Some reservoirs have been formed by the raising of dams on water courses, others are natural water bodies, the levels of which have been raised to various degrees both for direct supply of water or else for regulation of supply with water being extracted from the river some way downstream. Many reservoirs which were constructed for one purpose e.g. industry, may now serve different or additional purposes e.g amenity or, as in many cases, have been abandoned entirely. Reservoirs range in size with many under 1 acre in area serving local needs, up to Llyn Trawsfynydd which extends to . Bala Lake (Llyn Tegid) is a natural lake which at , slightly exceeds Llyn Trawsfynydd in size, but whose height has been slightly raised.
