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How did flash locks help boats pass shallow places on medieval waterways?

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View of Utrecht's Oude Gracht canal lined with historic brick buildings, arched doorways at water level, and boats moored along the canal edge in the Netherlands. — canals and inland waterways The *Oudegracht* (Old Canal), Utrecht, Netherlands.

Flash locks, also known as stanches, were used on medieval waterways to help vessels navigate shallow areas. These structures were built into weirs or dams along a river or canal. By opening the lock suddenly, a torrent of water was released, creating a surge that carried the boat over the shallow section of the waterway.1