When a water-bearing rock readily transmits water to wells and springs, it is called an aquifer. Aquifers must be both permeable and porous and include such rock types as sandstone, conglomerate, fractured limestone and unconsolidated sand and gravel. 'Reservoir' is a large lake where water is stored to be used by a particular area. 'Estuary' is partly enclosed coastal body of water in which river water is mixed with seawater. 'Karst' is an area of land made up of limestone. Karst landscapes feature caves, underground streams and sinkholes on the surface.