Bacteria Monitoring

Is it safe to swim in the water?” This is a common question ShoreRivers gets asked throughout the summer months when members of our community are hoping to find a refreshing place to cool off from the seasonal heat. To better answer this question, and to identify potential pollution hotspots, ShoreRivers runs a seasonal bacteria monitoring program.

ShoreRivers monitors 28 sites where people frequently come in contact with water, including popular swimming locations, marinas, yacht clubs, and towns. We monitor weekly or bi-weekly from Memorial Day to Labor Day and before holiday weekends. By collecting, analyzing and distributing data on bacteria levels in the river we aim to provide the public with the information they need to make educated decisions about their contact with the water.

If you are interested in learning the latest results from our bacteria monitoring, visit or follow #swimmableshorerivers on Facebook or Instagram.

Sampling Protocol

ShoreRivers follows EPA’s standard protocols for collecting and analyzing samples for enterococci, a type of bacteria commonly found in human and animal waste. Enterococci can enter our rivers when rain washes it in from failing septic systems, animal fertilizer off farm fields, livestock waste, sewer spills or overflows, pet waste, or wildlife.

The protocol uses a pass/fail system to determine if enterococci levels are safe or unsafe for swimming. For a single sample taken at one location, 104 MPN*/100 ml* is the threshold used to determine if the concentration meet acceptable swimming levels. Maryland regulations identify this threshold to determine safety at designated swimming beaches throughout the state (Code of Maryland Regulations At a site where three or more samples are collected at once, a geometric mean is calculated and 35 MPN*/100 ml is used as the threshold for safe swimming.

Note that while enterococci is an EPA recommended indicator for human health, there are additional risks with open water swimming that we do not regularly monitor, including vibrio, toxic algal blooms, and other biological and chemical toxins. Bacteria levels can also change, particularly as a result of significant rain events. Ultimately, each individual is responsible for making the decision to swim in our rivers. For more information regarding lowering the risk associated with open water swimming, please review our tips for safe swimming below.

*MPN = Most Probably Number of enterococci colonies.

Tips for Safe Swimming

There are inherent risks associated with coming in contact with river water, but you can lower your risk by following these guidelines and staying informed about the current health of your river.

  1. Check Swim Guide for current information.

  2. Do not swim within 48 hours of a major storm event. A major storm event is any storm that creates runoff, which provides a vehicle for bacteria to be transported from our land to our rivers.

  3. Do not swim if you have open wounds, scratches, skin lesions, an ear infection, or a comprised immune system.

  4. Do not swallow river water.

  5. Always shower or rinse off after swimming in open water.

  6. When in doubt, stay out - Never swim in cloudy or murky water, or if the water has an odd smell or appearance described as being more green or more brown than usual. Avoid areas where oil slicks, fish kills, or ‘scum’ is visible in the water.