1) Usually a sluice or floodgate.
1539 one barrell of tarre for the clows, Bridlington
1540 joynenge to the water of Ouse … with one water sewer and a clowe, Selby. It gave rise to minor place-names: 1540 ij closes called the Clowe Closes … charged wyth one clowe and water sewer for thadvoydyng [sic] and clensyng the water out of all the closes, Selby. The clowes could be made of different materials: 1711 competent timber for making and repairing … the common goat stocks, the clowes and bridges, Wistow
1749 a brick clow, Beverley. It was usual to ‘draw’ a clow to control the flow of water: 1740 We present James Garnett for letting the water run down the dam stones … for neglect of drawing the Clows belonging the miln, Beeston. Dismantling the clows was one way of putting a mill out of operation: 1615 for pulling up the mill-clowes … with a piked staffe of thre yeardes longe, whereby the mil-stones were almost broken, Northallerton.