Characteristics of in-Channel Sediment Storage, Walnut Creek, Central Iowa, USA.

Phosphorus (P) has potential to act as the primary driver of algal blooms and accelerated eutrophication in freshwater systems, and may contribute to hypoxic conditions in coastal areas (e.g., Gulf of Mexico). There is a growing body of evidence that much of the P delivered to the Mississippi River from agricultural landscapes originates as sediment-bound P from streambed and streambank erosion. Thus, better understanding of in-channel processes (e.g., sediment storage) is needed to effectively reduce sediment and P loads in agricultural watersheds. Although in-channel storage is an important piece of sediment and P budgets, its complex nature and laborious measurement result in it rarely being quantified in the field. In addition, studies that categorize storage volume into distinct depositional features at the spatial scale are exceptionally rare.

For the current study, in-channel sediment storage was quantified within 13.5 km of Walnut Creek, a perennial, third-order stream located in Jasper County, Iowa. Stored sediment volume was categorized into 7 feature classes based on location within the channel and depositional processes. Feature class sediments were analyzed for total P, aggregate stability, bulk density, and texture to assess P dynamics and resuspension potential. Spatial trends in storage are being evaluated based on a number of factors including watershed position, stream reach sinuosity, stream gradient, and stream power. As part of a larger study, storage results will be coupled with stream hydrology and sediment chemistry analyses (e.g., P buffering capacity, equilibrium P concentration) to elucidate the timing and spatial distribution of P sources and sinks within agricultural watersheds. This knowledge will allow producers, land managers, and policy makers to more effectively locate in-channel conservation practices that yield the greatest P load reduction per conservation dollar spent.