The 1947 Thames flood was worst overall 20th century flood

Flood

A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...

 of the River Thames

River Thames

The River Thames flows through southern England. It is the longest river entirely in England and the second longest in the United Kingdom. While it is best known because its lower reaches flow through central London, the river flows alongside several other towns and cities, including Oxford,...

, affecting much of the Thames Valley

Thames Valley

The Thames Valley Region is a loose term for the English counties and towns roughly following the course of the River Thames as it flows from Oxfordshire in the west to London in the east. It includes parts of Buckinghamshire, Berkshire, North Hampshire, Surrey and west London...

 as well as elsewhere in England

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 during the middle of March 1947 after a severe winter.

The floods were caused by 117 mm (4.6 inches) of precipitation (including snow); the peak flow was 61.7 billion litres of water per day and the damage cost a total of £12 million to repair http://www.environment-agency.gov.uk/news/1721484.
War

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 damage to some of the locks

Lock (water transport)

A lock is a device for raising and lowering boats between stretches of water of different levels on river and canal waterways. The distinguishing feature of a lock is a fixed chamber in which the water level can be varied; whereas in a caisson lock, a boat lift, or on a canal inclined plane, it is...

made matters worse.

Other significant Thames floods since 1947 have occurred in 1968, 1993, 1998, 2000, 2003 and 2006.