The Boundary Waters Treaty of 1909
This treaty was negotiated by the United States and Great Britain to establish mechanisms for resolving issues over the use of water along the Canadian-American boundary. George Gibbons, a lawyer from London, Ontario, was a driving force throughout the negotiations. The treaty created the International Joint Commission, with equal Canadian and American representation, to prevent and settle disputes. At a time when the country had not yet achieved full national sovereignty, the treaty enhanced Canada's powers in its relations with both Great Britain and the United States.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.