The principal aquifers of the Dayton area are sand and gravel layers in the 150- to 250-foot thick glacial deposits filling the river valleys (Miami River and its tributaries), which were originally cut in bedrock by preglacial streams. The upper and lower aquifers are separated by a poorly permeable till-rich zone, which confines the water in the lower aquifer; recharge to the lower aquifer is by vertical leakage through the till zone. The upper aquifer is pumped extensively only at Rohrers Island well field of Dayton, where water levels are kept high by artificial recharge. Although few cities in Ohio are as abundantly endowed with ground-water resources as Dayton, the demand is estimated to rise to quantities that cannot be supplied by the year 2000. A comprehensive plan for conservation is needed to meet future growth.