Lake Erie is the fourth largest and shallowest of the five Great Lakes, and is the only one with its floor above sea level. Generally the eastern portion of the lake is deep, while the western part is shallow and has many islands. The northern shore of the lake is in the Province of Ontario, Canada, and the southern shore is shared by four states of the United States. The lake water comes from L. Huron through the St. Clair River and the Detroit River (via L. St. Clair) at the west end. The only natural flow-out is at the northeast end through the Niagara River; besides, the Welland Canal bypasses Niagara Falls leading to L. Ontario.

Extensive commerce is carried out between the harbors on the lake as well as to and from the other Great Lakes. L. Erie is thus quite important to the St. Lawrence seaway system. The western shore side is one of the most highly industrialized and densely populated areas in the United States. The lake reached a fairly high level of eutrophication similar to that of L. Ontario before the 1970's. On the other hand, L. Erie is known, together with L. Michigan, for abundant fish catch which is much greater than that in other Great Lakes.