Lock (water navigation)

A lock is a way of raising and lowering boats and ships between stretches of water of different levels on rivers and canals. There is a fixed chamber where the water level can be raised and lowered.

A pound lock, the most common sort, has a chamber with gates at both ends that control the level of water. 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.

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.