Teddington Lock is a complex of three locks and a weir on the River Thames in England at Ham in south west London. The lock is on the southern side of the river. The river downstream of the lock, known as the Tideway, is tidal, though the Richmond Lock barrage downstream limits the fall of water to maintain navigability at low tide. The boundary point between the Port of London Authority, which is the navigation authority downstream, and the Environment Agency, which is the navigation authority upstream is marked by an obelisk on the Surrey bank a few hundred yards below the lock. Though Teddington marks the tidal limit, in periods of very high fluvial flow the tidal influence can be seen as far upstream as East Molesey, location of the second lock on the Thames.