Altaelva (Northern Sami: Álttáeatnu; Kven: Alattionjoki) is the third longest river in Finnmark county, Norway. The river begins in the mountains and lakes in Kautokeino Municipality, near the border with Troms county and Finland, just south of Reisa National Park. The 240-kilometre (150 mi) long river then runs northward into Alta Municipality where it flows out into the Altafjorden in the town of Alta. The river has carved out Sautso, one of the largest canyons in Europe on its way from the high Finnmarksvidda plateau down to the sea.