These ponds are secured by impoundments (dams or embankment dams). It is usually found in calm, freshwater reservoirs, lakes, impoundments, gravel pits or quarries. Milling produces large chat waste piles and flat areas with tailings deposited in impoundments. After thickening, slurry is often stored in ponds or impoundments. This includes all types of landfills, surface impoundments, canals, and other containment facilities. The impoundments are typically lined with clay or synthetic liners. The area also features one large and several smaller hydro-electric impoundments. In addition there is one large, and several smaller hydro-electric impoundments in the area. Both are U.S. Army Corps of Engineers impoundments. The Supreme Court, however, has held that impoundments without Congressional authorization are unconstitutional.