Sirlith

The Sirlith (sometimes misspelled Sirilith) is a river of eastern Mordor, the name of which was created outside of J.R.R. Tolkien's Middle-earth canon.

Sirlith is the river flowing into the Sea of Nurnen's east shore from the north, and its source lay in Nan Morlith.

Etymology
Sirlith is Sindarin for "river of ash" - referring to the fact that it flowed through the plains of Lithlad.

External references

 * www.thelandofshadow.com/ragath-dur-ancient-map-of-mordor
 * Sirlith at the Middle-earth Role Playing Wiki