Locks and Dams
Dams are built on shallow rivers to hold back water and form deeper navigation "pools." for safe navigation, most pools in the United States are maintained at a constant minimum water depth of 9 feet. Dams make it necessary for river vessels to use a series of locks to "step" up or down the river from one water level to another. The three steps in the "LOCKING THROUGH" process are shown below.
For a boat going downstream, the lock is first filled by opening the filling valve. The drain valve and upstream and downstream gates are closed, so the level of the water in the chamber rises to the upstream level. The upstream gate opens and the boat moves in.
To lower the boat, the gates are closed behind it, the filling valve is closed, and the drain valve is opened. The higher water in the lock chamber drains to the downstream level within minutes. The downstream gate is then opened and the boat moves out on the lower water level. The process is reversed for a boat going upstream.