Karahnjukar comprises two hyaloclastite mountains on the eastern side of River Jokulsa a Dal, the higher one 835 meters above mean sea level. The main canyon of Hafrahvammar is 5 km long, but the total length between The Desja River and The Trollagil Gorge is about 10 km. Its greatest depth is about 200 meters. Approximately half of it is called Dimmugljufur (The Dark Canyon).
There it is, very narrow and spectacular, probably the wildest one of the country. The river has been harnessed for electrical production. A 4wd track takes you there from the farm Bru and then further south to the junction further west near the bridge across river Kreppa and directly west to a more northerly junction of the same route between the main road and the Mt Kverkfjoll area.
Asphalted roads from valley Fljotsdalur, passable by all vehicles, continue to the dam site through spectacular landscapes. The creation of the country’s largest water reservoir started on Sepember 28th 2006. The bridge, used by the contractors and passers by, is under water. A new connection across the canyon on the dam itself was opened for traffic in the summer of 2007.