The Errol Dam Outflow Pool is a section of the Androscoggin River downstream of the Errol Dam. The dam was built in 1853 to create Lake Umbagog, which was formed by flooding three separate lakes connected by marshes and swamps. The outflow pool has a minimum flow of 522 cubic feet per second, which is regulated by the dam and the powerhouse. The pool is part of the Androscoggin River watershed, which covers 1,046 square miles and drains parts of New Hampshire and Maine.

The Androscoggin River is a river in Maine and New Hampshire. It is 178 miles long and joins the Kennebec River at Merrymeeting Bay in Maine before its water empties into the Gulf of Maine on the Atlantic Ocean. The river originates from Umbabog Lake in Coos County, New Hampshire, and flows through scenic landscapes, historic towns, and industrial areas.

The name "Androscoggin" comes from the Eastern Abenaki term meaning "river of cliff rock shelters" (literally "thus-deep-dwelling-river"); or perhaps from Penobscot meaning "river of rock shelters".