It rose from the Pools of Ivrin in the Ered Wethrin, flowed south and then southeast, flowing through a gorge in a series of rapids where it crossed the hills of the Andram or Long Wall, finally meeting Sirion in the Land of Willows Nan-tathren, not far above the Mouths of Sirion. Narog's tributaries were the River Ginglith in the north and the River Ringwil in the Taur-en-Faroth.
In the Etymologies, the river's name was stated to mean rushing, violent. However, in Parma Eldalamberon 17, two possible Khuzdul origins were mentioned: Narâk and Naruka, both of unknown meaning. Neither are canon, but the material in Parma Eldalamberon is younger.