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Most rivers are sustained by both groundwater and runoff. Surface runoff is a major contributor after a rain, but runoff also can result from snow or ice melting, which can last through much of the summer at high altitudes. Additionally, some runoff may be captured and retained for a time by wetlands or impoundment areas, and may feed the river's flow for some period of time after the last precipitation has fallen.
Groundwater will continue to sustain a stream's flow even when there is no precipitation. Water contained in an aquifer can flow to the surface as a spring or seep, or can fill a lake or river basin from beneath. Aquifers occur wherever geological conditions are right, and an aquifer can be the headwaters of a river.
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