Streambank erosion rates and loads within a single watershed: Bridging the gap between temporal and spatial scales

The importance of streambank erosion to watershed-scale sediment export is being increasingly recognized. However few studies have quantified bank erosion and watershed sediment flux at the basin scale across temporal and spatial scales. In this study we evaluated the spatial distribution, extent, and temporal frequency of bank erosion in the 5218 ha Walnut Creek watershed in Iowa across a seven year period. We inventoried severely eroding streambanks along over 10 km of stream and monitored erosion pins at 20 sites within the watershed. Annual streambank recession rates ranged from 0.6 cm/yr during years of hydrological inactivity to 28.2 cm/yr during seasons with high discharge rates, with an overall average of 18.8 cm/yr. The percentage of total basin export attributed to streambank erosion along the main stem of Walnut Creek ranged from 23 to 53%. Large variations in individual site, annual rates and percentage of annual load suggested that developing direct relationships between streambank erosion rates and total sediment discharge may be confounded by the timing and magnitude of discharge events, storage of sediments within channel system and the remobilization of eroded material.