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Stretch of level water impounded between two canal locks

A canal pound (from impound), 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 kilometres/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 mile (64 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 mile (103 km)) 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 8 mile (13 km).

Making a channel that would not leak water is not easily done. Gravel soil or light soil will leak for a few years until it settles and hardens. Puddle or clay would be used to line the channel, but that was not foolproof. The Union Canal and the Schuylkill canal where the canal prism (the shape of the canal as seen in cross section) was blasted through limestone leaked so badly, they had to line the channel with timber. After some years, the canal bed would settle and harden, and puddling would no longer be needed.

One main concern to the engineer was the canal's water supply. Pumping would contribute greatly to operating expense, especially at the summit level, hence gravity fed water is preferred. Feeder canals could be 20 mile (32 km) to 30 mile (48 km) in length, if needed. The canal bed would have to slope so that the water would not flow too fast down the canal and impede the progress of upstream boats. If a dam was not built, often water had to be pumped. The Chesapeake and Ohio put a steam pump near the South Branch, near mile 174, which had a capacity of about , since the water from Dam No. 8 was insufficient. This problem is particularly acute on summit pounds (see below). On the Delaware Division canal, the Delaware River was made to pump its own water into the canal, using an undershot water wheel which was connected to another wheel which had buckets attached, thereby lifting the river water up to the canal.

Where ice would form during winter, the canal prism would usually be closed and drained. During this time, sand bars would be dug out, locks and other structures would be fixed. Exceptions to this would include wartime necessities, i. e. the Chesapeake and Ohio Canal was left open in the winters of 1861–1862 because of the American Civil War.

American canals often had a man called a "level walker" (Chesapeake and Ohio Canal), "bank watchman" (Erie Canal), towpath walker 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. Other duties included checking the waste weir gates to see if they were letting out the correct amount of water, checking aqueducts for damage, as well as being called in the night to search for missing persons supposedly drowned in the canal. For culverts and flood gates, a row of heavy planks, interlocked with tongue and groove, would be driven across the canal (similar to a cofferdam) above and below the break, and would swell when wet. Canal companies would often post bounties for muskrats, e. g. the Middlesex Canal. 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 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 mile (8 km) long, between Poyntz Pass and Terryhoogan, but its water supply was a cause of problems over many years. Pennsylvania's Union Canal suffered likewise on its summit level for lack of water. To rectify that, engineers had to put three large dams across Swartara creek, a mile long dam over Cattail Creek, and the water then passed to a powerhouse where the water was pumped (using water wheels and additional steam pumps if needed) 95 feet up, whereupon it flowed 4 miles through an aqueduct. 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 (Lake Gatun) 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 (19 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. Climbing the hill that leads to Devizes requires 29 locks. 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 mile (1 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.

Canals have 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 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.

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.

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. Ramps at the side of canals next to the towpath are quite common in England particularly where horses were known to fall in. For example there is one on the Hanwell Lock Flight of locks on the Grand Union Canal.

Canal levels sometimes have sand bars or shallow areas where the boat will get stuck. It is common on English canals for boaters to carry a barge pole to help punt the boat off an obstruction. Some boats carried long iron tipped poles to push themselves off, although some canals forbade their use. Sometimes boatmen would ask the lockkeeper of the lock above for a swell, to raise the water in the level, so that they could get off the sand bar.