The Rocky Branch rises in the Presidential Range Dry River Wilderness Area of the White Mountain National Forest on the southernmost slopes of Mount Washington. The river drops rapidly to the south through a valley between Montalban Ridge (with the peaks of Mount Isolation and Stairs Mountain) to the west and smaller mountains to the east. Turning more to the southeast, the river leaves the national forest and enters the town of Bartlett, New Hampshire, where it joins the Saco River after passing under U.S. Route 302.