Rivers are kept running by the addition of water from rainfall or melting snow. Even when it does not rain, underground reservoirs (water stores) keep rivers supplied with water and flowing. When it rains, a lot of water seeps and sinks into the ground to surface somewhere else as a spring. Over thousands of years, rivers can carve huge valleys out of solid rock with a wide floodplain, which is a flat area that catches the overflow when the river is full.

Rivers keep flowing because gravity is constantly pulling the water down the path of least resistance (downhill). A river flows because there is a water table to support it. A river is nothing more than an outward manifestation of the water table.

A river that does not run dry at any time of year is carrying surplus water from precipitation that collects in the permeable rocks of hills and mountains that surround the watershed. Most rivers get their water from the mountains, where there is far more precipitation than over the lowlands.

Credit: SidmartinBio

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