Near-Channel Sediment Sources Now Dominate in Many Agricultural Landscapes: The Emergence of River-Network Models to Guide Watershed Management

Abstract:

Detailed sediment budgets for many agricultural watersheds are revealing a surprising story – that sediment is no longer primarily sourced from upland fields, but instead from near-channel sources. This is the case for the Minnesota River Basin (MRB) where an intensification and expansion of agricultural drainage combined with increased precipitation has (1) reduced surface runoff and erosion, (2) amplified streamflows, and (3) accelerated both near-channel sediment generation and sediment transport. Bluffs and streambanks in the MRB are now the dominant sources of sediment, but these features are not easily incorporated into traditional watershed-scale, sediment-transport models. Instead, we are advancing a network-based modeling framework that explicitly considers sediment sources, transport, and storage along a river network. We apply this framework to bed-material sediment transport in the Greater Blue Earth River Basin, the major sediment-generating subbasin of the MRB, where a recent sediment budget has quantified the locations and rates of erosion and deposition of major sediment sources and sinks (i.e., bluffs, streambanks/floodplains, agricultural fields, and ravines) over millennial and decadal timescales. With the river network as the basis of a simple model, inputs of sediment to the network are informed by the sediment budget and these inputs are tracked through the network using process-based time delays that incorporate uniform-flow hydraulics and at-capacity sediment transport. We explore how this sediment might move through the network and affect the variability of bed elevations under cases where the mechanisms of in-channel and floodplain storage are turned on and off. We will discuss timescales of movement of sediment through the system to better inform legacy effects and hysteresis, and also discuss targeted management actions that will most effectively reduce the detrimental effects of excess sediment.