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The River Thames rises at Thames Head in Gloucestershire ST9899 on the slopes of the Cotswolds and flows generally eastward to its mouth near Southend in Essex. At 215 miles long it is the second longest river in Britain after the Severn, and the longest entirely within England. Link

Marsh Lock is a lock and weir situated on the River Thames near Henley-on-Thames. The lock is close to the Berkshire bank, but accessed from the Oxfordshire side via two long walkways, the downstream one being near Mill Meadows. The first pound lock was built by the Thames Navigation Commission in 1773.
The weir consists of a series of iron watergates running from the lock to the Oxfordshire bank near the mills. It is situated between the two walkways.