Mount Monadnock lies on the divide between the Connecticut River and Merrimack River watersheds. The northern slopes of the mountain drain to Howe Reservoir, then via Minnewawa Brook to the Ashuelot River, the Connecticut River, and ultimately Long Island Sound. Gleason Brook and Mountain Brook flow off the western slopes, then via Shaker Brook to the South Branch of the Ashuelot River. The southwestern slopes of the mountain drain to Fassett Brook, then Quarry Brook, before reaching the South Branch of the Ashuelot.