Spring, a natural flow of water issuing at the surface of the earth. A spring occurs where underground water held in porous rock, called an aquifer, discharges at the surface through a natural opening, usually on a hillside slope. The flow may be large and steady, or it may fluctuate widely and even dry up for part of the year. If small and sluggish, the flow is usually called a seep.
In many cases, springs issue from the saturated zone of ground water, which is supplied and maintained by surface water draining downward through the earth. Springs also form above the main body of groundwater when the descending water encounters an impervious (nonporous) local layer and is diverted sideways to an outlet. In some cases springs may also be of artesian origin, where the water is forced vertically to the surface under its own pressure.
Because of varying origins and seepage through different kinds of soluble and non-soluble rocks, springs vary greatly in the purity and chemical content of their water. Some springs produce exceptionally pure water; some yield highly mineralized water and are called mineral springs. In some localities, mineral springs are believed to have medicinal value. Such springs—and the resorts that have been developed at some of them—are commonly called spas. The springs are usually hot springs, or thermal springs, and originate in high-temperature areas within the earth, especially areas of surface or subsurface volcanic activity.
