Rabbit River Turbidity Total Maximum Daily Load Report

The Rabbit River lies within the Bois de Sioux Watershed of the Red River Basin of the North. The Bois de Sioux Watershed is comprised of portions of Otter Tail, Grant, Wilkin, and Traverse Counties and is approximately 589 square miles in area, 321.4 square miles of which is located in Minnesota with the remaining area lying in the states of North Dakota and South Dakota. Land use in the watershed is dominated by agricultural crop production (2001 estimate of 85.43 percent). Much of the land is extensively drained for that purpose. This TMDL report used a flow duration curve approach and the Soil and Water Assessment Tool (SWAT) to determine the pollutant loading capacity of the Rabbit River under various flow regimes. This approach was used to calculate the general allocations necessary to achieve water quality standards for the impaired stream reach identified in this study. SWAT is a hydrologic model developed by the Agricultural Research Service (ARS) to predict the impact of land management practices in agricultural watersheds over long periods of time. The primary contributing sources of the turbidity impairment appear to be agricultural land soil erosion and streambank erosion in part caused by the extensive hydrological modification that has taken place across the watershed. The degree of turbidity impairment can most often be correlated to higher flows, with sediment reductions near 90 percent needed to achieve the turbidity water quality standard during most and high flow conditions. The numeric target for the TSS equivalent of 25 NTUs was calculated to be 47 mg/l.