Flash lock
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Early locks were designed with a single gate, known as a flash lock. The "gate" was a set of boards, called paddles, supported against the current by upright timbers called rymers. Boats moving downstream would wait above the lock until the paddles were removed, which would allow a "flash" of water to pass through, carrying the boats with it. Upstream boats would be winched or towed through the lock with the paddles removed. Considerable skill was involved both in removing the paddles in a timely manner and navigating the boat through the lock.
Flash locks were commonly built into small dams or weirs where a head of water was used for powering a mill. The lock allowed boats to pass the weir while still allowing the mill to operate when the gate was closed.
