These are bridges across a canal that you can move out of the way. They may lift up, like a draw bridge, or rotate out of the way. They are counterbalanced so that they can be operated by one person with no motorized assistance. You'll need to drop one of your crew ashore to operate the bridge for you and reposition it after you've passed through. (Photo: Wrenbury lift bridge, on the Llangollen Canal.)
For the ultimate resource in its subject area, visit Movable Bridges in the British Isles. For a historical overview, see A Brief History of Movable Bridges.
For an uncommon (on canals) mechanised lift bridge, see the wikipedia page on the Leamington Lift Bridge, located on the Union Canal in Edinburgh.
Tom Boddington has a good page on the Macclesfield Canal bridge 47 swivel bridge