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Floodplains are those portions of the land that adjoin watercourses and are periodically inundated. They are created when high levels of running water deposit loose soils and gravel that have been picked up from ravines and stream corridors. Floodplains are valuable in the Severn River area because they provide habitat for wildlife, serve as buffers between communities, and help protect water quality by collecting sediment from stormwater runoff.
The slope of a floodplain varies with the volumes of runoff and the types of soil collected. The upper floodplain is often composed of loose, sandy material which easily absorbs the normal volumes of runoff from nearby steep slopes and ravine troughs. The lower portion may consist of finer soil which supports dense plant growth. Here, stormwaters spread on the broader plain, velocities are reduced, and soil particles are deposited amid the vegetation. 
Plant growth in the floodplain follows a rather specific pattern. Most stream corridors on the Severn are too small to encompass the entire range of the well-known species. Those floodplains with the most variety are found along Severn Run and the watershed of Little Round Bay. In these areas the lowland forest includes river birch, sycamore, red maple, sweet gum, pin oak and willow oak. The dense shrub layer includes clethra, arrowwood and greenbrier.

The existing floodplain equilibrium may be upset by development in the watershed. Increased stormwater volumes erode the sandy material from the upper floodplain, enlarge stream channels and cause accelerated deposition on fertile floodplain terrain.