Lake Alftavatn is actually a part of the river Sog, the 19 km long discharge of Lake Thingvallavatn. It has an area of 2,2 km² and it is accessible by roads on both sides.

The catch consists of nice lake char, 1-6 pounders and large sea trout. The salmon runs through the lake but is seldom caught. The lake has not been netted for a long time. Travellers used to ford the river there in spite of the width of the lake. The distance from the capital is about 70 km.