Lock (water navigation)

A lock is a part of a navigable waterway system that makes a water "channel" deep enough for vessels to use. The lock controls pool depths, for example in a lock and dam system across a waterway.

The lock is a place where boats that travel up or down a river or canal can be moved to the next higher or lower level. Locks are built in places where the level of the water in the river or canal suddenly changes. This may be because of a waterfall there, or because a dam or a weir has been built, or because some other thing is in the way. The lock is like a big chamber with gates at each end. They have lock gears which empty or fill the chamber with water. Locks help a river to be more easily navigable (easier for boats to travel up and down), or for canals to be built across country that is not level.

A pound lock, the most common sort, has a chamber with gates at both ends that control the level of water.^[1] They were first used in China during the Song Dynasty (960–1279 AD). On rivers locks were built from turf. Most of them have been rebuilt in brick or stone.^[2]

A plan and side view of an empty canal lock. A lock chamber separated from the rest of the canal by an upper pair and a lower pair of mitre gates. The gates in each pair close against each other at an 18° angle against the water pressure on the "upstream" side of the gates when the water level on the "downstream" side is lower.

Principle of operation of a pound lock

For a boat going upstream:			For a boat going downstream:
1–2.	The boat enters the lock.		8–9.	The boat enters the lock.
3.	The lower gates are closed.		10.	The upper gates are closed.
4–5.	The lock is filled with water from upstream.		11–12.	The lock is emptied by draining its water downstream.
6.	The upper gates are opened.		13.	The lower gates are opened.
7.	The boat exits the lock.		14.	The boat exits the lock.

All pound locks have three elements:

• A watertight chamber connecting the upper and lower canals, and large enough for one or more boats. The position of the chamber is fixed, but its water level can vary.
• A gate (often a pair of "pointing" half-gates) at each end of the chamber. A gate is opened to allow a boat to enter or leave the chamber; when closed, the gate is watertight.
• A valve (traditionally, a flat panel (paddle) lifted by winding a rack and pinion mechanism) which allows water to drain into or out of the chamber. Larger locks may use pumps.