Karst plains are characterized by a thick layer of soluble bedrock (limestone or dolomite), often topped by a veneer of sandstone. Cracks in this overlying roof allow rain water and snowmelt to percolate into the soluble bedrock; over thousands of years, the water opens channels in the limestone, eventually producing a network of caves and underground streams. In some areas, the cave roof collapses, forming a sinkhole at the surface, another source of entry for precipitation; indeed, in karst areas, much of the surface drainage is into these sinks and surface streams may be very limited.
The watershed of these underground stream networks is often very extensive; since they pass beneath surface divides, they may encompass the territory of several surface watersheds. Eventually, the underground flow discharges into a surface stream, usually as a spring along the walls of a river valley; in some areas, the discharge is via a vertical shaft, producing what is commonly called a blue hole.
The flow from karst springs may be steady or vary through the seasons, depending upon the size of the network or aquifer that feeds them. Though common throughout much of our country, karst springs are especially abundant in the dolomite and limestone regions of southwest Missouri, central Kentucky and Tennessee and northern Florida. Wakulla Springs, south of Tallahassee, is one of the largest and most productive karst springs on the planet; giving rise to the Wakulla River, it discharges over 250 million gallons of water each day, flowing from the vast, flooded cave system of the Woodville Karst Plain.
