Lake St. Clair measures about 22.5 nautical miles (42 km; 26 mi) from north to south and about 21 nautical miles (39 km; 24 mi) from east to west. Its total surface area is about 430 square miles (1,100 km2).
This is a rather shallow lake for its size, with an average depth of about 11 feet (3.4 m), and a maximum natural depth of 21.3 feet (6.5 m). However, it is 27 feet (8.2 m) deep in the navigation channel which is dredged for lake freighter passage by the U.S. Army Corps of Engineers.
The lake is fed by the St. Clair River, which flows to the south from Lake Huron and has an extensive river delta where it enters Lake St. Clair. This is the largest delta of the Great Lakes System. Other rivers which feed Lake St. Clair are the Thames River and Sydenham River which originate in Southwestern Ontario, and the Clinton River which originates in Michigan.
The outflow from Lake St. Clair travels from its southwestern end into the Detroit River, and then into Lake Erie. – WikiPedia