Habitat description
Gently-sloping warm rivers are rivers characterized by large drainage areas, slopes with >2% gradient, average summer water temperatures >22°C, and a stream order >4. These mainstem rivers have wide floodplains with substrates dominated by gravel, sand, and silt.
Associated habitat types
Characteristic communities and species
These rivers are home to a variety of freshwater and salt-tolerant fishes. Inland rivers can include American eel, brown bullhead, chain pickerel, pumpkinseed, redbreast sunfish, spottail shiner, and banded killifish. Those rivers more closely associated with the coast can also include alewife, blueback herring, killifish, and mummichogs. Popular game fishes also found in gently-sloping warm rivers include large and smallmouth bass and several catfish species. Some of these rivers even see migrations of striped bass. Freshwater mussel species that typically inhabit low gradient warmwater rivers include eastern lampmussel, eastern elliptio, and alewife floater. In a subset of river segments, state-listed yellow lampmussel, tidewater mucket, and eastern pondmussel may also occur. These rivers also support rich and distinct dragonfly and damselfly assemblages that may include species of majestic, hanging, and merican clubtails, shadowdragons (Neurocordulia), prince baskettail, black-shouldered spinyleg, swift river cruiser, fawn darner, powdered dancer, blue-fronted dancer, and stream bluet, among others.
Ecological processes
As tributaries come together to form larger rivers, stream channels get deeper and wider. The cooling effect from groundwater and shaded banks weakens and results in warm water temperatures. Wide floodplains allow river channels to form well-defined meanders that at times may separate from the primary channel to create oxbow lakes. Wetlands, like freshwater marshes, are often associated with these rivers. Riverbanks are usually dominated by emergent vegetation and river margins and backwaters can support beds of submerged vegetation. Mainstem rivers that reach the ocean will be shaped by incoming and outgoing tides and may support tidal freshwater and brackish habitats. Brackish areas (areas where fresh and saltwater mix) are some of the most productive habitats, benefiting from both upstream and marine nutrients. In freshwater sections of the river, primary productivity is largely dependent on within-channel components such as algae and aquatic vegetation.
Threats
- Climate change: Changes in precipitation patterns due to climate change are strengthening the impact of peak flows and droughts. Channel scouring will likely become more significant in winter and spring. Water depth continues to decrease significantly in summer and fall. Reaches with the slowest moving water may experience extended periods of algal and submergent vegetation blooms which can result in anoxic (low oxygen) conditions in late summer and fall. Increasingly warmer conditions may spread diseases that can result in localized fish and mussel kills occurring more frequently.
- Invasive species: Invasive species have changed the structure and function of large river streambanks and floodplains. Species like purple loosestrife and common reed offer little by way of benefits, including foraging, to birds. Cyanobacteria invasive milfoil, and Asian clams may also proliferate within the stream channel.
- Dams & urbanization: Due to their location at the lowest elevations, conditions in large low-gradient rivers generally reflect the cumulative effects of land and water use throughout their watersheds. Watersheds with multiple dams and high percentage of urban development are most likely to have the most degraded large rivers. Dams can disrupt aquatic species migrations, streamflow, nutrients, and sediment patterns. Urbanized areas can also disrupt streamflow patterns from impervious ground cover and channelization as well as be sources of wastewater and pollutants that degrade water quality and instream habitats.
- Stormwater and wastewater: Most of our gently-sloping warm rivers receive multiple stormwater inputs from large urban centers and wastewater inputs from local or even regional wastewater treatment plants. Stormwater management can reduce inputs from road-related pollution (salt, oil, etc), and properly-maintained wastewater treatment plants that can reduce or eliminate combined sewer overflows can greatly reduce direct discharge of pollutants into these rivers.
Restoration & management recommendations
The condition of large rivers reflects the cumulative effects of land use in the watershed that drains into it. Consequently, restoration of hydrologic and morphological processes should consider the upstream watershed. Coordinated, comprehensive restoration efforts will benefit from watershed-scale conservation plans.
