The height of the river at Lechlade is 237 ft., the average fall between Lechlade and London, 1434 m., being rather less than 20 in.
The Thames forms part of the Gloucestershire-Wiltshire boundary to a point below Lechlade; thence for a short distance it separates Gloucestershire from Berkshire; after which it separates successively Oxfordshire and Berkshire, Buckinghamshire and Berkshire, Middlesex and Surrey, and finally, at its estuary, Essex and Kent.
Between Lechlade and Oxford the main channel sends off many narrow branches; the waters of the Windrush are similarly distributed, and the branches in the neighbourhood of Oxford form the picturesque "backwaters" which only light pleasure boats can penetrate.
The Thames is navigable for rowing-boats as far upwards as Cricklade, except in dry seasons, and for barges at all times as far as Lechlade, 18 m.
At Inglesham, threequarters of a mile above Lechlade, the Thames and Severn canal has its junction with the Thames.
The uppermost lock is St John's, below Lechlade; the lowest is Richmond, but this is a half-tide lock, keeping the water above at a level corresponding to half that of flood tide.