Provincial Watersheds, Historical

Note: This data has been replaced by the Ontario Watershed Boundaries (OWB). We are no longer updating this data. It is best suited for historical research and analysis. A watershed, also known as a catchment basin, includes all land that is drained by a watercourse and its tributaries. Watersheds are split into four categories: * primary * secondary * tertiary * quaternary These divisions are based on the federal framework originally known as the Water Resources Index Inventory Filing System. Ontario has 3 primary watersheds: * Great Lakes – St. Lawrence * Southwestern Hudson Bay * Nelson River Secondary watersheds are subdivisions of primary watersheds in Ontario. Most secondary divisions are either large river systems or groupings of small coastal streams. Tertiary watersheds are subdivisions of secondary watersheds in Ontario. Quaternary watersheds are subdivisions of tertiary watersheds. There are more than 1000 quaternary watersheds in Ontario.