In J. R. R. Tolkien's fictional Middle-earth, the Celduin or River Running was a 600–Númenórean miles long river that ran from the Lonely Mountain south through the Long Lake where it was joined by the Forest River and thence through the easern outskirts of Mirkwood, then south east through apparently uninhabited regions of Rhovanion to its confluence with the Carnen and finally in a long south-eastward loop to the great inland Sea of Rhûn, past the land of Dorwinion.

It is not exactly clear what the name of the river is past the point where Celduin and Carnen meet, but the map suggests that Carnen becomes the name of the combined river. Although the area was not mapped by Tolkien the Carnen must have flowed past many villages of Men, as the Men of Dale and Esgaroth traded with someone in the region.