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A canal pound, or 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 and Mersey Canal at Preston Brook (Dutton Stop Lock No 76) and the start of the Leeds and 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.
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 canal, which is one of three which cross the Pennines, is 32 miles (51 km) long, but the summit pound is just 0.8 miles (1.3 km). To the north and east, 36 locks descend to Sowerby Bridge, while to the south and west, another 56 locks descend to Castlefield Junction, on the edge of Manchester. The summit pound is 600 feet (183 m) above sea level, and is one of the highest summit pounds in Britain. In order to keep it in water, the company built a total of eight reservoirs near the summit pound. Hollingworth Reservoir was at a lower level than the summit, and so a steam engine was installed to pump the water up to a 4-mile (6.4 km) feeder, which delivered it to the summit pound. One reason for the present restriction on boat movements over the summit is that the water rights and associated works were sold to various local authorities in 1923 under the terms of the Oldham and Rochdale Corporations Water Act, as the subsequent increase in leisure traffic was not anticipated. The first canal to be constructed with a summit pound in the United Kingdom was the Newry Canal, completed in 1741. which linked Newry to Lough Neagh. The summit pound was 5.2 miles (8.4 km) long, between Poyntz Pass and Terryhoogan, but its water supply was a cause of problems over many years.
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 11.6-mile (18.7 km) Fenny Stratford pound on the Grand Union Canal, between Cosgrove Lock, which starts the ascent to the Braunston summit to the north, and Fenny Stratford lock, which starts the ascent to the Bulborne summit to the south. Every sump pound needs somewhere to discharge the surplus water, and in this case, a large viaduct and aqueduct immediately to the south of Cosgrove Lock carries the canal over the River Great Ouse, which serves that function.
A lock pound lies between two locks which are only a short distance apart. Water levels in the pound are liable to fluctuate as the locks are used. Boats entering the pound from the lower level remove a lockful of water from the pound, while those using the upper lock add a lockful of water. Because the maximum level in the pound is normally controlled by a bywash weir at the lower lock, some of the water from the upper lock may be lost over it, and if the lower lock has a deeper fall, there is a net loss of level when a boat passes through the pound.
A side pound is 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. An example of this is the Caen Hill locks on the Kennet and Avon Canal. 29 locks are required to climb the hill that leads to Devizes. The first seven and the last six have conventional pounds, but the middle sixteen have large side pounds, enabling all 16 to be fitted into a distance of around 0.6 miles (1.0 km).
Serving a similar function are the side ponds on lock flights such as the Foxton flight. The ten locks are organised as two staircases of five chambers each, where each lock can discharge water into the pond below it and receive water from the one above it. Although connected to the locks by sluices, they are still often called side pounds, as they are maintained at the level at which an intermediate pound would be if one were present. The term side pond is also used to refer to a water saving basin, which is maintained at a level between the upper and lower level of a single lock. Most of the locks on the Grand Union Canal between Whilton Locks and lock 45 at Bulbourne, the junction with the Wendover Arm, were built with two such side ponds, although they are currently disused.