The St. Mary River in southwestern Alberta is a dynamic waterway flowing through the foothills and prairie landscapes of Division No. 3, draining a diverse range of terrain from the Rocky Mountain front ranges down through the dry mixedgrass and foothills fescue subregions — some of the driest country in the province. The river is divided into two distinct fisheries by the St. Mary Reservoir, each with its own character, regulations, and target species. The upper river, above the reservoir, offers classic trout and whitefish water in a foothills setting, while the lower river, below the reservoir, transitions into a prairie fishery with a different suite of species and regulations. The river supports a rich aquatic ecosystem, including several species of conservation interest, and rewards anglers who take the time to understand its seasonal rhythms and unique character.