Control lock
（重定向自Tide lock）
![T.J. O’Brien Lock and Dam: A control lock between Lake Michigan and the Illinois Waterway. Note that there is little or no elevation change (aka: head) between the upper and lower ends of the lock.](/Images/godic/202502/16/T_J_OBrien_Lock_and_Dam5042.jpg")
A control lock, guard lock or stop 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.