A spring is a component of the hydrosphere. Specifically, it is any natural situation where water flows to the surface of the earth from underground. Thus, a spring is a site where the aquifer surface meets the ground surface.
A spring may be the result of karst topography where surface water has infiltrated the Earth’s surface (recharge area), becoming part of the area groundwater. The groundwater then travels through a network of cracks and fissure—openings ranging from intergranular spaces to large caves. The water eventually emerges from below the surface, in the form of a karst spring.
The forcing of the spring to the surface can be the result of a confined aquifer in which the recharge area of the spring water table rests at a higher elevation than that of the outlet. Spring water forced to the surface by elevated sources are artesian wells. This is possible even if the outlet is in the form of a 300-foot-deep (91 m) cave. In this case the cave is used like a hose by the higher elevated recharge area of groundwater to exit through the lower elevation opening.
Non-artesian springs may simply flow from a higher elevation through the earth to a lower elevation and exit in the form of a spring, using the ground like a drainage pipe.
Still other springs are the result of pressure from an underground source in the earth, in the form of volcanic activity. The result can be water at elevated temperature such as a hot spring.
The action of the groundwater continually dissolves permeable bedrock such as limestone and dolomite, creating vast cave systems.
Types of spring outlets
• Seepage or filtration spring. The term seep refers to springs with small flow rates in which the source water has filtered into permeable earth.
• Fracture springs, discharge from faults, joints, or fissures in the earth, in which springs have followed a natural course of voids or weaknesses in the bedrock.
• Tubular springs are basically underground cave systems formed by underground water.
Spring discharge, or resurgence, is determined by the spring’s recharge basin. Factors that affect the recharge include the size of the area in which groundwater is captured, the amount of precipitation, the size of capture points, and the size of the spring outlet. Water may leak into the underground system from many sources including permeable earth, sinkholes, and losing streams. In some cases entire creeks seemingly disappear as the water sinks into the ground via the stream bed. Grand Gulf State Park in Missouri is an example of an entire creek vanishing into the groundwater system. The water emerges 9 miles (14 km) away, forming some of the discharge of Mammoth Spring in Arkansas. Human activity may also affect a spring’s discharge–withdrawal of groundwater reduces the water pressure in an aquifer, decreasing the volume of flow.