Water in a stream may move several feet per minute, but water in an aquifer may move only several feet per month. This is because groundwater must overcome more friction or resistance to move through small spaces between rocks and soil underground. There are exceptions to this rule. An example is underground streams in limestone caverns where the water may move relatively fast. The exchange of water between surface water bodies and aquifers is important.
Rivers usually start as small streams and get larger as they flow downstream.
The water they gain is often groundwater. Such a stream 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 or recharged by water from the losing stream. A stream that 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.