there are a few natural lakes, mostly in the west, such as llyn llywenan, the largest on the island, llyn coron, and cors cerrig y daran, but rivers are few and small. there are two large water supply reservoirs operated by welsh water. these are llyn alaw to the north of the island and llyn cefni in the centre of the island, which is fed by the headwaters of the afon cefni.