The Upper Souris River was placed on the Environmental Protection Agencies (EPA) impaired waters list for low dissolved oxygen (DO). A Total Maximum Daily Load (TMDL) study was conducted to determine possible causes of DO depletion. From sampling and site visits it was determined nonpoint sources contributed the majority of organic loadings to the Upper Souris River. Through preliminary testing, it was determined that sediment oxygen demand (SOD) played a key role in depleting DO levels during winter months and required further investigation. River profile surveying, water quality sampling, and laboratory testing of SOD were carried out to determine parameters required for water quality modeling. SOD tests were conducted to determine impacts of sediment organic contents and temperature on SOD rate. Sediment oxygen demand rates ranged from 0.37 to 1.22 g O2/m2/d. The QUAL2K model was calibrated to simulate DO variations along the study reach under ice covered conditions.