A channel contained within a lock wall which allows the water level to be controlled without spilling it out into the air as happens when the wicket or butterfly valve is used. Locks such as those in the Cascade Locks Park have large vertical lifts so that when a boat is traveling upsream, using the wickets or butterfly valves in the upper gate to fill the lock would cause a stream of water to fall onto the deck of the boat waiting below. To prevent this, deep locks had a two-foot square opening equipped with a sluice gate on the upstream side of the upper gate. When the sluice gate was opened, water could flow through an internal passageway in the stonework of the lock, exiting at the botom of the lock below the waiting boats hull.
