Sediment Oxygen Demand (SOD) is an important variable in the oxygen balance of rivers, in particular during the winter under low flow and extensive ice-cover. In 1989, a method to measure SOD in-situ was developed and used to measure winter SOD rates in the Athabasca River. The main objectives of the 1990 study were to determine if SOD rates increased over the winter and to determine the longitudinal trend of SOD in the Athabasca River and the Wapiti-Smoky River system.