Lake Nírië was once five times its current size, extending all the way to the fouls waters of the Imsavild swamp. In the Demon Spawn War, the rift Nírië Whirlpool was created to flood the central valley of the Isterien mountains. This was done to wipe out one of Demogorgon's armies that had dug themselves in so well that Covenant forces could not attack them without suffering terrible losses. The whirlpool, a rift drawing water from this lake and spewing it into lake Sondicar is still active. This is a quick way of reaching Sondicar and its two elven cities, so some use this rift as method of travel.
Lake Nírië's major sources of inflow are a pair of easterly flowing rivers that begin at the base of the Harongrand plateau.