Streambank slumping and its contribution to the phosphorus and suspended sediment loads of the blue earth river, minnesota

ABSTRACT:

Suspended sediment transported in river systems has the ability to degrade water quality and impart negative impacts downstream as material is deposited. This study examines the significance of valley wall slumping as a source of both suspended sediment and phosphorus in the Blue Earth River. Several streambanks on the Blue Earth River in south central Minnesota were repeatedly surveyed from 1997 to 2000 to determine annual rates of streambank slumping. Volumetric changes between digital elevation models from successive topographic surveys were converted into changes in mass, and, subsequently, annual erosion rates were calculated based on surface area. An erosion rate constant was derived based on surface area of surveyed sites and applied to an inventory of eroding escarpments along the entire river to estimate the contribution of streambank slumping to the total suspended sediment (TSS) and total phosphorus (TP) loads in the Blue Earth River. Erosion rates for surveyed sites ranged from 544 to 3,995 t ha⁻¹ yr⁻¹ (242 to 1,782 tons ac⁻¹ yr⁻¹), with an average rate of 2,154 t ha⁻¹ yr⁻¹ (961 tons ac⁻¹ yr⁻¹). The derived erosion rate constant for slumping sites was 0.23 t m⁻² yr⁻¹ (0.024 tons ft⁻² yr⁻¹). Between 75,804 and 106,213 t (83,536 to 117,047 U.S. tons) of sediment is delivered to the river annually from all eroding escarpments inventoried, with between 28,047 and 39,299 t (30,908 and 43,307 U.S. tons) transported by flow as TSS. Streambank slumping accounts for 31% to 44% of the TSS load at the mouth of the Blue Earth River. The percentage of the TP load originating from streambank slumping is estimated to be from 7% to 10%, with annual contributions of 12 to 17 t P (14 to 19 U.S. tons).