structural:expansion
Conversion of marshland

After the river took its present-day course, many of the banks of the Thames Estuary and the Thames Valley in London were partly covered in marshland, as was the adjoining Lower Lea Valley. Streams and rivers like the River Lea, Tyburn Brook and Bollo Brook drained into the river, while some islands, e.g. Thorney Island, formed over the ages. The northern tip of the ancient parish of Lambeth, for example, was marshland known as Lambeth Marshe, but it was drained in the 18th century; the street names Lower Marsh and Upper Marsh preserve a memory.

Until the middle of the Victorian era, malaria was commonplace beside the River Thames, even in London, and was frequently lethal. Some cases continued to occur into the early 20th century. Draining of the marshes helped with its eradication, but the causes are complex and unclear.

The East End of London, also known simply as the East End, was the area of London east of the medieval walled City of London and north of the River Thames, although it is not defined by universally accepted formal boundaries; the River Lea can be considered another boundary. Most of the local riverside was also marshland. The land was drained and became farmland; it was built on after the Industrial Revolution.

Canvey Island in southern Essex (area 18.45 km, 7.12 sq mi; population 40,000) was once marshy, but is now a fully reclaimed island in the Thames estuary, separated from the mainland of south Essex by a network of creeks. Lying below sea level, it is prone to flooding at exceptional tides, but has nevertheless been inhabited since Roman times.