The Souris River, a slow moving praire river, originates in the Canadian province of Saskatchewan. The Souris River enters into the United States near Sherwood, North Dakota. The upper portion of the Souris River, from the border with Saskatchewan, Canada to Lake Darling, a total of 43 .4 miles, has been on the North Dakota Section 303(d) Environmental Protection Agency (EPA) list of impaired waters since 1998 (NDDoH, 2008). Low dissolved oxygen (DO) concentration in the river has been problematic, resulting in fish kills on the upper reach of the Souris River. The primary goal of the project is to determine source(s) of contaminations that cause seasonal variation in dissolved oxygen concentration. Objectives ofthis research include: 1) studying the nature and extent of the impairment using existing data, 2) executing a comprehensive year long sampling plan, and 3) analyzing sample data to identify the major sources of DO impairment. Results from this study will be used by North Dakota State Department of Health (DoH) for development of a Total Maximum Daily Load (TMDL) report to the EPA. The reach was expanded into Canada, totalling 52 river miles, between Glen Ewen, Saskatchewan, CA and Mouse River Park, ND. This reach was sampled 29 times during the period of this study. During winter months, DO under the ice was constantly below 1 mg/L. During the summer diurnal swings of DO caused the stream to fall below the designated 5 mg/L standard in the early morning hours. Sediment oxygen demand was determined to be the main stressor. Two contaminant sources that contribute to DO variation on the Souris River were identified, livestock ranching practices and the physical alteration of the river. Ranching is the leading cause of impairment. Livestock in the riparian area contribute organic inputs and stream bank erosion. Physical alterations lead to DO variation in two ways. Upstream impoundments eliminate scouring flows allowing sediment to build up. Downstream impoundments back up water into study reach contributing to sedimentation. Low head dams create hydrologic flow issues which lead to dissolved oxygen depletion.