Small Dams Have Large Impacts on Water Quality

Little is known about how small dams might impact water quality. The information below, gathered from a single study investigating this topic, provides an important piece of the puzzle.

Dams and their impoundments (the body of water created behind a dam) can impact water quality of the streams on which they are located. Good water quality is essential for a healthy stream. Too-high temperatures and too-low oxygen levels can cause substantial harm to important stream wildlife. From 2014 to 2016, DER-supported scientists conducted a study to better understand the implications of small dams on water quality and inform future stream restoration efforts.

Table of Contents

Key Takeaways

  • Temperature and dissolved oxygen were measured upstream and downstream of select dams in Massachusetts from 2014 to 2016.
  • Dam impoundments consistently had lower water quality compared to their upstream and downstream waters.
  • Negative effects from dam impoundments continue downstream. Two-thirds of sites studied had warmer stream temperatures downstream of dams and one-third had lower dissolved oxygen levels.
  • Dams with the largest negative impacts on water quality were found on high-gradient, coldwater streams in small watersheds.


A map of Massachusetts with many red dots spread across the state, representing dams.

Massachusetts has more than 3,000 dams, most of which are small, former mill dams. To date, the impacts of these dams on stream water quality has been poorly understood.

In 2014, a team of scientists from the University of Massachusetts-Amherst, with support from the Massachusetts Division of Ecological Restoration, began a two-year study investigating the impacts of dams on temperature and dissolved oxygen in streams across the Commonwealth. Temperature was studied at 30 dams and dissolved oxygen was studied at 12 dams. The results of this project can help natural resource managers better understand the cumulative impacts of dams on streams, as well as the potential for dam removal to improve water quality.

Observed Water Quality Impacts

Temperature1The surface water of the dam impoundments observed in this study was warmer than the water upstream of the impoundments. After spilling over the dams, the warm impoundment water elevated downstream temperatures at two-thirds of sites, with average August temperatures up to 5.3°C higher downstream than upstream of the dams. These elevated temperatures persisted for up to 4.5 km downstream of the dams. The highest stream warming was observed downstream of the dams with the widest impoundments (relative to the natural stream width) on the coldest streams.

Dissolved oxygen: Two-thirds of the dam impoundments observed in this study had lower surface dissolved oxygen than upstream levels. Downstream dissolved oxygen levels were found to be minimally impacted, suggesting that the aeration that occurs when water spills over the dam may help alleviate the negative effects from the lower dissolved oxygen levels in the impoundment. Coldwater streams with a high gradient (having a steep slope and rapid flow of water) in small watersheds and dams with shallow impoundments had the greatest reduction in dissolved oxygen levels both within the impoundments and downstream of the dams. Further investigation is ongoing on this topic.

A photo of water flowing over a dam with text "Impoundment: warmer surface water, low dissolved oxygen observed at two-thirds of studied sites" above the dam and the text "downstream: warmer water observed at tow/thirds of studied sites, low dissolved oxygen observed at one-third of studied sites" below the dam.

Dams and their impoundments had the most negative impact on the water quality of high-gradient, coldwater streams in small watersheds.


Elevated stream temperatures and low dissolved oxygen levels associated with small dams can result in increased stress and death to fishes, mussels, stream insects, and other aquatic organisms. As streams warm, native coldwater species can be replaced by non-native generalist species, as has occurred throughout Massachusetts. This study found that the warming effects of dams was most pronounced in the naturally coldest streams, suggesting that temperature-sensitive, coldwater species such as eastern brook trout are especially susceptible to warming impacts by small dams.

All aquatic organisms require oxygen to survive. Hypoxic (low dissolved oxygen) and anoxic (no dissolved oxygen) zones that can form in dam impoundments are inhospitable to aquatic species. Very low oxygen levels can also reduce leaf litter breakdown and change nutrient cycling in streams, which may alter energy availability to aquatic organisms.

Dam Removal Can Improve Stream Water Quality

Dam removal restores natural river processes. This study highlights the negative impacts on stream temperature and dissolved oxygen associated with dams. Between 2000 and 2018, 60 dams were removed in Massachusetts, and more are being removed every year.2 The insights from this study may help DER and partners identify and prioritize dam sites for future restoration.

Funding for this research was provided by the U.S. Department of the Interior through a grant from the National Fish and Wildlife Foundation’s Hurricane Sandy Coastal Resiliency Competitive Grant Program, the Massachusetts Division of Ecological Restoration, the University of Massachusetts Amherst’s Department of Environmental Conservation, and the U.S.D.A. Forest Service.

Check out this webpage for more information about dam removal in Massachusetts.


1Zaidel, P. A., Roy, A. H., Houle, K. M., Lambert, B., Letcher, B. H., Nislow, K. H., & Smith, C. (2021) Impacts of small dams on stream temperature. Ecological Indicators, 120, article 106878.

2American Rivers Dam Removal Database.

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