by Sally Soule, former NRWA Environmental Education Director

Overview
Citizens in the Nashua River watershed depend on its waterways for drinking water, recreation, waste removal, and aesthetics. Plants and animals also depend on the waterways. In order to protect aquatic habitats and waterways from degradation, it is essential that watershed residents understand the connection between land use, water quality, water quantity and aquatic habitat. A waterway’s physical, chemical and biological features determine aquatic habitat. Biologically diverse waterways are considered to be an indicator of healthy aquatic ecosystems.

Human actions on the land and in the river directly affect aquatic habitats. Sedimentation caused by stream bank erosion and runoff can smother substrates necessary for aquatic life. Sedimentation can also lower water clarity, which eliminates sunlight needed by aquatic plants for photosynthesis. Polluted runoff can contribute nutrients that accelerate plant growth and lower dissolved oxygen levels. The removal of vegetation along riverbanks can lead to erosion and increased temperatures due exposure of the waterway to sunlight. Bacteria from failing septic systems and agriculture can introduce pathogens that can threaten human health and recreational uses of the river.Alterations of natural flow regimes in river and streams and water levels in other non-flowing hydric habitats can degrade their value for aquatic and other water-dependent organisms.

Physical Setting
The Nashua River watershed drains 538 square miles in the central highlands of southern New England. The river has two main branches: the North Nashua River, which is formed by the junction of the Whitman River and Flag Brook in West Fitchburg, MA, and the South Nashua River, which flows from Wachusett Reservoir. The North and South Nashua Rivers meet in Lancaster, MA and form the Nashua River’s Main Stem. Two major tributaries, the Squannacook and Nissitissit Rivers, join the Main Stem and contribute a significant amount of flow. These tributaries are fed by a series of smaller streams that originate in the watershed’s upper reaches. Smaller waterways such as Catecunamaug, Unkety and Mulpus Brook also flow into the Main Stem. The Main Stem flows north to join the Merrimack River in Nashua, NH.

The river’s headwaters and tributaries begin in terrain that is formed by rolling hills, which are more rugged to the north and west. The Main Stem flows through the Nashua River valley, which follows a broad flood plain, bordered by hills. The landscape shows the effects of the movement and melting of glaciers. During the last ice age, rocky soil was deposited in the uplands, while the lowlands were buried in sand and gravel carried by glacial melt water. The watershed has a broad range of soil types, which support a diversity of plant communities. This topography helps shape the aquatic habitat of the watershed’s tributaries and Main Stem.

The climate of the watershed is typical for central New England. Temperatures range from ninety-five degrees Fahrenheit to minus twenty degrees Fahrenheit. Precipitation averages about forty-four inches per year, although slightly less falls in the summer. Approximately forty-four percent of the precipitation is lost as evapotranspiration and the remainder is divided between surface runoff and ground water discharge.

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Nashua River Main Stem

Physical Description
The Main Stem flows slowly at a low gradient over flood plains, which contain glacial deposits of sand and gravel. River depths range from less than two meters to over ten meters. The substrate of the Main Stem is generally silty with layers of fine organic material. Some sand deposits are present. Riffles in fast flowing, shallow areas exist at several locations including the Rt. 111 bridge in Hollis, New Hampshire and downstream from the Ice House Dam in Ayer.

The aquatic habitat and hydrology of the Main Stem are influenced by several impoundments created by dams. The impoundments slow the river’s flow even further and create lake or pond-like areas prone to warm water, low levels of dissolved oxygen, algal blooms, and dense aquatic plant growth. Several impoundments are known to contain infestations of nuisance aquatic plants. The impounded areas also serve as a sink for sediments; some of which may be leftover from point source discharges in the 1960s and earlier.

Biology
The Main Stem supports an abundance of fish species associated with warm water fisheries such as largemouth bass, bullhead, and perch. Large woody debris are common in the river and are important habitat features that provide shade for fish, perching places for birds, and resting areas for turtles. Several oxbows on the Main Stem offer habitat for waterfowl and mammals. Kingfishers, herons, beavers, and muskrat are common along the waterway; mink and river otter have also been seen.

Riparian Corridor and Land Use
The riparian corridor is dominated by wooded or open land interspersed with areas of residential and industrial development. Significant portions of land along the Main Stem are protected by state, federal, or local agencies. Some areas of the river are densely forested with oaks, pines, and maples. In these reaches, the overstory provides a canopy that shades the river and keeps water temperatures relatively low. These are important areas for fish. In places where the river is wide, the riverbanks support herbaceous vegetation such as cattail, woolgrass, and purple loosestrife (invasive).

