Ífingr
In Norse mythology, Ífingr is a river that separates Asgard, the realm of the Æsir, from Jötunheimr, the land of jötnar, according to stanza 16 of the poem Vafþrúðnismál from the Poetic Edda:
John Lindow in Norse Mythology 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.