Evaluating Causal Factors That Influence the Spatial and Temporal Variability of Streambank Erosion in Iowa

Highlights Critical shear stress and erodibility were quantified for Iowa streambanks at regional and seasonal scales. Variations in critical shear stress and erodibility were apparent across landform areas. Values of critical shear stress in northern Iowa were less than those in southern Iowa. Seasonal variations in critical shear stress and erodibility are due to soil moisture and freeze-thaw cycles. Abstract. Streambank erosion exhibits high variability both spatially and temporally, and few studies have attempted to characterize the causal factors of the variability at regional and seasonal scales. This study combines geotechnical field measurements with historical trends of soil moisture and temperature to quantify the spatial and temporal variability of critical shear stress and erodibility in streambanks of the Major Land Resource Areas of Iowa (MLRAs). The data suggest that till-derived soils in north-central and northeast Iowa lack cohesion compared to loess-derived soils in southern Iowa. Thus, critical shear stress values from the northern parts of the state were significantly less than those from western and southern Iowa. A multivariate regression equation was developed using properties of texture and bulk density to quantify critical shear stress values for all stream bank soil series in the state. The cumulative distribution functions of these values visually reflect the separation between the critical shear stress values of the different MLRAs, with the loess-derived soils in western and southern Iowa having similar, overlapping patterns, in contrast to MLRAs in north central and northeast Iowa with the till-derived soils. Temporally, the critical shear stress reaches minimum values during March when soil moisture is high and there are several freeze-thaw events. This is most apparent with bank soils in eastern Iowa. Keywords: Cohesion, Critical shear stress, Erodibility, Sediment, Streambank erosion.