Gently-sloping Cold Streams

Slow-flowing in small watershed valleys with a strong connection to ground water. Substrates are dominated by sand and gravel with some adjacent floodplain wetlands and the potential for rooted aquatic vegetation.

Habitat description

Small, coldwater streams with meandering channels closely connected to groundwater, usually running through relatively wide alluvial floodplains where substrate is often dominated by sand and gravel. Over time, the stream channel moves back and forth in the floodplain. Aquatic invertebrates, typically associated with the current stream channel can also be found throughout the floodplain in wetlands and substrates associated with historic stream channels if the floodplain has not been developed or otherwise altered. While some higher elevation stream reaches can be low gradient, most low gradient streams reside in valley bottoms and are in regular contact with groundwater. Beavers are often found in these streams [except for on Cape Cod] and can create extensive stream-associated wetlands. These wetlands tend to increase solar exposure and cause stream warming but coldwater habitat can still exist throughout the reach unless groundwater is intercepted or the floodplain becomes excessively developed.

Associated habitat types

It is typical to have coldwater stream reaches of higher gradient in sequence with low gradient reaches as dictated by topography. Beaver-induced wetland habitat is also typically associated with these habitats. Young forest will be present as Beavers clear portions of the floodplain. In the absence of beavers, floodplain forests and shrublands are often associated with small low gradient coolwater streams. 

Characteristic communities and species

Fish communities in these reaches can be very diverse and highly varied. Western watersheds will Include coldwater species like brook trout, longnose suckers, and our only occurrences of lake chubs. These streams can have additional cool and warmwater species like white suckers, creek chubs, longnose and blacknose dace and tessellated darters. Eastern watersheds can have coldwater species, mostly brook trout, but will also include cool and warmwater species like fallfish, common shiners, bridle shiners and redfin pickerel. Mussels include Eastern Elliptio, Eastern Pearlshell, and at fewer segments Creeper and Triangle Floater. Typical dragonfly species include Ocellated and Fawn Darners, Maine Snaketail, Eastern Least Clubtail, Pygmy Clubtails, and several Striped Emerald species where streams flow through wetlands.   

Ecological processes

Beavers are often the primary ecosystem engineers in this habitat as they bring large woody habitat and organic material into streams that have seen 300 years of tree clearing and stream habitat simplification. Organic inputs increase the productivity of the system and, in the case of large wood, encourage sinuosity. Unlike their high gradient counterparts, these streams can retain beaver dams for many years, eventually providing enough substrate even for mature trees to grow on old dams. The low energy and power that allows these dams to persist, however, also allows encroachment by invasive species. These reaches serve a critical role in flood storage, retaining water in adjacent side-channel wetlands.

Threats

Especially in areas that have seen excessive urbanization, low gradient coldwater streams will be subject to serious climate change alterations. Climate change models show increased warming of groundwater, which will severely limit the ability of these streams to support coldwater fish assemblages in the future. While these groundwater-fed systems are inherently resistant to total drying, channelization, water withdrawal, and watershed encroachment can cause water levels to drop, making them more susceptible to increases in remaining groundwater temperature and limiting the extent of coldwater refugia. Development in the floodplain further reduces the amount of, and connection to, adjacent wetland habitats, reducing flood storage, groundwater storage and climate resilience.

Restoration & management recommendations

Restoration actions of all streams and rivers are designed to restore form and function, which will then be maintained largely through stream power and riparian corridor protection. Low gradient systems respond immediately to certain restoration actions like lateral floodplain connectivity but lack the stream energy and power to remove accumulated sediments associated with dam removal. In many cases, sediment management will be needed to expedite restoration processes.

• Floodplain Restoration: Many streams in this category have been historically channelized, severing the connection between stream channel and floodplain. Restoring the connection is particularly important for gently sloping habitats and can have immediate flood storage benefits. Increased flood connection will also enhance groundwater infiltration and promote climate resilience and water quality.
• Large Woody Habitat: The low energy of these streams makes them an excellent choice for the reestablishment of large woody habitat. The material will stay in place, provide variability in stream current, and will over time facilitate the reestablishment of sinuosity, reversing the historical trend of channelization, and further enhance flood storage. Large woody habitat also provides microhabitats and additional nutrients and substrates for a diverse aquatic community.
• Dam Removal: Low gradient streams were historically less of a target for damming as they could provide little vertical head and had little power. However, in places where dams exist, their removal can connect large reaches of stream between dams.
• Culvert Replacement: Undersized culverts should be replaced where possible to improve passage for fish and other wildlife and reduce erosion and sedimentation.

Examples

• Center Brook

• Quashnet River, Red Brook

• Great Brook

• Manhan River

• Fort River

Additional resources

Wood is Good