Tidal River

The sanctuary is located in a transition zone, where the freshwater of the Potomac River mixes with saltwater of the Chesapeake Bay through daily tides that fluctuate from 1–2 feet. The water is largely fresh, with a salinity that typically ranges from 0.1–0.5 parts per thousand. The sanctuary waters support a variety of fish populations, such as largemouth bass, blue and channel catfish, striped bass, shad, river herring, and sturgeon.

From the late spring to early fall, the shallow waters near the shoreline contain large beds of submerged aquatic vegetation (SAV), or underwater grasses. They form grassy meadows and weed beds that serve as important feeding grounds and spawning and nursery habitat for juvenile fish, crustaceans, mollusks, and other animals. There are approximately a dozen different species of SAV that can be found in the sanctuary, including wild celery, water stargrass, coontail, and several species of naiads. A non-native grass called hydrilla, which was introduced to the Potomac River in the 1970s and 80s, is also dominant. While hydrilla offers some benefits and is protected in the Chesapeake Bay, it can grow very thick and crowd out native grasses.