Poros was a small river in southeast Gondor flowing into the Anduin. Estimated at being at least 400 Númenórean miles long, it began in the Ephel Dúath. It then flowed southwest for approximately 300 miles when it turned westward toward the Anduin delta.
It formed the border between Harondor and Ithilien. It could be forded at the Crossings of Poros, through which the Harad Road ran. It was the site of the Battle of the Crossings of Poros which occurred between Gondor and the Haradrim in TA 2885.
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The Poros formed the northern border of the region of Harondor (South Gondor), and the southern border of Ithilien. During the later Third Age it was the effective southern border of Gondor.
Around T.A. 1944 some Haradrim had crossed the Poros to South Ithilien, but they were defeated by the Southern Army.
In T.A. 2885, the Haradrim crossed the Poros and invaded Ithilien with great strength. Reinforced by Rohan, Steward Túrin II won a victory at the crossings of Poros, though the Rohirrim princes Fastred and Folcred were slain. The Haudh in Gwanûr was made where the Poros was crossed by the Harad Road.