Many reservoirs are fed by underground water supplies flowing in aquifers.
An aquifer is an underground layer of water-bearing porous stone, earth or gravel. Groundwater in aquifers flows to the surface naturally and usually discharges into reservoirs, springs, seeps and they can also form wetlands. The permeability of the geologic layer of which an aquifer is formed determines the importance of an aquifer as a source of groundwater.