The St. Marys River is the natural outlet of Lake Superior. Near Sault Ste. Marie, Ontario and Michigan, a rock ledge at the head of the St. Marys River Rapids is the natural control of the Lake Superior outflows. The rock ledge acts like a weir, which permits flows to increase and decrease relative to Lake Superior's level. This self-regulating feature, along with Lake Superior's immense storage capacity, makes the lake a highly naturally regulated water body.
This natural outflow control began to change as early as 1822, when water was diverted from just above the rapids for operation of a sawmill. A ship canal was constructed in 1855. Subsequently, various expansions to these facilities took place, culminating in the flow control facilities we see today. They now consist of three hydropower plants, five navigation locks and a 16-gated control structure (called the Compensating Works) at the head of the St. Marys Rapids. (Click on the adjacent map to see an enlarged version; select various locations to see photos of these structures.)
Since the Compensating Works (see photo) were completed in 1921, Lake Superior outflows have been regulated. This regulation is carried out by the International Lake Superior Board of Control in accordance with conditions specified by the International Joint Commission. Lake Superiors outflows are adjusted monthly, taking into consideration the water levels of both Lake Superior and lakes Michigan and Huron (which are considered one unit hydrologically). The objective, called systemic regulation, is to help maintain the lake levels both upstream (Superior) and downstream (Michigan-Huron) close to their long-term seasonal averages, and to prevent any one of them from getting either extremely high or low. The regulated outflow is achieved by adjusting the flows through the three hydropower plants, after the amounts required for lockages, the St. Marys Rapids fishery and industries at Sault Ste. Marie are met. A minimum of one-half of one gate is kept open at the Compensating Works to maintain water in the rapids for fish spawning. More gates are opened when flows in the river exceed the capacities of the hydropower plants. Hydropower diversions and lockage data are compiled monthly by the Lake Superior Board.