Just upstream a few meters from Hraunfossar, this is Barnafoss ("Children's Falls").  There used to be a natural stone arch spanning the river at this point but the following sad story explains the name and why the arch is no longer there:

Once there lived a widow on the farm Hraunás. She was well off and among her possessions was the farm Norðurreykir in Hálsasveit. She had two children. They were quite young when this story took place. There came a time when evening services were to be held at Christmas at Gilsbakki. The Mistress of Hraunsás and all her folk, except the children, attended the service. The children were told to stay at home and play. The moon was shining and the weather was fine. When the people came home the children had disappeared. Their footsteps led to the stone arch over the river. Their mother had the arch destroyed, saying that no man would ever be allowed to cross the falls alive. She later donated to the Church at Reykholt the farm Norðurreykir, in memory of her children. (Kristleifur Þorsteinsson II (1972) Hvítá.276)