By Jennifer Merrill, Ph.D.

We all know saltwater and freshwater habitats are home to very different animals. Those that travel from the ocean’s salty waters to the uppermost freshwater stream reaches, such as salmon and eels, are superheroes of the aquatic world. Our freshwater home aquariums pale, literally, in comparison to the collections we can grow in saltwater aquariums.

Nemo and Dory don’t belong in the streams of our region of North America. Yet, Stroud Water Research Center and others have found salt concentrations in some of the smallest streams in Pennsylvania have surpassed those of ocean water. 

Scientists have known that salt in freshwater systems has been a threat since the 1960s, yet our use of road salt has increased more than fivefold since then. Researchers have documented increased salinity’s acute and long-term impacts on macroinvertebrates, amphibians, and fish in streams, rivers, and lakes. 

Summer Salt

Contamination of fresh water from road salt is a year-round threat. Why? Groundwater in many areas is recharging from salt-laden surface water. That groundwater is a primary source of streamflow during the summer and fall when precipitation is less frequent. Temperature also affects how deadly salt in fresh water is for aquatic macroinvertebrates.

Research by John Jackson, Ph.D., and Dave Funk demonstrated the influence of temperature on salt toxicity to four species of mayflies. Higher salt concentrations were less lethal to mayflies exposed to colder temperatures, offering a reprieve for the mayflies.

That’s the good news. The bad news is that salt measurements in our streams can exceed that threshold in urban and suburban watersheds.

More bad news: Mayflies are more sensitive to salinity in higher summer temperatures, and road salt is now detected year-round because of the high rates of salt pollution in winter. 

Four measurements collected during the 2025 Winter Salt Stream Snapshot exceeded 3,000 milligrams of chloride per liter, a concentration that Jackson and Funk found was lethal to at least half of all aquatic species they tested. 

Cut the Salt

So what can we do? Nobody wants to leave ice on roadways or sidewalks. But we can be more careful and protect our fresh water. As with many environmental challenges, prevention is the most effective solution. Salt is expensive and difficult to remove from water. Here are some ways you can help:

• Be conservative with your salt use. 
• Remove snow and ice. Focus on removing snow and ice, not melting it with chemicals.
• Pick it up. Some municipalities sweep streets in the spring to pick up road salt and other debris, but many don’t, especially in rural areas. What appeared to be a small mound of rock salt in my suburban neighborhood filled three five-gallon buckets. In fifteen minutes I was able to clean up 15 gallons of salt to return to the salt shed. 
• Share your concern. Take photos of the road salt you remove and share them with your social media networks. Show how you are making a difference.
• Test a stream. Order a free Freshwater Test Kit from the Stroud Center and add your data to our map of stream saltiness.

Whether you are motivated by your concern for the environment or for protecting our drinking water, your actions can help. 

Additional Reading

• Myers, D.T. Freshwater salinization: Saltier rivers are a growing concern. 2024. National Parks Service, U.S. Department of the Interior.
• Jackson, J.K., and D.H. Funk. Temperature affects acute mayfly responses to elevated salinity: implications for toxicity of road de-icing salts. 2018. Philosophical Transactions of the Royal Society B: Biological Sciences 374(1764).
• Dugan, H.A., and S.E. Arnott. The ecosystem implications of road salt as a pollutant of freshwater. 2023. WIREs Water 10(2): e1629.