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. A book is available on the Pontcysyllte Aqueduct Restoration: Winter 2003 to Spring 2004.
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.
An article on aqueducts, with numerous photos, appeared in the April 2001 issue of Waterways World magazine.
Europe's largest aqueduct is in Germany (though this article refers to it as a "water bridge"). Here's another photo.