 The high ground forms the catchment area for numerous rivers and streams . There are about 483 kilometres ( 300 mi ) of named rivers on Exmoor . The River Exe , for which Exmoor is named , rises at Exe Head near the village of Simonsbath , close to the Bristol Channel coast , but flows more or less directly due south , so that most of its length lies in Devon . It reaches the sea at a substantial ria ( estuary ) on the south ( English Channel ) coast of Devon . Historically , its lowest bridging point was at Exeter , though there is now a viaduct for the M5 motorway about 3 kilometres ( 2 mi ) south of the city centre . It has several tributaries which arise on Exmoor . The River Barle runs from northern Exmoor to join the River Exe at <unk> , Devon . The river and the Barle Valley are both designated as biological Sites of Special Scientific Interest . Another tributary , the River <unk> , flows from the <unk> Lake . 
