New Hampshire Rivers Management and Protection Program
Souhegan River Nomination
VII. RESOURCE ASSESSMENT
Briefly describe the significant geologic resources of the river and its corridor, including any unique or visually interesting features such as waterfalls, unusual rock formations, and areas of rapids. If you are unable to include such features, then simply describe the bedrock geology map. Consider geologic resources on the basis of natural history, visual, and economic interest. Indicate if the state geologist or a national or state resource assessment has identified these geologic resources as significant at a national, regional (New England), state, or local level.
During the glacial period, glacial Lake Merrimack extended up the Souhegan River to Milford center leaving behind fine sands and silts that underlie the floodplains of the river. Additionally, streams flowing from melting glaciers deposited sediments in layers of similar sized grains. These stratified drift deposits are often excellent sources of groundwater. The 1987 USGS study, Hydrogeology of Stratified Drift Aquifers and Water Quality in the Nashua Regional Planning Commission Area, South Central New Hampshire, identified the Souhegan aquifer as one of the potentially most productive sources of high quality groundwater in the region. Three of the corridor communities rely on this aquifer for their existing and future water supplies.
The River flows through a gorge in Greenville with steep sides. The land on which the gorge is located was donated to the NH Fish and Game Department for preservation purposes. The Horseshoe in Wilton is another geologically significant area that serves as the local swimming hole.
Mammals and birds found in the Souhegan River corridor are those commonly found in southern New Hampshire. These include raccoons, skunks, muskrats, beavers, porcupines, white tail deer, woodchucks, squirrels, mice, bats, rabbits and other indigenous species adapted to living near humans. The more rural areas of the watershed may also provide habitat for larger animals that require extensive habitat areas, or species that require solitude such as moose, black bear and lynx. Depending on the season, the River corridor is host to a wide diversity of bird species. Gulls, doves, woodpeckers, chickadees and jays would be found throughout the year while other species such as warblers, sparrows, wrens, swallows, robins and several species of raptors are only seasonal residents. Other species including a variety of ducks, geese and herons nest in the area or migrate through the corridor.
|Animal Species||Location||E orT||N or S|
|Eastern Hognose Snake||T||S3|
Based on an agreement with the NH Natural Heritage Inventory, we do not disclose the location of threatened and endangered species in written materials.
|Significant Habitat||Diversity Rating|
The western reaches of the river corridor are very undeveloped and provide access between a number of large public and semi-public protected resource areas. In addition, the River corridor serves as a travel corridor in the developed areas of the region between the protected open spaces.
Typical plant species in the river corridor include those commonly found in southern New Hampshire and include white pine, hemlock, red maple, red oak, sycamore and numerous species of grasses and shrubs.
|Plant Species||Location||E orT||N or S|
|Long's Bitter Cress||E||S1|
|Bird's Foot Violet||E||S2|
|Stiff Tick Trefoil||SH|
|Exemplary Natural Ecological Community||Location||Rank|
|Southern New England High-energy Riverbank Community||S?|
|Southern New England Floodplain Forest||S2|
Native species of fish in the Souhegan River include small mouth bass, sunfish, pumpkinseeds, yellow perch, suckers and dace. In addition, the River is stocked annually by the NH Department of Fish and Game with brown trout, rainbow trout and brook trout.
|Fish Species||Location||E orT||N or S|
|Significant Habitat||Diversity Rating|
|Atlantic salmon nursery habitat||identified by US Fish and Wildlife Service as|
the best Atlantic salmon nursery habitat in the
The brook, brown and rainbow trout in the Souhegan River rely almost entirely on a stocking program. Over 5,000 rainbow, brown and brook trout are stocked in the Souhegan River annually. When released, the trout are of a legal size for angling, representing what is called a "put and take" program.
The Souhegan River is an important part of the Merrimack River anadromous fish restoration program and is considered one of the most productive rivers in the watershed. The upper reaches of the Souhegan and its tributaries provide the appropriate habitat - gravelly, sloping bottoms, water temperatures, oxygen levels and food sources - for excellent growth and survival of Atlantic salmon frye. On average, 100,000 Atlantic salmon frye are stocked in the Souhegan River annually. The dams on the River are equipped with downstream passage only at this point since natural reproduction is not expected. The Merrimack River Basin Fish Passage Action Plan for Anadromous Fish, January 1988, calls for the construction of upstream passage at the Merrimack Village dam when a specific number of shad pass through the Amoskeag dam. All other upstream passage is deferred.
In addition, the River is integral to the extremely successful US Fish and Wildlife Services Adopt-a-Salmon-Family Project that uses a watershed approach for environmental education. Classes are given Atlantic salmon to raise during the year which are then released into the Souhegan River in the spring. At present, the Souhegan River is the main release site for the program that currently involves approximately 25 schools in Massachusetts and New Hampshire.
