Ífingr

In Norse mythology, Ifing (Old Norse, Ífingr) is the name of a river that separates Asgard, the realm of the gods, from Jotunheim, the land of giants, according to stanza 16 of the poem Vafthrudnismal from the Poetic Edda:

John Lindow in Norse Mythology (2001) states in reference to Ifing that a river on which ice will never form is one that runs swiftly and therefore is extremely difficult to ford (thus forming an effective barrier between the worlds of gods and giants).

...
Wikipedia