See quot. 1840.
1821. It [the canal] contains nine locks. The first, after the guard-lock, has a descent of six feet.—T. Dwight, ‘Travels,’ ii. 353.
1840. [A] guard-lock, in canalling, is employed in maintaining the level of a canal, by preventing the encroachment of water, &c.—H. S. Tanner, ‘Canals and Railways of the U.S.,’ p. 250. (N.E.D.)
