A control lock, or guard lock differs from a normal canal lock in that its primary purpose is controlling variances in water level rather than raising or lowering vessels. A control lock may also be known as a tide lock where it is used to control seawater entering into a body of fresh water.

Examples

The T. J. O’Brien Lock and Dam at Chicago, Illinois is a guard lock that controls the outflow of water from Lake Michigan

Lake Michigan

Lake Michigan is one of the five Great Lakes of North America and the only one located entirely within the United States. It is the second largest of the Great Lakes by volume and the third largest by surface area, after Lake Superior and Lake Huron...

 into the Illinois Waterway

Illinois Waterway

The Illinois Waterway system consists of of water from the mouth of the Calumet River to the mouth of the Illinois River at Grafton, Illinois. It is a system of rivers, lakes, and canals which provide a shipping connection from the Great Lakes to the Gulf of Mexico via the Mississippi River. The...

 while locking vessels through between the waterway and Lake Michigan.

Lock 8 near the south end of the Welland Canal

Welland Canal

The Welland Canal is a ship canal in Canada that extends from Port Weller, Ontario, on Lake Ontario, to Port Colborne, Ontario, on Lake Erie. As a part of the St...

 at Port Colborne, Ontario

Port Colborne, Ontario

Port Colborne is a city on Lake Erie, at the southern end of the Welland Canal, in the Niagara Region of southern Ontario, Canada...

, Canada

Canada

Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

 is a guard lock. Due to the large expanse of shallow water in Lake Erie

Lake Erie

Lake Erie is the fourth largest lake of the five Great Lakes in North America, and the tenth largest globally. It is the southernmost, shallowest, and smallest by volume of the Great Lakes and therefore also has the shortest average water residence time. It is bounded on the north by the...

, changes in wind direction and force create water level changes as great as 11 feet (3.4 m) at Port Colborne. Lock 8 controls the water level in the canal, keeping it independent of the fluctuations of Lake Erie, but allows ships to enter Lake Erie regardless of its level.