The source of Geiraldslækur is on the west side of Stórisandur, south of Geiraldsgnúpur. The stream flows partly parallel with Fitjá og then into Austurá midway between Arnarvatn stóra and the heath-fence. The river is a spawning area for brown trout and therefore has good fishing. There is quite a walk to the stream and therefore it is not as popular as Austurá, which again could be desirable for spawning fish. Geiraldslækur is sensitive to droughts. If water levels become very low, the fish move into Austurá and the fishing becomes better there.