Grímsá, a river flowing from the Reyðarvatn lake down the Lundarreykjadalur valley in numerous falls and rapids.
Farthest down river are the waterfalls Hrísbrekknafoss and Jötnabrúarfoss, which is as far as salmon goes.
Higher up are the waterfalls Kleppagilsfoss, Selmýrafoss, Kleppafoss, Kerlingafoss, Kálfsgilsfoss and many others.