Hraunhafnarvatn is the largest lake on Melrakkaslétta at 3.4 km². The lake is a maximum 4m deep and is 2 m above sea level. The river Hraunhafnará flows into it at the southern end. Hraunhafnarvatn is by the main road which runs beside the lake on a gravel reef between the lake and the sea. The lake contains both Arctic char and brown trout, which have prospered well, as net fishing has been practiced. There is a good population of Arctic char, at most 1/2 – 1 pound but up to three pounds, as well as brown trout, which can be up to 6 pounds. The best fishing spots in Hraunhafnarvatn are by the gravel reef and within the area along the lake towards Hraun-hafnará. Out in the estuary of the river there are often nice brown trout.