 Due to stiff penalties applicable on the non-tidal river, which is a drinking water source before treatment, sanitary sewer overflow from the many sewage treatment plants covering the upper Thames basin is rare in the non-tidal Thames, which ensures clearer water compared to the river's tideway.Below Teddington Lock (about upstream of the Thames Estuary), the river is subject to tidal activity from the North Sea.Before the lock was installed, the river was tidal as far as Staines, about upstream.