Wanderlust World Wanderlust World The River Thames is fundamentally a freshwater river—yet its lower reaches function as a tidal estuary. This dual character defines its ecology, history, and management.
Source: Thames Head, Gloucestershire (Cotswolds) — spring-fed, limestone aquifer.
Upper/Middle reaches: Rural, agricultural, clear chalk-stream character (trout, grayling, water crowfoot).
London entry: Teddington Lock (west London) marks the **tidal limit** — the furthest point where sea influence reaches. Above Teddington: fully freshwater, managed by Environment Agency for flow, quality, abstraction.
Below Teddington: River becomes tidal — water level rises/falls ~7m (23 ft) twice daily. Salinity gradient: freshwater at Teddington → brackish central London → near-full seawater at estuary mouth.
Ecology: Unique estuarine species: European eel, smelt, flounder, seals (regular to Westminster), porpoises (occasionally). 125+ fish species recorded.
Management: Port of London Authority (navigation), Environment Agency (quality), Thames Estuary Partnership (coordination).
Londoners see a tidal, sometimes saline river — leading to "Thames is saltwater" misconception. But the *river* (fluvial flow) is freshwater; the *estuary* is marine-influenced. The distinction matters: freshwater abstraction (London's water supply), wastewater treatment standards, flood defense (Thames Barrier), and ecological targets all depend on this classification.
Before embankments (1860s), the tidal Thames was wider, shallower, marshier. The "Great Stink" (1858) prompted Bazalgette's sewer system. Today, the Thames is among the cleanest urban rivers globally — salmon returned 1974, seahorses 2017.