Gjöll is the river that separates the living from the dead in Norse mythology. It one of the eleven rivers traditionally associated with the Élivágar, rivers that existed in Ginnungagap at the beginning of the world.
According to Gylfaginning, Gjöll originates from the wellspring Hvergelmir in Niflheim, flowing through Ginnungagap, and thence into the worlds of existence. Gjöll is the river that flows closest to the gate of the underworld. The river is said to be freezing cold and have knives flowing through it. Within the Norse mythology, the dead must get over Gjallarbrú, the bridge which crosses Gjöll to get to Hel. Gjallarbrú, which was guarded by Móðguðr, was crossed by Hermód during his quest to retrieve Baldr from the land of the dead. 
Gjöll has a parallel with similar mythological rivers from Indo-European cultures such as the GreekStyx. Gjöll is also the name associated with the boulder to which the monstrous wolf Fenrir is bound.