Contributions to Suspended Sediment and Total Phosphorus Loads from Three Distinct Holocene Alluvial Deposits Comprising Streambanks in Central Iowa, USA.

There is a growing body of evidence that much of the phosphorus delivered to the Mississippi River from agricultural landscapes originates as sediment-bound P from streambank and streambed erosion. A detailed study was initiated in 2014 with the goal to identify and quantify in-channel sediment-P sources (i.e., streambanks, bed) and sinks (i.e., storage) in Walnut Creek, a perennial, warm-water stream that drains a 5218 hectare, agriculturally-dominated watershed in central Iowa. A primary project objective is to quantify contributions to suspended sediment and total P loads from three distinct Holocene alluvial deposits that comprise Walnut Creek streambanks.

Streambank stratigraphy is comprised of the Camp Creek, Robert’s Creek, and Gunder members of the DeForest Formation, with each individual member being deposited during a distinct time range during the Holocene. These members represent 11, 10, and 8% of the channel’s total eroding streambank surface area, respectively. Colluvial material on the lower portions of the streambanks represents the remaining 71% of eroding bank surface area, and is being investigated as well. Member-specific erosion rates are being quantified via erosion pins, channel cross section measurements, and terrestrial LiDAR scans. Erosion estimates are combined with member attributes (i.e., thickness, bulk density, texture, total P concentration) and times-series stream gauge station data to provide an estimate of contributions to annual suspended sediment and total P loads. Coupling temporal, site-specific streambank recession measurements and stream discharge data with time-lapse photography is assisting to identify the dominant erosional processes (e.g., hydraulic, slumping, subaerial) associated with individual members.