Marshes are areas of shallow water, thickly vegetated by characteristic herbs and grasses, without shrubs or trees. Approximately 100 "brackish-water estuarine river marshes" totaling 118 acres have been inventoried along the Severn. Generally small, these marshes range from one-quarter acre to 15 acres. Fringe marshes along the sandy shoreline exhibit limited diversity and are usually dominated by saltmarsh cordgrass, which protects the shoreline from wave-induced erosion. Marshes provide habitat for many creatures, but most importantly, their rich plant life forms the foundation of the estuarine food chain.
Marshes at the heads of creeks and coves are laid on silty sediments and accumulated organic matter. Among the numerous plants are narrowleaf cattail, arrow arum, wild rice, sedges, cordgrass, saltmarsh waterhemp, and the invasive reed phragmites. Flowering plants include cardinal flower, blueflag, rose mallow, seashore mallow, and saltmarsh fleabane. Sullivan's Cove marsh, a sanctuary operated by the Nature Conservancy, is a particularly large and rich marsh on the north shore of Round Bay. Contrary to popular expectation, mosquitoes are not a major problem in Severn marshes, which can be accessed in small boats at high tide.