- Dam removal: Dams affect all large rivers in the state, both directly when they are in the river itself or indirectly when located in tributaries. Reaches downstream of the dam are likely to be cemented (brick-like arrangement of cobbles) because fines like sand and silt are now retained behind dams, starving downstream reaches of sediment. Changes to the substrate make it harder for species needing loose gravel for reproduction to persist. Reaches downstream of dams in Massachusetts can also be warmer and have lower oxygen concentrations than upstream of the dam. When dam removal is not feasible, passage structures (e.g., bypass channel) can further improve conditions by allowing some exchange of species, sediment, and nutrients between habitats upstream and downstream of the dam.
- Water quantity restoration: Many large rivers in Massachusetts have large dams that can change the quantity and seasonality of water flowing downstream during different times of year. One of the best ways to mitigate the effects of hydropower dams is to operate them as run-of-river (RoR),. Although still impactful, RoR will mimic natural hydrologic patterns better than peaking operations because inflows and outflows from reservoirs will be about equal. Instream flow protection that promotes healthy rivers, particularly under changing climatic conditions, should be at the forefront of water withdrawal management efforts.
- Water quality restoration: An ongoing challenge in Massachusetts is the large quantity of wastewater effluent entering some rivers. While wastewater treatment has greatly improved, many municipalities have combined sewer overflows (CSOs) that result in raw or partially untreated sewage flowing into waterbodies when stormwater overwhelm existing infrastructure. Warmwater rivers are prone to increasing algal blooms and high bacterial concentrations when wastewater overloads mainstem rivers. Separation of stormwater and wastewater systems can help prevent spills and restore water quality. Stormwater management can also be improved to better treat effluent from increasingly large storm events. Stormwater management can reduce temperature inputs to streams, and reduce pollutants like salt, oil, and other road residues.
- Floodplain restoration: Rivers benefit from unrestricted connections to their floodplains. Floodplain restoration actions may include revegetation, riparian forest management that favor growth of large trees, “daylighting” of mainstem reaches and tributaries in urban areas, and replacement of paved surfaces with permeable materials. Whenever possible, paved surfaces—particularly in floodplains—should employ improved stormwater managementto allow water to infiltrate into the ground instead of being diverted directly into stream channels.
- Channel and shoreline restoration: Channel and shoreline restoration along warm rivers can take several forms and is closely tied to floodplain restoration. Specific actions include removal of hard structures from streambanks (e.g., riprap, concrete), removal of dykes and berms that constrict channel movement, revegetation of banks, and contouring of stream banks to lower their elevation. Warm rivers are usually large systems that require intact vegetated buffers of widths greater than 200 ft (and up to 3000 ft) to protect water quality and nutrient cycling.
- Invasive species management: Prevention is the best management strategy for reducing invasive species in large rivers and their floodplains. Aquatic invasive species are easily spread from waterbody to waterbody by several vectors including boats, escaped hydroponics, and aquarium reasesand even waterfowl. Once established they are largely impossible to eradicate. Boaters should remove plants and animals, warm wash and thoroughly dry boats between sites. Live wells, bait buckets and cooling water should be emptied far from shore. It is illegal to empty aquaria into Massachusetts waterbodies. 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 such as purple loosestrife, common buckthorn, autumn olive, and other invasive plants once they are established. A clear vision of desired outcomes should be established prior to commencing eradication efforts.
Examples
Examples of large low-gradient warmwater rivers in Massachusetts include the Blackstone, Charles (downstream Bellingham; pictured above), Concord, Mystic, Nashua, Neponset (downstream Norwood Memorial Airport), Quinebaug, Taunton, and Connecticut Rivers.
Additional resources
Boat Brochure for Preventing Spread of Invasive Species
Aquatic Invasive Species in Massachusetts
Stop Aquatic Hitchhikers Handout
Managing Aquatic Invasive Plants
Strategic Recommendations for Managing Invasive Plants in Massachusetts