The St. Clair Watershed (HUC 04090001) has a land area of approximately 5,400 square miles and is located in Michigan and Ontario, Canada. The hydrology of the watershed is dominated by the St. Clair River, which flows into Lake St. Clair and eventually into Lake Erie. The watershed receives an average annual precipitation of 29.7 inches, with a significant amount of this falling as snow. The snowpack in the watershed is influenced by lake-effect precipitation, which can result in heavy snowfalls in certain areas. Surface water quality in the watershed is influenced by agricultural activities, urbanization, and industrial discharges. The main reservoir in the watershed is Lake St. Clair, which serves as a source of drinking water, recreation, and fisheries. Climate trends in the watershed include increasing temperatures, changes in precipitation patterns, and more frequent extreme weather events.