The Thames is navigable from the estuary as far as Lechlade in Gloucestershire. Between the sea and Teddington Lock, the river forms part of the Port of London and navigation is administered by the Port of London Authority. From Teddington Lock to the head of navigation, the navigation authority is the Environment Agency. Both the tidal river through London and the non-tidal river upstream are intensively used for leisure navigation. All craft using the river Thames must be licensed.
The river is navigable to large ocean-going ships as far upstream as the Pool of London and London Bridge. Today little commercial traffic passes above the docks at Tilbury and central London sees only the occasional visiting cruise ship or warship, moored alongside HMS Belfast. There is a regular traffic of aggregate or refuse vessels, operating from wharves in the west of London. The tidal Thames links to the canal network at the River Lea Navigation, the Regent's Canal at Limehouse Basin, and the Grand Union Canal at Brentford.
The non-tidal River Thames is divided into reaches by the 45 locks. The locks are manned for a greater part of the day, but can be operated by experienced users out of hours. This part of the Thames links to existing navigations at the River Wey Navigation, the River Kennet and the Oxford Canal.
There is no speed limit on the Tideway downstream of Wandsworth Bridge, although boats are not allowed to create undue wash. Upstream of Wandsworth Bridge a speed limit is in force for powered craft to protect the riverbank environment and to provide safe conditions for rowers and other river users. The speed limit of 8 knots (15 km/h) applies to powered craft on this tidal part and 4.3 knots (8 km/h) on the non-tidal Thames. The Environment Agency has patrol boats (named after tributaries of the Thames) and can enforce the limit strictly since river traffic usually has to pass through a lock at some stage. There are pairs of transit markers at various points along the non-tidal river that can be used to check speed - a boat travelling legally taking a minute or more to pass between the two markers.
Until sufficient crossings were established, the river provided a formidable barrier, with Belgic tribes and Anglo-Saxon kingdoms being defined by which side of the river they were on. When English counties were established their boundaries were partly determined by the Thames. On the Northern bank were the traditional counties of Gloucestershire, Oxfordshire Buckinghamshire, Middlesex and Essex. On the southern bank were the counties of Wiltshire, Berkshire, Surrey, and Kent. However the 214 bridges and 17 tunnels that have been built to date have changed the dynamics and made cross-river development and shared responsibilities more practicable. The 1974 boundary changes moved some of the boundaries away from the river, so that, for example, some of Berkshire became Oxfordshire, some of Buckinghamshire became Berkshire, and some of Middlesex became Surrey. On occasion – for example in rowing – the banks are still referred to by their traditional county names.
Many of the present road bridges on the river are on the site of earlier fords, ferries and wooden structures. The earliest known major crossings of the Thames by the Romans were at London Bridge and Staines Bridge. At Folly Bridge in Oxford the remains of an original Saxon structure can be seen, and mediaeval stone structures such as Newbridge, and Abingdon Bridge are still in use. Kingston’s growth is believed to stem from its having the only crossing between London Bridge and Staines until the beginning of the 18th century. During the 18th century, many stone and brick road bridges were built from new or to replace existing bridges both in London and along the length of the river. These included Putney Bridge, Westminster Bridge, Datchet Bridge, Windsor Bridge and Sonning Bridge. Several central London road bridges were built in the 19th century, most conspicuously Tower Bridge, the only Bascule bridge on the river, designed to allow ocean going ships to pass beneath it. The most recent road bridges are the bypasses at Isis Bridge and Marlow By-pass Bridge and the Motorway bridges, most notably the two on the M25 route Queen Elizabeth II Bridge and M25 Runnymede Bridge.