Lake Saint Clair is about 26 mi (41.8 km) from north to south and about 24 mi (38.6 km) from east to west. This is a rather shallow lake for its size with an average depth of about 11 ft (3.4 m), and a maximum natural depth of 21.3 ft (6.5 m). However, it is 27 ft (8.2 m) deep in the navigation channel which has been dredged for lake freighter passage. Lake Saint Clair is fed with fresh water flowing out of Lake Huron to its north via the St. Clair River, which has an extensive river delta, the largest one within the Great Lakes System. Also, the Thames River and Sydenham River flow into Lake Saint Clair from Southwestern Ontario, and the Clinton River flows into it from Michigan. The outflow from Lake Saint Clair flows from its southwestern end into the Detroit River, and thence into Lake Erie.
The tarry time (i.e the time between entering and leaving) of the water in Lake St. Clair averages about seven days, but this can vary from as little as two to as many as thirty days, depending on the direction of the winds, the water circulation patterns, and the seasonal amount of water that is flowing out of Lake Huron. If the water flows through the navigation channel, which is maintained by the U.S. Army Corps of Engineers, the time the water remains in the lake is about two days.