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.
Sinkholes also play an important role in connecting groundwater and surface water.
Lakes commonly occupy the depressions created by sinkhole collapse. Some streams lose their entire flow at low-flow stages to the upper Floridan aquifer through swallow holes in the stream bed. Springs and wells bring the water from the aquifer up to the surface. Groundwater is generally safer than surface water for drinking because of the filtration and natural purification processes which take place in the ground. These processes however, become ineffective owing to sewage, fertilizers, and toxic chemicals, which may seep into the groundwater supply.