A canal pound, aka reach, is the stretch of level water impounded between two canal locks. Canal pounds can vary in length from the non-existent, where two or more immediately adjacent locks form a lock staircase, to many miles.

The longest canal pound in the United Kingdom is between the stop lock on the Trent & Mersey Canal at Preston Brook (Dutton Stop Lock No 76) and the start of the Leeds & Liverpool Canal near Leigh (Poolstock Bottom Lock No 2), a distance of 39.5 miles (63.6 km).
Another long pound is on the Kennet and Avon canal between Wootton Rivers Bottom Lock and Caen Hill top lock

History

Pounds came into being with the development of pound locks to replace the earlier flash locks. A key feature of pound locks was that the intervening level between locks remained largely constant, as opposed to the variable levels created by the opening of flash locks.

Summit pound

A summit pound is formed at a summit

Route summit

A route summit is the highest point on a transportation route crossing higher ground. The term is often used in describing railway routes, less often in road transportation...

 on the canal, and where all the defining locks descend from the pound. Summit pounds are particularly important in canal design, as every boat entering or leaving the pound causes a loss of water. Summit pounds therefore need an independent form of water supply, which may take the form of weir

Weir

A weir is a small overflow dam used to alter the flow characteristics of a river or stream. In most cases weirs take the form of a barrier across the river that causes water to pool behind the structure , but allows water to flow over the top...

s on adjacent river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

s, reservoirs or pumping station

Pumping station

Pumping stations are facilities including pumps and equipment for pumping fluids from one place to another. They are used for a variety of infrastructure systems, such as the supply of water to canals, the drainage of low-lying land, and the removal of sewage to processing sites.A pumping station...

s. Common practice during canal design was to make summit pounds as large as practically possible, in order that losing a lockful of water would not lower the water level too significantly. The Rochdale Canal

Rochdale Canal

The Rochdale Canal is a navigable "broad" canal in northern England, part of the connected system of the canals of Great Britain. The "Rochdale" in its name refers to the town of Rochdale, Greater Manchester, through which the canal passes....

 is a good example of a canal with a relatively short summit pound, which requires restrictions on lock workings at certain times.

Sump pound

The inverse of a summit pound is a sump pound. In contrast to a summit pound, a sump pound is a point where every boat entering or leaving the pound causes an addition of water.

the longest one is the 11 mile Fenny Stratford pound on the grand union canal

Lock pound

A pound which lies between two locks which lie only a short distance apart. Water levels in the pound are liable to fluctuate as the locks are used.

Side pound

A particular type of extremely short lock pound, which is extended sideways to make up for the short distance between locks so as to avoid excessive level fluctuations. Side pounds should not be confused with side ponds (without the u).