Habitat description
Steep Cool Streams have a stream order between 1-4, slopes greater than 2 degrees, steep valley walls, fast-moving water, and have summer water temperatures between 18-22°C. These fast-moving streams include riffles, pools, and runs flowing through narrow floodplains. Rocky streambeds, made of bedrock, boulder, cobble, and gravel give shelter to aquatic insects during dry periods. Shade from nearby forests keeps water cool, while leaves and other natural debris add nutrients. Unlike Steep Cold Streams, these streams depend less on groundwater and are more likely to dry out or warm up in hot weather. These streams tend to be somewhat larger in size than their coldwater counterparts and might only be able to support coldwater species in particularly wet summers.
Associated habitat types
Steep Cold Streams, Gently Sloping Cool Streams and Rivers, all riparian forest types.
Characteristic communities and species
- Fish: Suckers*, dace*, and shiners* dominate these streams depending on location. Brook trout* and brown trout appear during cooler months but are usually absent in summer.
- Invertebrates: Dragonflies include ocellated darner*, Maine, riffle*, and brook snaketails*, southern and northern pygmy clubtails, and twin-spotted spiketail. Freshwater mussels are limited due to the lack of fine substrate.
*Massachusetts’s Species of Greatest Conservation Need
Ecological processes
These habitats depend on high-energy water flow and frequent disturbances to maintain their form and function. These streams have a weaker connection to groundwater compared to Steep Cold Streams and may receive warmer water from impoundments. The weaker groundwater connection can make them particularly susceptible to drought. Associated upland habitats provide not only shade to maintain the coolwater habitat through summer months, but nutrients through leaf fall that provide the dominant energy input to feed modest algal and significant invertebrate communities that break local nutrient sources down. These streams tend to be somewhat larger in size than their coldwater counterparts and might only be able to support coldwater species in particularly wet summers. Gravity moves cobble and gravel downstream, creating habitats for sensitive species. Beavers construct temporary dams that are often removed by strong water flows, adding nutrients and large woody material that support channel formation and productivity.
Threats
Steep cool streams face multiple threats that jeopardize their health and stability. Climate change is increasing the frequency and severity of extreme weather events such as floods and droughts, shifting precipitation patterns, and raising summer temperatures, all of which reduce water availability and stress aquatic species. Land use changes, including urbanization, floodplain development, channelization, improperly sized culverts, and dams, disrupt natural stream processes, fragment habitats, and increase erosion and sedimentation, particularly along steep valley walls. Water withdrawal, channelization, and watershed encroachment lower water levels and groundwater inputs, making these systems more vulnerable to temperature increases and diminishing the availability of coldwater refugia. While headwater reservoirs can provide water quality protection for public supply, they also trap significant portions of annual rainfall, altering downstream flows. Additionally, invasive species, including those colonizing streambanks, can outcompete native organisms and fundamentally alter community composition, further degrading habitat quality.
Restoration & management recommendations
Many, if not all Steep Cool Streams were at one point capable of sustaining brook trout or other coldwater fish and have been degraded by the threats listed above. Restoration and management could return more of that coldwater community if implemented effectively. In some cases, known coldwater fish communities are present in these streams and are listed in 321 CMR 5.00 as Coldwater Fish Resources.
- Dam removal: Dams are ubiquitous across Massachusetts and have historically been used to generate power, store water, and manage flows. Dam removal provides multiple benefits, including improved fish passage, enhanced connectivity of climate refugia, and reduced liability for landowners. Depending on the site, sediment removal or natural sediment release can restore downstream habitats. In cases where full removal is not feasible, partial removal or bypass structures can help restore some natural functions, including sediment transport and fish migration. Steep streams were often targeted for dam construction due to their capacity to generate vertical head. Removal of dams in these systems can mitigate severe erosion risks, improve habitat connectivity, and restore natural channel-forming processes. In areas with bedrock, partial removal may achieve significant ecological benefits, but in areas prone to erosion, full removal is recommended to prevent downstream degradation.
- Culvert replacement: In steep streams, culverts are often installed at improper elevations, becoming perched over time and impeding fish passage. Proper culvert replacement in these systems reduces erosion risks and allows the high-energy flow to maintain substrate dynamics while preserving habitat connectivity.
- Channel and shoreline restoration: In high-gradient systems, restoration aims to reestablish natural channel processes and sediment transport. Historical channelization and armoring have disrupted these dynamics, leading to increased downstream erosion. Efforts should focus on removing hard structures, creating side channels, and establishing benches to stabilize banks and enhance habitat complexity.
- Floodplain restoration: Floodplain restoration enhances ecological functions by reconnecting streams and rivers to their natural floodplains. These actions improve water storage, groundwater infiltration, and climate resilience while supporting biodiversity and water quality. In steep terrains, floodplain restoration focuses on mitigating erosion and stabilizing slopes. Effective strategies include revegetation, stormwater management, and removing impervious surfaces to reduce sediment transport. Maintaining shaded canopies further protects these systems from temperature increases.
- Water quality: Protecting and restoring water quality is critical across all habitats. Effective strategies include maintaining riparian buffers and shaded canopies to reduce sedimentation and mitigate warming, addressing sources of pollution such as urban runoff and wastewater effluent, minimizing nutrient overloading to prevent algal blooms, and implementing site-specific actions like reengineering stormwater systems and improving wastewater treatment to address localized threats.
- Invasive species management: Preventing the introduction of invasive species is essential to maintaining the ecological integrity of streams and rivers. Aquatic invasive species are easily spread from waterbody to waterbody by boats and waterfowl. Stream users should avoid emptying bait buckets and/or aquaria into waterways as it is illegal to do so into Massachusetts waterbodies. For existing invasions, early detection and aggressive removal efforts can minimize spread and impact. On floodplains, new invasive species should be eradicated as soon as detected and disposed of accordingly (e.g., placed into black garbage bags and brought to a landfill). Aggressive removal over the course of several years may be necessary to curtail the spread of invasive species. Replacing invasive plants with native vegetation will improve habitat quality and biodiversity.