In Norse mythology, Élivágar (Ice Waves) are rivers that existed in Ginnungagap at the beginning of the world. The Prose Edda relates:

“The streams called Ice-waves, those which were so long come from the fountain-heads that the yeasty venom upon them had hardened like the slag that runs out of the fire,-these then became ice; and when the ice halted and ceased to run, then it froze over above. But the drizzling rain that rose from the venom congealed to rime, and the rime increased, frost over frost, each over the other, even into Ginnungagap, the Yawning Void. ”
The eleven rivers traditionally associated with the Élivágar include the Svöl, Gunnthrá, Fjörm, Fimbulthul, Slidr, Hríd, Sylgr, Ylgr, Víd, Leiptr and Gjöll (which flows closest to the gate of Hel and is spanned by the bridge Gjallarbrú), although many other additional rivers are mentioned by name in both Eddas.