Potomac River Reaches 94 Degrees As Fish Kill Reported In Maryland

SENECA, Md. — Record-breaking water temperatures likely contributed to a fish kill in the Potomac River near Seneca, where Maryland environmental officials recorded an extraordinary 94-degree water temperature following days of extreme heat across the region.

The Maryland Department of the Environment (MDE) confirmed the fish kill in a social media post, saying, “We’re aware of reports of a fish kill in the Potomac River near Seneca. Our biologist recorded water temperatures at a record 94°F. This can trigger a summer turnover event, when layered warm and cold water suddenly mix and oxygen-poor water rises to the surface, stressing fish.”

Although the fish kill occurred in Montgomery County, the Potomac River flows south along the borders of Charles and St. Mary’s counties before emptying into the Chesapeake Bay, making the conditions noteworthy for residents, anglers and boaters throughout Southern Maryland.

The fish kill follows several days of dangerous heat across Maryland and the Mid-Atlantic. Air temperatures climbed into the upper 90s and low 100s across much of the state, with parts of Montgomery County reaching about 100 degrees and nearby Washington, D.C., reaching 102 degrees during the recent heat wave.

The National Weather Service issued multiple heat alerts as high temperatures and humidity pushed heat index values well above 100 degrees. Scientists say prolonged periods of extreme heat can quickly warm rivers and streams, reducing the amount of dissolved oxygen available for aquatic life.

What Is A Summer Turnover?

According to MDE, a summer turnover occurs when warm surface water and cooler, deeper water suddenly mix. When that happens, oxygen-poor water from the bottom rises to the surface, causing dissolved oxygen levels to drop dramatically.

Fish rely on dissolved oxygen to survive. Warm water naturally holds less oxygen than cooler water, so when high temperatures combine with a turnover event, fish can become stressed or die within a short period.

State officials have not announced how many fish were affected or identified the species involved.

Maryland’s Fish Kill Trends

MDE’s most recent annual fish kill report provides additional context for this week’s Potomac River incident. According to the department’s 2025 Fish Kill Summary, Maryland received 89 fish kill reports statewide last year, confirming 80 fish kills that resulted in an estimated 273,353 fish deaths.

Southern Maryland accounted for a significant share of those reports. Calvert County recorded 10 fish kills in 2025, the second-highest total in the state behind Anne Arundel County’s 17. Charles County reported four fish kills, while St. Mary’s County recorded two.

The report found that natural causes were responsible for most fish kills statewide. Low dissolved oxygen was identified as the leading cause, contributing to 28 of the 48 fish kills attributed to natural conditions. MDE also noted that nearly three-quarters of all reported fish kills occurred between May and September, when warmer water temperatures and seasonal stratification are most likely to reduce oxygen levels.

Report A Fish Kill

Residents can report fish kills or other environmental concerns to the Maryland Department of the Environment’s Fish Kill Investigation Section through MDE’s Fish Kill Reporting page or by calling 443-482-2731 or 800-285-8195 during business hours. After hours, weekends and holidays, reports can be made through the Maryland Emergency Management Agency at 866-633-4686.

Learn More: MDE Fish Kill Reporting Page | 2025 Fish Kill Summary Report | National Weather Service | Maryland Department of the Environment

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