Hempstead Lake began in the early 1800s when Oliver's Eagle Flour Mill, Long Island's largest gristmill, was built, and the original mill pond was enlarged into a lake in 1870 with a 22-foot-high dam to create a 400-million-gallon reservoir for Brooklyn residents. Because the sandy, permeable soils allowed most of the water to seep into the ground, the reservoir project was soon abandoned, and the lake still has difficulty retaining water. Long Island's drinking water comes from an aquifer of glacier meltwater more than 10,000 years old that is replenished when rain and melting snow seep into the ground. Lakes, rivers, and wetlands form where the underground water table reaches the surface, so Hempstead Lake's water level rises and falls with that water table. During droughts, the water is often 2 to 3 feet below the lake bottom, causing the lake to dry up. Demand for water in the surrounding communities, along with roads, buildings, and parking lots that reduce seepage into the ground and sewage treatment plants that discharge treated water into nearby bays, has reduced recycling back into the aquifer, and the water table in Nassau County has dropped 15 to 20 feet since the 1960s. During a drought in 2002, Hempstead Lake nearly dried up, some wildlife including turtles was moved to nearby lakes, and after rains recharged the aquifer the lake refilled and fish were restocked from other ponds on Long Island.