The Saint Mary River also provides water for irrigation in Southern Alberta. The Saint Mary River Irrigation District (SMRID) is Canada’s largest irrigation district delivering water through 2,060 kilometres (1,280 mi) of canals and pipelines to approximately 1,505 square kilometres (370,000 acres) of land south of the Oldman and South Saskatchewan Rivers between Lethbridge and Medicine Hat.
The irrigation project was started in 1898, and on September 4, 1900, the first water was brought to Lethbridge by the project. In December 1945 the Canadian Pacific Railway transferred control of the projects to the government of Alberta, creating the Saint Mary and Milk River Development (SMRD). A dam was created in 1946 on the St Mary's River, to service the irrigation system, and water finally reached Medicine Hat in 1954 upon the completion of the St Mary Main Canal.