Without due care the London would floods would get worse over time. Whilst some places like the low-lying Isle of Grain, Canvey Island, Hoo peninsular, Benfleet and Isle of Sheppey would flood more frequently, others further up stream like Plummeted, the Greenwich peninsular, West Ham CB and the Isle of Dogs would once more become permanent swamps and tidal marshlands.
Most of the Thames was surrounded by at leads a few feet of flood plain or marshland. Large tracts were swamps or mud flats were running all the way from the River Lea’s outflow to Canvey Island and the Isle of Grain.
Others existed along the river Roding in Redbridge LB and at the River Lea as far as Enfield Lock. These included Plumstead Mars, Rediff Marsh, Westham Abby Marsh, the Plaistow Level and Rainham Marsh. Lambeth was a moorland, with a coastal swamp (Lambeth Marsh) and inland swamp (Lower Marsh) in the 1300’s.
The old Isle of Dogs, Stepney Marsh, Blackwall Level, Bugsby's Marsh and North Greenwich Marsh were drained in the 17th century and joined up as the then farming peninsulas that are now the Isle of Dogs and the Greenwich peninsula. The marshes were mostly reclaimed by the 1880's and totally drained by World War 2, but would revert if the dykes, drains and embankments failed. Thames Embankment (at Westminster) was overwhelmed and part of the Chelsea Embankment did collapse in the flood of 1923.
Teddington, Chiswick, Putney, Hammersmith, Westmister, Southwark, Barns, Battersea and Richmond (in Surrey) flood on occasion and some incidents have occurred as far north as Oxford, but any permanent mass flooding by the Thames would not get past about Battersea.
Other moorlands and bogs
Moorefield was a bog from which the Wallbrook takes most of its water from. The River Brent has a habit of flooding in Hendon, Perivale, Brentford and Hanwell. Parts of Hampstead heath were marshland and the River Wandle used to flood Wandle Park, Carshalton Pond and Waddon Marsh when they were farmland.
The Thames Flood Barrier and the Canvey Island Sea Wall
The Thames Flood Barrier was not built in 1962 and the Victorian Canvey Island Sea Wall was in decay. The Victorian (1880's) wall was falling apart and the 1947 and 1954 wall were only localised. The 1970's and 1980's wall did not exist.
Other major rivers
The River Quaggy in Bromley LB, the River Roding (which once had a thin marsh on each bank south of Epping) around Epping, Ilford, Romford and of Uttlesford DC; the River Lea in south east Hertfordshire and London's East End, the River Brent in northern and western London, the River Colne for those in Hillingdon LB and Spelthorne DC and the River Mardyke in Upminster UDC.