Sometimes a river is made entirely non-tidal by constructing a sea lock directly into the estuary. A river improved by these means is often called a Waterway or River Navigation (see example Calder and Hebble Navigation). A weir will increase the depth of a shallow stretch, and the required lock will either be built in a gap in the weir, or at the downstream end of an artificial cut which bypasses the weir and perhaps a shallow stretch of river below it. In large scale river navigation improvements, weirs and locks are used together. When a stretch of river is made navigable, a lock is sometimes required to bypass an obstruction such as a rapid, dam, or mill weir – because of the change in river level across the obstacle.
Locks on the Rideau Canal, Entrance Valley, near Parliament Hill, Ottawa, Canada 1410–1475) constructed 18 pound locks on the Naviglio di Bereguardo (part of the Milan canal system sponsored by Francesco Sforza) between 14. Yet the first true pound lock was built in 1396 at Damme near Bruges, Belgium. This pound lock serviced many ships at once in a large basin. In medieval Europe a sort of pound lock was built in 1373 at Vreeswijk, Netherlands. The water level could differ by 4 feet (1.2 m) or 5 feet (1.5 m) at each lock and in the Grand Canal the level was raised in this way by 138 feet (42 m). The gates were 'hanging gates' when they were closed the water accumulated like a tide until the required level was reached, and then when the time came it was allowed to flow out. The distance between the two locks was rather more than 50 paces, and the whole space was covered with a great roof like a shed. They replaced earlier double slipways that had caused trouble and are mentioned by the Chinese polymath Shen Kuo (1031–1095) in his book Dream Pool Essays (published in 1088), and fully described in the Chinese historical text Song Shi (compiled in 1345): Pound locks were first used in China during the Song Dynasty (960–1279 AD), having been pioneered by the Song politician and naval engineer Qiao Weiyue in 984. In contrast, an earlier design with a single gate was known as a flash lock. A pound lock has a chamber with gates at both ends that control the level of water in the pound. 10.7 A combined system: Three Gorges DamĪ pound lock on the Keitele– Päijänne Canal at Äänekoski in Central FinlandĪ pound lock is a type of lock that is used almost exclusively nowadays on canals and rivers.
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