The River Rimril was located in eastern Amn where it emerged from the Snakewood and flowed southwest to Eshpurta. There it merged with Trifin Creek and together they formed the Amstel River. Serpent's Tail was a local name for this river because it flowed out of the south end of Snakewood.
During the Shoon Empire, this river flowed through the elvish land of Arundath, the Quiet Forest (as Snakewood was then known). Shoon VII was collecting unicorn horns and slaughtered all the elves who tried to protect them. It was said that the River Rimril flowed red with their blood. For years, fishermen could still find some jewelry and trinkets on their hooks.