A sudden thaw occurred over New Year's Eve, followed by unusually heavy rain, doubling the volume of water coming down the river.

The sudden rise in water level coincided with a high spring tide and a storm surge caused by a major extra-tropical cyclone in the North Sea.

The storm surge raised the water levels in the Thames Estuary, measured at Southend, to 1.5 metres (4.9 feet) above normal.

The situation was worsened by capital dredging that had been carried out between 1909 and 1928, deepening the river channel by about 2 metres (6.6 feet) to allow access for deeper-draughted vessels to the Port of London.

[6] Extensive flooding resulted as the river overflowed the Embankments from the City of London and Southwark up to Putney and Hammersmith.

Imperial Chemical House (designed by Sir Frank Baines, the Principal Architect of the Government's Office of Works) was one of the new buildings constructed in the area in 1927–29.

The current headquarters of MI5, Thames House, was built to a very similar (though not identical) design in 1929–1930 on the other side of Horseferry Road, with the two buildings forming a landmark pair of office blocks facing the river.

By then, much of London's shipping had moved to Tilbury Docks further downriver, greatly reducing the navigational difficulties that a barrier would present.

[10][11] Near Lambeth Bridge the embankment gave way, sending a wall of water through a generally poor and run-down area; there, nine people drowned (and one died of a heart attack brought on by shock) in their basement apartments, unable to escape from where they were living.

There were miniature waterfalls at Cleopatra's Needle and the Royal Air Force Memorial, and the training ship President floated at street level.