Sitting right south of Parliament Hill and the downtown is the eastern terminus of the Rideau Canal, just over 200 km long. Opened in 1832, it provided a protected inland route west from Montreal to Kingston on Lake Ontario that avoided the St. Lawrence, where the U.S. lurked on the south bank of the river. During the War of 1812, this was a real concern to the British (Canada was still a British colony then). It's never been used for military purposes, though it was heavily used for shipping until 1849 when locks were built on the St. Lawrence. Today it's popular with pleasure craft.
The canal freezes of course during the winter, and becomes a 7;8 km. long rink, officially the 2nd longest in the world since Winnipeg opened a rink on the Assiniboine and Red Rivers a km. longer. But that one is only 2-3 metres wide, whereas the Rideau rink is the full width of the canal, so it's now officially the largest if not the longest skating rink in the world. And once, in Ottawa for a meeting, I took my skates and skated the whole thing!