CHANNEL CROSS-SECTIONAL ENLARGEMENT ON THE MINNESOTA RIVER, MINNESOTA AND ITS ROLE IN SEDIMENT PRODUCTION

The Minnesota River has experienced a large increase in stream flow over the past century. Increases have occurred across the entire flow duration distribution, with post-1975 flow at Mankato, site of the longest record in the basin, being roughly twice the pre-1975 flow at any given exceedance probability. Because the Minnesota River and it major tributaries flow through alluvial channels that meander across relatively undeveloped floodplains, the system presents an opportunity to study the geomorphic response of alluvial rivers to change in formative discharge. In this study, we used aerial photographs to measure channel width along sub-reaches approximately 10 meander bends in length along the main stem Minnesota River and major tributaries. Overall width increases were on the order of 0.4 percent per year between 1937 and 2015. At-a-station analysis of width vs. discharge shows that width has increased even at low stages. Furthermore, width change is largest in reaches that have undergone the most lateral channel migration. Channels thus appear to be enlarging through differential rates of erosion and deposition, rather than simply by upward adjustment of the vegetated zone. Because observed width change is consistent with that predicted by a simple hydraulic geometry relationship between width and discharge, it appears likely that width has responded rapidly to the increase in flow. A regional hydraulic geometry relationship for cross-sectional area as a function of drainage area indicates that most of the cross-sectional area in the basin is located along the main stem or trunk reaches of large tributaries. Higher-order channels are thus disproportionally important as a potential source of widening-related sediment. However, while widening probably represents a large gross source of sediment, much of this sediment, particularly coarser sand-size material, is probably stored elsewhere in the basin. Including widening in a sediment budget compiled for a 167-km reach of the lower Minnesota River implies that both sand and silt/clay likely undergo significant amounts of net storage. However, the storage is probably not sufficiently large to allow overall channel depth to adjust rapidly, meaning that depth adjustment probably occurs on timescales that are longer than those required for width adjustment.