A canal pound, reach, or level (American usage), 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. The longest level on the Erie Canal in New York was the 60 mile level (actually 64.2 miles) between Henrietta and Lockport.
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.
On some American canals, some pounds acquired nicknames due to the mileage between locks, e.g. the "Eight Mile Level" on the Chesapeake and Ohio Canal was between Riley's Lock (Lock 24) and Edward's Ferry (Lock 25), a distance of about eight miles.
American canals often had a man called a "level walker" (Chesapeake and Ohio Canal), "bank watchman" (Erie Canal) or inspector whose job was to walk along the pound (level) with a shovel, checking for leaks and repairing minor ones before the leak could cause major damage, and calling the section work crew for major ones. His rounds were about 20 to 24 miles daily. Muskrats would cause leaks by burrowing in the canal, as well as competitors such as stage coach lines or teamsters who would sabotage the canal by digging holes in the bank.
Types of pound
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 Morris Canal in New Jersey used Lake Hopatcong to feed its summit pound through a feeder canal. The lake was large enough, that traffic could enter the lake from the canal. Lock 1E was east of the summit pound, and Lock 1W was west of the summit pound. The Panama Canal also uses a lake as its summit 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 11.6-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.
Regulating Water Level
Canals had several devices used to keep the water level from rising too high in the pounds, causing floods, washouts, and other damage.
Waste weirs are used to regulate the height of the water in the canal. Any water higher than the highest board in the waste weir would flow over the board, and out of the canal pound. Boards could be added or taken out to adjust the height of water. Waste weirs often had paddle valves at the bottom, allowing the canal to be completely drained for repairs, emergencies, or at the end of the boating season for winter. On some canals (e.g. the Chesapeake and Ohio Canal) with the exception of short levels, almost every level had a waste weir between the locks.
At locks, there was often a bypass weir which fed water to the flume, allowing water to bypass the lock into the lower pound. This could be adjusted, similar to the waste weir by adding or removing boards to control the water level.
Spillways and Informal Overflows (Mule drinks)
Spillways were also used, allowing the water to spill over and into the surrounding rivers. These may be positioned over the towpath or on the berm side of the canal.
An "informal overflow" on American canals is a dip in the towpath which functions as a spillway, but usually lacking concrete or formal structure. The boatmen sometimes called these "mule drinks", since the mules would drink from the water flowing over the towpath. Informal overflows were often replaced by a waste weir or lined with concrete or masonry to make a more "permanent" spillway.
See Lock (Water Transport) for more info.
If the pound was many miles long, sometimes the canal would be designed with stop gates, so that if there was a break in the canal the whole pound would not lose all its water. Isolating the break would allow the company to make repairs sooner. For instance, on the 14 mile level of the Chesapeake and Ohio canal, there were stop gates put before and after Big Pool so that it (the Big Pool lake) would not have to be drained  in case of breaks or repairs.
Animal Escape Hole
On the Erie Canal, in the three mile long rock cut above Lockport, if a mule or horse fell into the canal pound, there were some escape "holes" cut in the towpath, a depression covered over by wooden boards, so that the animal could go into the "hole" and then be rescued. This was as a result of the bank being practically vertical, making it impossible to retrieve one that fell into the canal.
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