In its 1914 Order of Approval allowing increased hydropower development in the St. Marys River, the International Joint Commission established the basic objectives for and limits to the regulation of Lake Superior’s outflow. The conditions for regulation given in the original Order acknowledged the needs of various interest groups on Lake Superior and the St. Marys River, including navigation, hydropower and riparian owners.
Since 1978, the Commission has issued several supplements to the original Order of Approval. As a result, the Order now specifies that the level of Lakes Michigan and Huron must also be considered when determining the outflow from Lake Superior. In addition, the Order addresses concerns for the fishery in the rapids.
Various regulation plans have been developed and used to determine Lake Superior outflows. Each of these plans has adhered to the operating conditions contained in the Commission’s Order. The main objective of the present regulation plan is to determine a flow that will bring the levels of Lake Superior and Lakes Michigan and Huron to nearly the same relative position within their respective ranges of actual historic levels. At the same time, the plan tries to prevent the level of Lake Superior from rising above or falling below certain water levels specified in the Order. The plan also contains provisions to safeguard against high levels in the harbor below the locks, provides a fixed minimum release, limits winter flows, and employs a forecast of future water supply conditions.
The ability to regulate the outflow from Lake Superior does not mean that full control of lake levels is possible. This is because the major factors affecting the water supply to the Great Lakes; over-lake precipitation, evaporation and runoff; cannot be controlled; neither can they be accurately predicted over the long term.
The regulated release of water from Lake Superior is made through the various structures located on the St. Marys River. The allocation of flow to these facilities is determined monthly, based on the outflow specified by the regulation plan and the conditions given in the Order of Approval. This water is used for domestic water supply, navigation through the locks, hydropower production, and to maintain fish habitat in the rapids.