Overview:
The smallest lake in the Great Lakes system, Lake St. Clair is not considered to be one of the "Great" lakes.

Lake St. Clair is shallow, averaging 10 feet (3 meters) deep. St. Clair's maximum depth is only 21 feet (6.4 meters), a mere fraction of Lake Superior's maximum depth of 1,332 feet (406 meters).

The northeastern portion of Lake St. Clair is an extensive delta system, the largest within the Great Lakes.

Figures:

LENGTH: 26 miles / 41.8 km.

BREADTH: 24 miles / 38.6 km.

AVERAGE DEPTH: 10 ft. / 3 m.

MAXIMUM DEPTH: 21 ft. / 6.4 m. *

VOLUME: About 1 cubic mile / 4.17 cubic km.

WATER SURFACE AREA:
U.S.: 162 sq. miles / 420 sq. km.
Canada: 268 sq. miles / 694 sq. km.

DRAINAGE BASIN AREA: 4,890 sq. miles / 12,616 sq. km.

SHORELINE LENGTH (including islands):
U.S.
Mainland: 59 miles / 95 km.
Islands: 84 miles / 135 km.
Canada
Mainland: 71 miles / 114 km.
Islands: 43 miles / 69 km.

OUTLET: Detroit River to Lake Erie

NAME: French explorers discovered the lake in 1679, calling it Lac Sainte Claire in honor of Sainte Claire of Assisi whose feast day fell at that time. It was Sainte Claire who established an order of Franciscan nuns called the Order of the Poor Claires. Government officials and mapmakers later changed the spelling to the present form of Saint Clair, or St. Clair. This led to some confusion as to the true origin of the name. Another theory is that the lake was named after the first governor of the Northwest Territory: General Arthur St. Clair.

* Deepest measurement outside dredged navigation channel, which has a depth of 27 feet / 8.2 meters.
Reference: Lake Erie and Lake St. Clair Handbook (1993), edited by Stanley J. Bolsenga and Charles E. Herdendorf, WA