Llyn Ogwen lies at a height of about 310 metres above sea level and has an area of , but is a very shallow lake, with a maximum depth of only a little over 3 metres. It is fed by a number of streams from the slopes of the mountains which surround it, which include Tryfan and Pen yr Ole Wen. The largest of these streams is Afon Lloer, which flows from Ffynnon Lloer.