The water that flows along a river

All rivers have a starting point where water begins its flow. This source is called a headwater. The headwater can come from rainfall or snowmelt in mountains, but it can also bubble up from groundwater or form at the edge of a lake or large pond.
The water that flows along a river, ultimately to the sea, as part of the water cycle. It includes water from overland flow, throughflow, interflow, soil pipes, percolation, and groundwater, as well as precipitation that falls directly into rivers and lakes.

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