Wetlands are areas that remain wet for significant periods and form unique ecosystems for plants and wildlife where water accumulates faster than it drains away; some are inundated year-round, others hold water only briefly in spring, and most are covered with water for less than a month during summer. They are vital to clean water supplies because they filter water before it passes into other bodies of water. Historically, wetlands were often seen as unappealing, useless, disease-ridden places, so many were drained, cleared, and converted to crop production, while in urban areas others were filled for houses, industrial facilities, office buildings, and sanitary landfills. Early farming practices in Maryland often disturbed wetlands and lowered the state's total percentage of wetlands by 45-65% over time, and the Chesapeake Bay watershed experienced substantial losses between the mid 1950's and the late 1970's, averaging more than 2,800 acres a year. Greater understanding of their importance to healthy water supplies has led to modern agricultural practices aimed at preserving Maryland's wetland resources. This Piedmont wetland filters water flowing into the Potomac River and then into the Chesapeake Bay. Frederick County contains approximately 420,000 acres of wetland that provide water filtration and habitat for a variety of birds, mammals, reptiles, and amphibians, while also offering recreational opportunities such as hunting, fishing, and birdwatching and enhancing the health, vitality, and open space character of the landscape. Plants that grow in wetlands are called hydrophiles, and Maryland has 1,655 species of wetland plants, including skunk cabbage, jewelweed, arrow arum, spatterdock, reeds, broadleaf, arrowhead, cowslip, cardinal flower, blue iris, royal fern, cattails, and swamp rose. Some of these plants were used by people: Native Americans harvested broadleaf arrowhead, or wapato, for its tubers, reeds were used by Native Americans and early settlers in the Middletown Valley for thatching and for making household goods such as baskets, and jewelweed was valued for relieving skin irritations. Wetlands maintain clean, healthy waterways by moderating overland water flow to streams, reducing runoff, pollution, and erosion, sequestering pollutants so they can biodegrade when possible, absorbing heavy rainfall to reduce storm and flood damage, and trapping water so it seeps slowly into the earth and recharges groundwater supplies, which helps landscapes absorb and recycle water and reduces the likelihood and severity of droughts. A wetland is identified by three attributes: hydrology that saturates the area at some point during the growing season, hydric soils with low-oxygen conditions caused by water being present long enough during that season, and predominantly hydrophyte vegetation.