Aquatic Habitat Concerns

• Recreational fishing is a common activity on the lower Main Stem. However, large boats and motors can churn up bottom sediments, which affect water clarity, and accelerate streambank erosion. Large motor boat noise and/or wakes can also intimidate wildlife from using or remaining in what is otherwise suitable habitat.
• At this time there are no fish ladders to ensure passage for fish at any of the dams on the Nashua River Main Stem. Anecdotal evidence suggests that fish attempt to move from one impoundment to the next, but are thwarted by several dams.
• Nonpoint source pollution continues to threaten the river’s chemistry and habitat; sedimentation and nutrients are key concerns.
• Invasive species such as purple loosestife and water chestnut pose a threat to the river’s riparian corridor and aquatic habitat.

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Nashua River Tributaries

General Physical Description
Many of the river’s main tributaries begin in the moderate to steep topography of the north and west regions of the watershed. Bedrock outcrops and predominantly till or hardpan soils dominate the watershed’s upper reaches. These characteristics generally create impervious areas, which prevent infiltration, and lead to heavy run off, “flashy” peak flows, and dry weather flows that are substantially below average. Flooding, erosion, and sediment transport are also evident and indicative of the topography and geology. Many tributaries possess desirable riffle/run/pool habitat ratios, which are critical for aquatic life. Riffles are mixing areas that provide food and oxygen to the stream life; runs consist of deeper, swifter stretches of the stream where fish can travel; and pools offer important resting places for fish and wildlife.

Most tributaries flow into the river’s Main Stem from the north, south, and west. Very few flow to the river from the east and those that do such as Unkety, James and Nonacoicus Brooks originate in more level terrain, have slower flow regimes, and are associated with wetlands.

Tributaries to Wachusett Reservoir such as the Stillwater and Quinapoxet Rivers generally possess exemplary aquatic habitat: riffle/run flow regimes, cobble substrates minimally affected by sedimentation, high percentage of canopy cover, and sustained flow. Fortunately, the Metropolitan District Commission's protection of the tributaries has a side benefit of protecting aquatic and riparian habit as well.

Biology
Many of the watershed’s tributaries are biologically diverse. The Squannacook and Nissitissit Rivers are both considered by the state to be Outstanding Resource Waters. This designation affords protection for the high level of aquatic habitat found in both waterways. Cold water fish species such as brook trout can be found in many tributaries, and cool temperatures afforded by forest canopy and low levels of dissolved oxygen also support many species of aquatic insects and invertebrates. Several species of freshwater mussels can be found in the basin’s tributaries. Mussels are generally considered by to be biological indicators of water quality and aquatic habitat. Many freshwater mussels species are thought to be in decline due to habitat loss and alteration. Mammals such as otters, moose, and beaver are also present in and near many tributaries.

Biological diversity is compromised by polluted runoff, point source pollution (combined sewage overflows), sedimentation, loss of vegetated buffers, and channelization and alteration of natural flow regimes in several tributaries that flow through highly urbanized areas such as the North Nashua River, and some portions of Monoosnoc, Falulah, and Phillips Brooks.

Riparian Corridor and Land Use
Many tributaries such as the Whitman, Squannacook and Nissitissit Rivers flow through forested lands with little residential or commercial development. For these waterways the diversity of vegetation types provides ample cover and food for invertebrates, which provide food for fish and other wildlife. Riparian areas adjacent to tributaries serve as natural buffer zones, which protect water quality and provide habitat.

A few streams, including the North Nashua River, flow through highly developed areas where streamside vegetation is sparse and the waterway is exposed to direct sunlight and urban impacts.

Aquatic Habitat Concerns

• Water withdrawals may adversely affect many of the basin’s smaller tributaries such Mulpus, Catecunemaug, and James Brooks.
• Sedimentation from road runoff and development may be present in smaller tributaries.
• Alteration of streamside vegetation adversely affects urban tributaries in certain reaches.

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Summary
Aquatic life in rivers and streams depends on the quality and quantity of available habitat. The Nashua River watershed’s rivers and streams contain a range of aquatic habitats that support a diversity of plant and animal life. For example, the Squannacook and Nissitissit Rivers are renowned for their recreational cold water fisheries and pristine beauty. During the past one hundred years as land use in the basin has shifted from agriculturally based activities to residential and industrial uses the landscape and ecology of the Nashua River watershed has changed dramatically. Presently, habitat loss, polluted runoff, and increased water withdrawals threaten aquatic species in the Nashua River and its tributaries. Protection of the Nashua River watershed’s riparian corridors and the adjacent upland from unplanned development is essential for maintaining high levels of aquatic habitat.

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