The Thames River is the most important river in the United Kingdom, flowing through London and many other cities. Originating in western England, it is 346 kilometers long and drains an area of 13,000 square kilometers. In London, the river widens and forms a 29 kilometer estuary entering the North Sea. Along the Thames are many historically significant buildings and sites that have helped nurture British civilization and culture, including Westminster Abbey, St. Paul's Cathedral, and the Tower of London. The river contains 27 bridges spanning its waters, including the iconic Tower Bridge, and remains an important transportation route as well as cultural symbol for the United Kingdom.