Lock 21

Lock 21 is a submerged historic canal lock located off the southwestern end of Macdonell Island in the Long Sault Provincial Park, near Ingleside, Ontario, on the St. Lawrence River. Guard Lock 21 was built in 1885–86 as part of a second enlargement to the Cornwall Canal system, designed to circumvent the Long Sault rapids; the lock is 270 feet long between its gates, 45 feet wide, and had 14 feet of clearance over the sill. Operated first by the Department of Railways and Canals until 1936, then by the Department of Transport, the entire area was eventually flooded to serve as headwater for the Hydro dam at Cornwall. Divers can explore the old lock gates, the sluice-gates of the weir (all closed except the southernmost, which can be swum through), various mechanisms that operated the lock, a rail trailer, and the foundations of the lockkeeper's cabin.

The site reaches a maximum depth of around 19–20 m (65 ft), with the upper structure of the lock encountered at roughly 9 m (30 ft). The lock walls, walkways, and weir structures provide the main relief in what is otherwise a river environment with a strong 2–3 knot current running through the St. Lawrence.

The sheltered areas of the lock attract a variety of species that use the structure to escape the current, including round goby, bass, sunfish, perch, carp, walleye, St. Lawrence eels, crayfish, and spotted loaches.