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.
This weir retains the river above it at half-tide level, in order to cover the mud-banks which had been bared at low tide between Richmond and Teddington by the lowering of the low-water level, owing to the removal of various obstructions in the river below.
In 1851 the river was diverted eastward into a new channel (called the New Cut) and its old channel was locked and floated, thereby forming the North Dock with an area of 112 acres and a half-tide basin 500 yards long covering 22 acres.
Lying parallel to the sea front and to the west of the entrance channel from which it runs at right angles, it has an area of 13 acres with a half-tide basin of 4 acres and a lock 300 ft.
