The River Culm rises at a spring () near Culmhead and flows west through Hemyock, then Culmstock to Uffculme before joining the River Exe on the north-western outskirts of Exeter. The name of the river is thought to mean 'knot' or 'tie', in reference to the river's twists and loops; or is derived from a Celtic river-name meaning "winding stream". The River Otter rises near Otterford, where a stream feeds the Otterhead lakes: (). It then flows south for through East Devon to the English Channel at the western end of Lyme Bay. The Permian and Triassic sandstone aquifer in the Otter Valley is one of Devon's largest groundwater sources, supplying drinking water to Taunton. The other rivers are the River Yarty and the Corry Brook.