- Class A
- Class B
- Class A
- Class B
At present, the Souhegan River supports its water quality classification, Class B, at all locations. The Souhegan Watershed Association's volunteer water quality monitoring program currently monitors 17 sites on the Souhegan for dissolved oxygen and bacteria. Due to the heavy rainfall in June, the 1998 sampling season was able to document high levels of bacteria during wet weather. Bacteria counts below Wilton and at the 122 bridge in Amherst exceeded the acceptable standard for swimming throughout the year. This is of concern since the Amherst site is a popular swimming area. Results from this program indicate problems with dissolved oxygen at the Pine Valley Mill site in Milford. Wet weather sampling by DES also documented dissolved oxygen problems in this area. The low dissolved oxygen levels are due in part to the presence of two dams in the downtown Wilton area.
Phosphorous samples were collected on the Souhegan River during the 1998 sampling season. Samples collected at the impact sites for both the Greenville and Milford wastewater treatment facilities exceeded the 0.05 mg/l level of concern limit throughout the season except in times of high water volume in June. Phosphorous levels continued to rise downstream of the Milford indicating other sources of phosphorous contribution, possibly the two golf courses and residential development in this stretch of the corridor.
Briefly describe the natural flow characteristics of the river, including natural periodic variation in flow (e.g., spring run-off and summer flow amounts) and frequency and duration of flood events. If applicable, describe purpose of and flow variations caused by impoundments, significant diversions, or channel alterations, including interbasin transfers. Indicate which segments of the river are free flowing.
Souhegan River flow data is only collected at one location, just above Wildcat Falls in Merrimack. The station operated as a full station until 1976 when it was converted to a partial station which is used only during periods of extreme weather, to estimate flooding conditions or drought severity. The monthly average flows for the Souhegan River as reported in the USGS publication Statistical Analysis of Stream Gauging Date, 1981, are presented in the table below. Flows range from a high of 818cfs in April to a low of 39 cfs in September. The 7Q10 flow, the lowest seven day sustained flow which occurs once in ten years, for the Souhegan River is 12.8 cfs. Flows in the Souhegan have been modified over the years by the construction of 12 flood control structures throughout the watershed to minimize flood damage. The flood control system is managed by the NH Department of Environmental Services.
AVERAGE MONTHLY FLOWS
SOUHEGAN RIVER AT MERRIMACK, NH
Month Mean Flow (cfs) October 45 November 202 December 274 January 218 February 284 March 553 April 818 May 392 June 219 July 110 August 60 September 39
Briefly describe, give the location and identify the type (e.g., floodplain, forested, etc.) and type of ownership (i.e., public or private) of significant areas of open space in the river corridor. Describe and include the location of any protected land parcels within the river corridor (e.g., state parks and forests, national forest lands, municipal parks and conservation easements).
Despite the rapid pace of development in southern NH, large areas of undeveloped land exist along the Souhegan River in each community, particularly in the western sections of the corridor. The Land Use Maps in both the corridor and watershed studies identify they types of land use along the River and the locations of publicly and privately protected open spaces. The major parcels of protected land are identified by community in the following discussion.
Merrimack: Three sites along the River are owned by the Town, the Eighty Acres site - predominantly forested includes Wildcat Falls; the Turkey Hill Bridge site - open and forested, provides car top access to the River; Davidson Avenue green space - predominantly forested. In addition, the Whippoorwill Boy Scout Camp is located on the River.
Amherst: There are three areas of protected land owned by a municipality along the River, the Scott and Sherburne site - predominantly floodplain; the Currier Land - predominantly floodplain; and the Curtis Well site - public drinking water supply owned by the Town of Milford mixed woods and fields.
Milford: Milford conservation lands include an unnamed piece east of downtown - floodplain, forest, field; the site east of the swinging bridge - open area and woods; Emerson Park - small developed park; the Keyes Memorial Park - floodplain, open recreation area; and an unnamed parcel adjacent to the fish hatchery. The NH Department of Fish and Game operates a fish hatchery on a large parcel of land along the river - mixed fields and forest.
Wilton: The Town Forest on the Souhegan River is the only substantial piece of Town owned land in the river corridor. The Society for the Protection of NH Forests owns a parcel along the River - forested. The NH DOT owns a 3.2 mile scenic easement on Route 31 in Wilton and Greenville along the Souhegan River.
Greenville: The NH Department of Fish and Game owns a large parcel that is predominantly forested and includes the gorge.
New Ipswich: There are a couple of small pieces of land owned by the Town along the River that are predominantly forested.