Before You Swim: Check Water Quality At Potomac River And Southern Maryland Beaches

SOUTHERN MARYLAND — Following a stretch of dangerous heat and with more showers and thunderstorms in the forecast this week, many Southern Maryland residents and vacationers will be looking to rivers, lakes and beaches to cool off. Before diving in, experts recommend checking the latest water quality conditions, as rainfall can temporarily increase bacteria levels and make some swimming areas unsafe.

A free online resource called Swim Guide helps visitors determine whether a swimming area recently met recommended water quality standards. The website and mobile app provide current recreational water quality information for more than 10,000 beaches, rivers, lakes and swimming areas across the United States and around the world, making it a valuable tool whether headed to a local swimming hole or traveling on vacation.

Created by the nonprofit Swim Drink Fish, Swim Guide was developed after its founders found that reliable beach water quality information was often difficult for the public to access. Today, the platform relies on more than 100 affiliate organizations — including riverkeepers, conservation groups, educational institutions and government agencies — that monitor waterways and upload current testing results.

In Southern Maryland and the surrounding Potomac River watershed, water quality information is provided by the Potomac Riverkeeper Network, which regularly collects water samples during the swimming season and publishes the results through Swim Guide.

The website allows users to search for a specific beach, browse an interactive map or locate nearby swimming areas. Each location includes the most recent sampling date, water quality status, weather information and beach details, helping visitors make informed decisions before entering the water.

Swim Guide uses a simple color-coded system based on bacteria testing.

• A beach is marked green when results show enterococci levels below 36 colonies per 100 milliliters of water — meeting recommended recreational water quality standards.
• A beach is marked red when results show enterococci levels above 36 colonies per 100 milliliters of water, and swimming is not recommended until conditions improve.
• A beach is marked gray when the results were collected more than one week ago and are no longer considered current.

Enterococci are bacteria found in the intestines of humans and animals. They wash into rivers, lakes and beaches through sewage overflows, stormwater runoff, pet waste, wildlife droppings and failing septic systems.

The bacteria themselves rarely make people sick. But their presence is a warning sign — if enterococci are in the water, other harmful germs could be there too. Testing for enterococci is a quick, reliable way to flag that risk without testing for every possible pathogen one by one.

Why Checking Before Every Visit Matters

Water quality can change quickly, particularly during the summer months.

The Potomac Riverkeeper Network warned that heavy rainfall can wash stormwater runoff, pet waste, wildlife waste and other pollutants into rivers and streams, causing bacteria levels to rise. While high temperatures alone do not determine whether a location passes or fails testing, periods of extreme heat are often accompanied by pop-up thunderstorms and changing water conditions.

Because of this, the organization recommends checking Swim Guide before every trip to the water rather than relying on previous visits. It also advises waiting at least 48 hours after rainfall of more than one inch before swimming, when bacteria levels are often at their highest.

Checking current conditions has become just as important as checking the weather forecast, especially during the hottest weeks of summer when more people head outdoors to cool off.

The latest available Swim Guide results, current at the time of publication, showed three monitored locations did not meet recommended recreational water quality standards:

• Mallows Bay – Grady’s Spit, Grady’s Spit, near Nanjemoy in Charles County, Maryland — sample taken July 1 by the Potomac Riverkeeper Network
• Shirley Boulevard in Port Tobacco, Charles County, Maryland — sample taken July 1 by the Potomac Riverkeeper Network.
• Lake Lariat in Lusby, monitored bi-weekly by the Calvert County Health Department; a history of agricultural runoff and algae blooms has contributed to past water quality issues at the lake.

Useful At Home And On Vacation

Swim Guide isn’t just for Southern Maryland. The platform lets travelers check water quality at thousands of beaches, lakes and rivers across North America and several other countries before heading into the water.

In addition to current testing results, Swim Guide provides maps, weather information, beach amenities and a pollution reporting feature that allows users to notify local monitoring organizations of suspected pollution or environmental concerns.

Swim Guide emphasizes that its information is intended to complement — not replace — official government advisories. Visitors should always follow posted beach closures, health advisories and other safety warnings.

Readers can explore current water quality conditions by visiting SwimGuide.org or downloading the free Swim Guide mobile app.

Quick Links: Swim Guide | Find a Beach on Swim Guide | Potomac Riverkeeper Network

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