The River Menalhyl (Cornish: Dowr Melynheyl, meaning river of the estuary mill) is a river in Cornwall, England, UK, that flows through the civil parishes of St Columb Major and Mawgan-in-Pydar. Its length is about 12 miles and it flows in a generally north-west direction. The name comes from the Cornish words melyn meaning mill and heyl meaning estuary - estuary mills.