This derivation is connected to the legend of Nimrodel. Though the Gilrain was for most of its length a fast, straight river, there was a region about halfway between its source and its mouth where it wandered somewhat, and formed a small mere before flowing on. According to the story, Nimrodel herself wandered beside the river as she descended from the White Mountains, and saw the reflected stars glinting in the mere's waters. She sat beside the pool and slept, and so she did not reach Amroth's ship before it sailed.

Note that the meandering region and the small mere are recorded in a textual description of the river, and were said to form as it broke through a ridge thrust out southwards from the body of the White Mountains. This description is hard to square with the canonical map of the river, however, which shows none of these features, and so they are omitted from the map accompanying this entry.