These rivers are mentioned in verse 29 of the Grimnismol:
Kormt and Ormt and the Kerlaugs twain
Shall Thor each day wade through,
When dooms to give he forth shall go
To the ash-tree Yggdrasil;
For heaven's bridge burns all in flame,
And the sacred waters seethe.
There are 2 prevailing interpretations of this verse:
- the verse is part of a larger set that prophecizes what will happen during Ragnarok
- when Thor leaves the heavens (i.e., when a thunder-storm is over) the rainbow-bridge becomes hot in the sun
The rivers themselves are believed to be just beneath the tree roots of Yggdrasil. As to your question, Yggdrasil's roots form very weird cartography, as this illustration details:
If we indeed take the rivers to be under the roots, then they are actually in the other realms. However, artist renderings do depict the rivers to be just under Bifrost, though there is no explicit reference to this in the Edda or Grimnismol:
No other information is known about these 2 rivers.