Hraunfossar is a very unusual sort of waterfall. Since the lava in this valley isn’t very old, it is porous. This means the water flows easily through the spongelike lava instead of over it. Above the waterfall you will find no river at all. Underneath the lava layer there is an older and much harder layer which the water flows on top of. These pictures illustrate it very well. There’s another closeup of the waterfall here.