Which treaty established the basic international boundary rules for the Great Lakes?

The Boundary Waters Treaty of 1909 established the basic international rules for managing the Great Lakes boundary waters.

The United States and the United Kingdom, acting for Canada, signed the treaty on January 11, 1909. It created the International Joint Commission, or IJC, to help resolve disputes and approve projects that could affect water levels or flows across the shared border.

The treaty covers boundary waters between the two countries, including the Great Lakes. Its central principle is that neither country should unilaterally alter shared waters in ways that cause injury across the border. The IJC can investigate issues, conduct studies, and issue orders of approval for certain water-related projects.

The treaty is different from the Great Lakes Compact, a later agreement among U.S. states and provinces concerning water withdrawals and diversions. It is also not a peace treaty ending a war. Its enduring role is cooperative oversight of shared international waters.