The River Cober (Cornish: Dowr Kohar) is a short river in west Cornwall, United Kingdom. It rises near Porkellis Moor in the former Kerrier District and runs to the west of the town of Helston before entering the largest natural lake in Cornwall – Loe Pool. The water is impounded by the natural barrier, Loe Bar, and the river system can be traced several kilometres out into Mount's Bay. Mining activity of over one hundred years in the river catchment, ceased in 1938, in the Wendron and Porkellis mining districts; and the engine house of Castle Wary (also known as Wheal Pool), near Nansloe can still be seen on the east side of the river below Helston. The lower reaches of river was canalised in 1946 and a causeway built over Loe Marsh in 1987.
A common belief is that until the 13th century the River Cober ran directly to sea, until its mouth became blocked by a deposition of sand which formed Loe Bar and created Loe Pool. Loe Bar was most likely created by eustatic sea level rise after the last ice age. The rising sea level pushed a large amount of sediment into the mouth of the river, blocking it and creating a barrier beach. Loe Bar consists mainly of flint, a rock not found in Cornwall; the nearest onshore source is in east Devon, 120 miles (190 km) away. The most likely source of flint is found offshore; the drowned terraces of a former river that flowed between England and France and is now under the English Channel. Poorly forged hooks with mooring rings are on view on an ancient wall by St. Johns as well as remains of barges/boats having been observed under that road, indicating that boats were moored under the site of Castle Green car park.
^ abLe. Messurier, B. and Luck, L. (1998) Loe Pool and Mount's Bay. No. 12 in The National Trust Coast of Cornwall series of leaflets.
^Bere, Rennie (1982) The Nature of Cornwall. Buckingham: Barracuda Books; pp. 15, 66, 69
^ abMay, V.J. Loe Bar. In May, V.J. and Hansom, J.D. (2003) Coastal Geomorphology of Great Britain, Geological Conservation Review Series, No. 28, Joint Nature Conservation Committee, Peterborough, 754 pp.