Sediment and phosphorus dynamics within the channel and floodplain of Walnut Creek, Iowa
Excessive loadings of sediment and phosphorus (P) to waterways are prime water quality impairments within both the agricultural Midwestern United States of America (USA) and globally. Streambanks, floodplains, and channel beds may all significantly influence watershed export of suspended sediment (SS) and total phosphorus (TP), yet mechanisms at the watershed scale are poorly understood. This study seeks to investigate the dynamic influences of streambank erosion, channel-floodplain connectivity, and in-channel storage on SS and TP export within Walnut Creek, a third-order, alluvial stream channel in central Iowa, USA. Channel cross sectional change data suggest that Walnut Creek is currently experiencing degradation and widening (stage IV of channel evolution) in response to historic land use and hydrologic alterations. Over study duration, Walnut Creek’s streambanks were estimated to contribute the equivalent of 4.0 to 43.9% of previously reported annual watershed SS loads, and the equivalent of 2.7 to 37.5% of TP loads. It was estimated that colluvial material, generated from streambank mass wasting and subaerial weathering and erosion processes, dominated bank SS and TP contributions to loads. An increase in channel cross sectional area of ~17% over 16 years has reduced the lateral connectivity between Walnut Creek and its floodplain. Overbank discharge threshold (i.e., discharge required to force streamflow to exit channel and inundate floodplain) increased 15% over the same time period, resulting in decreases in annual suspended sediment (-24%) and TP (-26%) fluxes to floodplain storage. Walnut Creek was estimated to store sediment at the rate of ~2.7 Mg per m channel length, and TP at the rate of 0.7 Mg per m channel length. Sinuous reaches (sinuousity > 1.2) stored a significantly greater (p < 0.001) volume of sediment than straight reaches, and also exhibited significantly greater (p < 0.001) sediment depth. In-channel storage may be a significant component of watershed sediment and TP budgets. Total in-channel sediment storage was estimated at 36,554 Mg, ~3.25 times greater than the 2015 watershed SS load. Rehabilitation strategies that decrease channel conveyance and velocities (e.g., introduced meandering) may increase streambank stability, restore channel-floodplain connectivity, and reduce watershed export of SS and TP.