CHANNEL EVOLUTION AS PHOSPHORUS AND SEDIMENT SOURCES IN CENTRAL IOWA

Within the Des Moines Lobe region of central Iowa, post settlement alterations to the landscape and fluvial systems have been dramatic. Among the most pervasive alterations are the conversion of the land surface to row crop agriculture, the introduction of subsurface drainage, and the straightening of waterways. These changes have modified the landscape in two phases. First, with the advent of extensive row crop agriculture widespread upland erosion lead to channel aggradation and deposition of alluvial material along the banks of the channels. Second, the combination of soil conservation practices, which led to a decrease in sediment delivery, and the previous implementation of subsurface drainage along with channel straightening have led to incising channels with unstable banks. These unstable banks often lead to high amounts of streambank erosion during peak flow events that present a serious threat to water quality in Iowa. In order to assess the magnitude of this erosion, bank recession rates were estimated using erosion pins within Onion Creek, a small stream draining an agricultural watershed in Boone and Story counties, IA. Pin measurements indicate that bank erosion mobilized more sediment than was exported between March 2012 and May 2015, and that bank erosion accounted for 35% of the cumulative particulate phosphorous exported. These results suggests that floodplain and channel-bed storage are important sinks within this system and that streambank erosion must be accounted for if we are to create accurate phosphorous and sediment budgets for the state of Iowa.