Lake Arnarvatn has an area of 2,15 km². It is rather shallow and situated at 440 m above sea level. It is fed by the brook Krummavatnslaekur, the discharge of the Lakes Arfavotn in the north. Its discharge runs off to the south. The lake char and the brown trout in the lake are large and they have been netted for a long time. This lake is rather centrally located on the moor and a bit further east, in Alftavatnskrokur, is a refuge hut. The 4wd track leading to the lake is relatively well passable. The distance from the capital over the mountain pass Kaldidalur is about 200 km.