This glacial river, a discharge of the icecap Vatnajökull in the south, is the most voluminous in the northern part of the country. Its discharge area, 7380 km², is the largest on in Iceland. About 1700 km² thereof are sub glacial. The river is 206 km long and the average flow over Dettifoss is 183 m³/sec.
The first bridge was built down in the lowlands in 1905 and the present one in the same place in 1947. The third bridge on road no. 1, further south, was built in 1956-57. Before the river was bridged, travellers were server by the farmers, who were obliged by law to ferry them across against reasonable payment.
Some people did not want to spend money on that or were too poor and forded the river on their own. Many were drowned. Annually the river carries with it approximately 5000 tons of mud, sand and gravel to the estuaries and they are the breeding grounds for the harbour and common grey seals, the great skua and black backed seagulls among other bird species.
Two ferries were used to serve the travellers, who had to get across River Jokulsa a Fjollum. One of them was operated by the farmer of Grimsstadir until the bridge was built near the ferry point in 1947. In 1880 a retreat for the travellers was built of stone and concrete at the ferry point on the western side of the river. Downstairs was a horse stable and upstairs was a stove and sleeping quarters. At that time, there were very few such well equipped retreats in the country.
However good this house may have been, very few people could rest there because it was haunted by a calf-like, hairy and frightening monster. The house became the talk of the town all over the country and it was difficult not to believe in the stories because of the multitude of witnesses, who had experienced the haunting. This house still stands a short distance to the north of the bridge if anyone wants to test the truth of these stories.