Thanks for coming to our project web page! This introduction to coldwater and dam removal was made possible by a grant from the Massachusetts Council of Trout Unlimited and the Greater Boston Chapter of Trout Unlimited, in partnership with NepRWA.

As part of the project, informational signs were posted by coldwater streams in the Neponset Watershed with links to this page. If you came here from one of those signs, welcome! For more information, please reach out to NepRWA River Restoration Director, Sean McCanty, or

The Fish Need Our Help

If keeping the Neponset River clean is important to those of us who live in its watershed, think of how important it is to those living in and on the water of the river and its tributaries. Rivers and streams provide a home for many living beings, from dragonflies to fish.

As a prized sportsman’s catch, brook trout are a special concern. NepRWA has partnered with Greater Boston Trout Unlimited (GBTU) in a joint effort to preserve the trout resources in the Neponset Watershed. This website outlines the challenges and opportunities facing us amid a changing climate.

Where Are the Trout?

Native brook trout in the Neponset River Watershed are classified as coldwater fish, because they flourish in water below 68°F.

A stream or river where coldwater fish live and reproduce is called a Coldwater Fish Resource (CFR). MassWildlife has identified seven CFR streams in the Neponset River Watershed.

  • Traphole Brook in Sharon/Norwood
  • Pine Tree Brook in Milton
  • Germany Brook in Westwood/Norwood
  • Purgatory Brook in Westwood/Norwood
  • Beaver Brook in Sharon
  • Tubwreck Brook in Dover
  • Mill Brook in Dover/Westwood

In addition to trout, coldwater fish include Rainbow Smelt, Longnose Suckers, and others.

Brook Trout, salvelinus fontinalis

Coldwater vs Warm Water

Trout are considered a “coldwater” species because they require water temperatures consistently below 68°F with lots of oxygen.

In contrast, warmwater species, like sunfish or bass, don’t have as many requirements. Coldwater streams mostly originate from groundwater springs, while warm waters can have their sources in lakes and ponds.

What Trout Need

Coldwater streams meet several needs for trout habitat, starting with dissolved oxygen. Because they are typically steep and fast-flowing, coldwater streams absorb air when they bubble over rocks as riffles or rapids.

Other key habitat elements of coldwater streams include:

  • a mix of sandy and rocky stream bed.
  • frequent deep and slow pools.
  • tree cover to keep the water cool.
  • plenty of large woody debris or root wads.

Female Brook Trout will lay their eggs in sandy or gravel areas that have cool groundwater discharging into the stream. These nests or “redds” are critically important to maintaining brook trout populations.

Large Woody Debris

Large woody debris (LWD) refers to large piles of dead wood along streambanks: from whole trees complete with root wads to large tree limbs.

LWD is a vital and naturally occurring component of healthy stream ecosystems. Besides fish habitat, the ecological benefits of LWD include streambank stability and wider biological diversity.

As a result, LWD removal should be considered only when there is compelling evidence that LWD is causing property flooding, contributing to significant streambank erosion, or poses a navigational hazard.

When cutting streamside trees or removing LWD, permission and approval must always be obtained first. For property owners, always contact your municipal officials in charge of Wetland regulations (typically town/city conservation agents) to determine if the proposed work is allowable and if it will require any local regulatory approval or permits.

For non-owners, you must also determine property ownership in the specific area where the work is to be done and obtain permission from the property owner to legally gain access and perform the work.

Large woody debris in stream. Photo credit Fishbio.

Threats facing Coldwater Trout

Any threat to the health of coldwater streams is also a threat to trout, including:

  • Pollution and other habitat degradation
  • Dams
  • Climate Change


The #1 pollution threat to coldwater streams is stormwater runoff, a term covering water from any size rainfall draining into our streams from roads, parking lots and other paved surfaces.

Pine Tree Brook outfall in MIlton.

Water running off from paved surfaces almost always carries pollutants, ranging from road salt and oil to lawn fertilizers and road dust.

Besides directly harming coldwater fish, pollutants encourage algae growth and raise nutrient levels in stream water, thereby reducing dissolved oxygen. Pet and animal waste, another common pollutant, adversely impacts bacteria-levels in streamwater.

Stormwater pollution is significantly worse in urban areas of the Neponset Watershed due to minimal vegetation. In less developed areas, rain moves slowly through grass and other vegetation, filtering out much of the harmful material. 

