 The high ground forms the catchment area for numerous rivers and streams . There are about 300 miles ( 483 km ) of named rivers on Exmoor . The River Exe , from which Exmoor takes its name , 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 . 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 . 
