The Fossá River cascades from the edge of the highlands into the narrow valley of Fossársdalur, at the head of the valley of the Þjórsá River. The river pours over steep cliffs in two falls, the larger of which was unnamed until the geologist Helgi Pjeturs named it Háifoss (High Falls) in 1912. It is 122 metres high and is the second highest in the country (after Glym in Hvalfjord). The other falls, slightly lower, appears a short way from Háifoss. A French engineer and colleague of Helgi, André Courmont, named it Granna.
The strata by the falls are nearly 2 million years old and are composed of thick, successive layers of lava lying on top of palagonite and compacted layers of volcanic ejecta that erupted before the ice age. The easiest route to the falls is along a line that lies northwest from the road to the Sultartanga power plant, by the Hólaskógur woods.