Sinkholes are closed depressions in the land surface formed by dissolution of near-surface rocks or by the collapse of the roofs of underground channels and caverns. Sinkholes are a natural, common geologic feature in places underlain by soluble rocks such as the limestone and dolomite that form the Floridan aquifer system. Under natural conditions, sinkholes form slowly and expand gradually. However, activities such as dredging, constructing reservoirs, diverting surface water, and pumping groundwater can accelerate the rate of sinkhole expansions, resulting in the abrupt formation of collapse-type sinkholes, some of which are spectacular. For more information, go to the Special Feature on sinkholes.
Rivers usually start as small streams and get larger as they flow downstream.
The water they gain is often groundwater. Such a steam is called a gaining stream. It is also possible for streams to lose water to the ground at some points, In these cases, aquifers are replenished by water from the losing stream. A stream which flows near the surface of an aquifer will lose water to the aquifer if the water surface in the stream is higher than the water table of the aquifer. A stream will gain water if the water surface of the stream is lower than the water table in the adjacent land.