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.
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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.
Pounds can be described in various ways according to their situation;
A summit pound is formed at a summit 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 weirs on adjacent rivers, reservoirs or pumping stations. 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 is a good example of a canal with a relatively short summit pound, which requires restrictions on lock workings at certain times.
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.
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.
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).