The rivers source is in the Brendon Hills which fall within the Exmoor National Park. The hills are quite heavily cultivated unlike their neighbouring upland areas of Exmoor and the Quantock Hills. The Brendon Hills are largely formed from the Morte Slates, a thick faulted and folded sequence of Devonian age sedimentary rocks. It then flows through an alluvial floodplain underlain by sub-alluvial gravels, underlain by rocks of the Mercia Mudstone Group. The floodplain is between and above the ordnance datum (mean sea level). The surrounding land is dominated by wet meadow and wooded areas including willow trees.