Just like bridges on roads and railroads, canals need to cross over things. On the U.K. canal system, you have the opportunity to cross over rivers, other canals, railroad tracks, and superhighways (motorways).
The photo shows the view from a boat crossing the Edstone Aqueduct on the Stratford-on-Avon Canal (in 1975). The canal here spans a dual track rail line.
Probably the most impressive is the Pontcysyllte Aqueduct on the Llangollen Canal, which is over 1000 feet long and passes 120 feet above the River Dee. Here is a variety of photographs of Pontcysyllte.
Impressive in another way is the Barton Swing Aqueduct which carries the Bridgewater Canal over the Manchester Ship Canal. This 234 foot aqueduct rotates out of the way to allow large ships to pass on the Ship Canal below. See Martin Clark's Barton Swing Aqueduct for photos and more information.
"A collection of photos and historical comment about the UK's aqueducts" can be found at Aqueducts of the Inland Waterways
Europe's largest aqueduct is in Germany (though this article refers to it as a "water bridge"). Here's another photo.