River Length: 34.8 miles from the convergence of the South Branch and West Branch of the Souhegan in New Ipswich to the Merrimack River.Watershed Area: 170 square miles.Towns in the Watershed: Ashby (MA), Ashburnham (MA), New Ipswich, Temple, Sharon, Greenfield, Lyndeborough, Greenville, Brookline, Wilton, Milford, Mont Vernon, Amherst, and Merrimack.Watershed Population: 35,000 people.Average Monthly Flows: high - 818 cubic feet per second in April; low - 39 cfs in September.Water Quality: Class B (fishable and swimmable) for its entire length.Tributaries: Furnace Brook, Richardson Brook, King Brook, Goldsmith Brook, Blood Brook, Gambol Brook, Purgatory Brook, Tucker Brook, Caesar's Brook, Beaver Brook, Great Brook, Hartshorn Brook and Stony Brook. Stony Brook in Downtown Wilton is the largest tributary.Groundwater Resources: Five municipal water supply wells are located within the watershed.Water Withdrawals: 16 registered water withdrawals: 11 surface water withdrawals for irrigation, hydropower, snowmaking with one municipal water supply, industrial supply, and fish hatchery.Water Discharges: 5 NPDES discharges: 2 municipal wastewater treatment facilities (Greenville, 0.25 MGD and Milford, 2.15 MGD design capacity) and 3 industrial discharges; 2 other discharges for aquaculture and sand/gravel extraction.Dams: Five operating hydro facilities on the river: 1 in New Ipswitch, 2 in Greenville, 2 in Wilton. Four other currently unused dams: 1 in New Ipswich, 1 in Greenville, 1 in Wilton, 2 in Milford. The Merrimack Village Dam that blocked the entrance to the river was removed in 2008. The two dams in Milford are being studied for removal also. You can read about the dam removal in Merrimack here. There are also 13 flood control dams on the tributaries. Three of the flood control dams are in the process (2021-2) of being expanded to accomodate the increased flows expected as climate change is happening. One of these dams (12A) may also be set up to be able to release water to the river during times of critical low flow conditions.Recreation: Canoeing, kayaking, fishing, swimming, hiking, picnicking.Fisheries: Stocked with Rainbow, Brook, and Brown trout for put-and-take fishery. Threatened and Endangered Species: 28 species of plants and animals on endangered or threatened list. Seven exemplary natural communities.
Watershed Map - Nashua Regional Planning Council