Habitat description
Large, low gradient, coldwater rivers dominated by runs and pools with occasional riffle habitat (stream order greater than 4, <2% slope). Average summer water temperatures are commonly <20°C. These are often mainstem rivers found at low elevations in valley bottoms. Substrate is dominated by silt, sand, and gravel with occasional cobble, boulder, and bedrock. The smaller substrate sizes are indicative of low-energy systems where slow-moving water allows deposition of fine sediment. Gravel and cobble can be embedded within a matrix of silt and sand. The stream channel may have noticeable but small meanders associated with gravel and sand bars.
Associated habitat types
Riparian and Floodplain, Steep cold rivers, Steep cold streams, Gently-sloping cool rivers
Characteristic communities and species
Because of their location in the watershed and habitat heterogeneity within these rivers, fish communities often include coldwater species along with more abundant coolwater and warmwater species. For example, fish communities in the Deerfield River are comprised of Eastern Brook Trout, Rainbow Trout, and Brown Trout, all coldwater species; alongside White Sucker, Longnose Dace, Creek Chub (coolwater species); and Brown Bullhead, Pumpkinseed and Chain Pickerel (warmwater species). In Massachusetts, coldwater rivers and streams are identified as those where at least one of 11 coldwater fish species are found. More information is available at: https://www.mass.gov/info-details/coldwater-fish-resources. Species commonly found in these habitats include otter, mink, beaver, and wood turtle.
Ecological processes
These habitats rely on high-energy water flows to maintain channel form but typically exhibit slow moving waters in summer/early fall. Beaver in these rivers build dams, lodges, and food caches that can stay in the same location for decades. Although beaver ponds are often seen as detrimental to coldwater rivers, they have been found to be vital for maintaining high biodiversity, providing nutrients to downstream habitats and for augmenting downstream flows during drought conditions. Furthermore, beaver ponds provide essential nursery areas to some wildlife like waterfowl. Food webs in these systems are largely dependent on nutrient input from upstream habitats and the floodplain. Heavily shaded upstream tributaries and groundwater seeps are vital sources of coldwater to maintain mainstem habitats.
Threats
Projected increases in temperatures and changes in precipitation patterns resulting from climate change are already shifting ecosystem composition and degrading habitats at different rates. Climate change will continue to threaten biota and human infrastructure in these systems by extending periods of warm, dry weather in summer and fall and increasing the magnitude of high flows in winter and spring. Droughts and warm weather will likely shrink or warm existing coldwater habitats making them less able to support coldwater species. Increasingly high flooding events can dismantle habitat structure by straightening channels, increasing entrenchment, and disconnecting rivers from floodplains. Erosion resulting from heavy rain events can change the composition of stream substrates towards more abundant fine sediment.
These systems are particularly vulnerable to urban development and dams in their watersheds. Adverse impacts to water quality (e.g., temperature, dissolved oxygen) and biota, through altered habitat conditions, increase concurrently with increasing amounts of impervious surfaces and number of dams. The lack of appropriate buffers, inputs from dam reservoirs and effluent from wastewater treatment plants can result in warm water inputs that alter the characteristics of these systems. Similarly, water withdrawal can reduce the volume of groundwater inputs sustaining coldwater habitats. Warmer, shallower conditions are also more conducive to invasive species spread.
Restoration & management recommendations
- Floodplain Restoration: Floodplains that are connected to rivers provide nutrients that support food chains as well as store waters that mitigate downstream flooding and are slowly released back to the river channel as flows recede. Restoration of the floodplain may include a variety of techniques including revegetation or removing impervious surfaces such parking lots in exchange for soft surfaces (e.g., gravel roads, settling ponds, rain gardens).
- Water Quality Restoration: Because large coldwater rivers are often found at lower elevations, protection of upstream coldwater sources and mitigation of warmwater inputs are imperative to maintaining functioning systems. Protection of upstream coldwater sources may include protection of riparian forests along coldwater tributaries, especially those with large native trees, protection of groundwater aquifers, and recovery of instream flows. Mitigation of warmwater inputs may include rerouting of wastewater and road drainage.
- Water Quantity Restoration: Large coldwater rivers are rare in Massachusetts and yet they are some of the most heavily impacted systems. Large rivers, in general, are often the target of hydropower-generating dams and reservoirs. Advocating passage for all migratory and resident species at these structures as well as for management of operations that mimic a natural hydrologic regime will go a long way towards bringing the function of regulated large rivers closer to a natural state.
- Dam Removal: Removal of dams in gently-sloping cold rivers can help maintain cold water temperatures, when reservoirs behind dams can no longer absorb solar radiation, and restore upstream and downstream fish passage. Channel/shoreline restoration, including revegetation, may be necessary once dams are removed.
- Channel and Shoreline Restoration: Depending on the site, a variety of techniques may be needed such as removing riprap to restore a more natural channel. Large logs and rock structures are often placed in slow-moving sections of rivers to increase habitat complexity. The structures provide areas where scour can create pools during high flows and cover from predation for a variety of species.
- Culvert Replacement: Improperly sized culverts can become a barrier to migratory species in a variety of ways, for example, when culverts become perched, clogged, or result in high stream velocities. Culverts in Massachusetts should be replaced by structures that follow the Massachusetts Stream Crossing Guidelines. Recent estimates are that culverts may need to be sized up even beyond recommendations in the Guidelines to be climate-smart.
The location and scale of relevant management efforts will be site-specific. For instance, removal of a dam located upstream of other dams is unlikely to open much new habitat to migratory fish species or significantly improve downstream conditions. In contrast, protection or restoration of a large tributaries may provide a major source of coldwater to a downstream river. While smaller projects can have significant positive impacts at the local scale, they should be considered within the context of land and water use patterns and comprehensive management plans at the watershed scale, including those traditionally focused on municipal or state-owned infrastructure.
Examples
Examples of large, low gradient coldwater rivers in Massachusetts include valley-bottom mainstem sections of the Deerfield and lower Squannacook Rivers (pictured above).