided by Richard Marston and two anonymous reviewers—they improved the paper significantly. Work was supported by the following organizations: The Minnesota Pollution Control Agency ; the National Center for Earth Surface Dynamics, a U.S. National Science Foundation -sponsored Science and Technology Center ( EAR-0120914 ) and NSF CBET-1209445 ; Seattle University ; and the Minnesota Corn Growers Association, which supported the initial 1938–2008 channel widening study on the lower Minnesota River and for which John Nieber, David Zumr, Jason Ulrich and Mikhail Titov all provided assistance.
- Channel width
- Hydraulic geometry
- Minnesota River
- Sediment budgets