Habitat degradation occurs in other ways besides pollution. For example, rain falling on sunbaked asphalt can warm up significantly more than if it fell on grass. This additional heat is carried into the stream, raising the overall water temperature. The runoff can cause heat stress on fish in addition to the pollutants it carries.


Historically, the New England economy depended on dams to power everything from grist mills to large textile factories.

Among the estimated 3,000 dams across Massachusetts today, the majority are small structures from earlier times that are no longer in use and often unmaintained.

Ponkapoag Brook Dam, Canton.

For conservationists, these old dams pose a particular challenge because most of them stretch across the entire stream or river. These so-called “run-of-river” dams create impoundments behind them – lakes, ponds, and reservoirs. Impoundments can be harmful to coldwater habitats for trout.

For example, because they are usually less shaded by trees than a river or stream, ponds and lakes raise water temperatures. And the slower moving water tends to lose dissolved oxygen, both from heat and from the decomposition of organic material on the pond bottom.

By blocking the flow of water, dams create problems both upstream and downstream.

Upstream, sediment builds up in the ponds and reservoirs behind dams. Downstream, the loss of sediment results in faster stream flow and increased erosion along stream banks. 

The upstream problem is especially critical given the age of many dams. Unless accumulated sediment is regularly removed from behind the dam, the impoundment pond will become shallower, further raising the water temperature. But many small dams are privately-owned and poorly maintained. Most do not require state inspection or any requirements for regular maintenance.

Besides these issues, dams also create breaks in the natural flow so fish cannot swim upstream. By fragmenting fish populations, dams put them more at risk of dying out.

Dams also pose a threat to humans. Typical run-of-river dams offer no protection from flooding.  Dams can collapse from water backing up behind them during storms. The surge of water and sediment from a collapsed dam can overwhelm properties and infrastructure downstream, as well as demolishing wildlife habitat.

Removing Dams

Today, dams are being reevaluated throughout the Commonwealth. While some dams have been repaired, others have been removed entirely. The litany of problems outlined above basically breaks down into two primary reasons to remove dams.

  • Restore coldwater trout fish and other wildlife habitat.
  • Remove the threat of flooding from old poorly-maintained dams.

Add the fact that dam removal often costs less than repairs, and you have a clear case for removing old, out-of-use dams.

A good example of a recent dam-removal success is the Mill Pond dam on Traphole Brook in Norwood.

With the completion of the project in 2022, water flowed through woody debris and over the rocky bottom channel for the first time in over 200 years.

Within days, trout were spotted checking out the new channel section, and one year in, trout were in high abundance within the restored section.

The list of dams under consideration for removal in the Neponset Watershed ranges from small dams like Mill Pond to large dams such as the Baker Dam in Lower Mills.

Baker Dam in Lower Mills on the Dorchester/Milton line.
Mill Pond Dam removal in Norwood.
A restored Traphole Brook after dam removal.

A Changing Climate

In the long run, climate change poses the biggest threat to trout and other coldwater species in the Neponset Watershed.

As the world grows warmer from greenhouse gas emissions, we have already seen significant shifts in climate patterns in our region.

Drought one year, followed by rainstorm after rainstorm the next. For humans, that means dealing with water use restrictions one year and flash floods the following year. 

Flooding in Norwood, 2020.

For coldwater fish like trout, climate change has brought a range of challenges, including:

  • reduced water flow and increased water temperatures in droughts.
  • increased salinity from road salts washing in during mild winters.
  • high stream flows during heavy rainstorms, causing erosion and habitat destruction. 

As we deal with climate change on a global scale, what can we do to help protect native brook trout in the Neponset Watershed?

  • Improve water quality by filtering and cleaning stormwater.
  • Conserve water in order to ease the drain on groundwater supplies.
  • Remove barriers to fish passage such as dams and small culverts.

We’re working to protect our streams!

In recent years NepRWA has undertaken a number of construction projects to protect our streams, including rain gardens, bioswales, and infiltration trenches, all with the aim of filtering and cleaning stormwater before it flows into our streams.

In partnership with Greater Boston Trout Unlimited (GBTU), we are now extending our efforts to ensure that streams in the Neponset Watershed stay lined with native vegetation to buffer and store water during storms.

For More Information on Coldwater Streams and Trout:

For More Information on Dam Removal:

Local Advocacy: