Hempstead Lake originated in the early 1800s when Oliver's Eagle Flour Mill, Long Island's largest gristmill, was built. In 1870, the original mill pond was enlarged when a 22-foot-high dam created a 400-million-gallon reservoir intended to supply water to Brooklyn residents, but the project was soon abandoned because most of the water seeped into the ground through the sandy, permeable soils. Long Island's drinking water comes from an aquifer containing meltwater from the glaciers that formed the island more than 10,000 years ago, replenished as 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. Heavy demand for water in surrounding communities, along with roads, buildings, and parking lots that reduce seepage into the ground and sewage treatment practices that discharge treated water into nearby bays, has reduced groundwater recharge, and the water table in Nassau County has dropped 15 to 20 feet since the 1960s. During a 2002 drought, 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.