What Salinity Tells Us About Our Waterways
July 2026
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When WET Creekwatchers collect their water samples, one of the many water quality parameters that we measure is salinity. This is a simple number that tells us a great deal about the health and character of our waterways.
Put simply, salinity is the measure of how much salt is dissolved in water. Here on Maryland’s Lower Eastern Shore, salinity is what gives our rivers, creeks, marshes, and bays their unique identities. It also determines which plants and animals can call each of these habitats home.
You might know from experience that water isn’t equally salty everywhere! In general, the closer you get to the Tangier Sound and Chesapeake Bay, the saltier the water becomes. Our monitoring data consistently reflects this pattern. Freshwater ponds like Mitchell Pond and upstream sites like East Downtown Zoo typically have very low salinity. Sites closer to the Bay, such as Pirates Wharf and Whitehaven, experience higher salinity.
However, these conditions are not fixed. Heavy rainfall can temporarily lower salinity by adding more freshwater to the system, while prolonged dry periods like we experienced throughout June this year can allow saltier water to move farther upstream. Over the long term, sea level rise is also changing the balance, allowing saltwater to reach areas that were once predominantly fresh.
So – why does salinity matter? Different aquatic species have evolved to live within specific ranges of salinity. Some can survive only in freshwater, while others require saltwater. Between those extremes is what we call brackish water (a mix of fresh and salt) that supports a wide diversity of life. Monitoring salinity helps us to understand not only today’s conditions, but how habitats are changing over time.
One of the best examples of adaptation is the humble mummichog. Although they are small and often overlooked, mummichogs are among the most resilient fish in our watershed. They can survive in fresh, brackish, and salt water environments and can tolerate rapid changes in salinity caused by tides and storms. This flexibility allows them to thrive in tidal marshes and creeks where conditions change daily. Their success benefits the whole ecosystem because they serve as an important food source for larger fish, birds, and other wildlife.
That said, not every adaptable species is good news! The blue catfish, introduced to the Chesapeake Bay watershed decades ago, has developed the ability to tolerate a broad range of salinity, too. This has allowed the species to move beyond freshwater rivers and expand into brackish portions of the Bay. Blue catfish are highly successful predators with huge appetites. They compete with native fish for food and prey on many species that keep our ecosystems balanced, including our beloved blue crabs and juvenile rockfish! Their ability to tolerate changing salinity has contributed to their rapid spread.
By tracking salinity year after year, we build a clearer picture of the health of our watershed and the future of the ecosystems we all value. Every data point helps to tell the story! Be sure to check out what’s going on in the watershed on our River Updates webpage. You can also learn more about salinity trends over time in the feature article from our 2023 Wicomico River Watershed Report.
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