The east face of South Twin drains into the Little River, thence into the Ammonoosuc and Connecticut Rivers, and into Long Island Sound in Connecticut. The northwest slopes of South Twin drain to the North Branch of the Gale River, another tributary of the Ammonoosuc River. Southwest of South Twin Mountain is the valley of Twin Brook, which flows into the Franconia Branch of the East Branch of the Pemigewasset River, through the Pemigewasset Wilderness, thence into the Pemigewasset and Merrimack Rivers, and into the Gulf of Maine in Massachusetts.