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This National Trail is 184 miles (294 km) long, following the River Thames from its source in the Cotswolds almost to the sea, finishing at the Thames Barrier in Greenwich.
The trail keeps to the banks of the river most of the route.
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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.