Question

Where does the water in rivers come from? What causes rivers to flow?

Answer 1

Short answer:
Rivers get their water from precipitation (rain and snow), melted snow and ice, groundwater that seeps out as springs or baseflow, and from lakes, wetlands and smaller tributaries in the drainage basin. Rivers flow because of gravity — water runs downhill from higher to lower elevations; the slope (gradient) and differences in hydraulic head cause and control the speed and volume of flow.

More detail:

Where the water comes from
- Precipitation: rain falling on the watershed is the main input. Some runs off the land surface into streams (overland flow).
- Snow and ice melt: seasonal snowpack and glaciers release stored water during melt, often a major source for rivers in cold or mountainous regions.
- Groundwater/baseflow: water that has infiltrated into the ground moves slowly through soil and rock and returns to streams as springs or seepage. This sustains flow between storms.
- Tributaries, lakes and wetlands: smaller streams and standing-water bodies in the drainage basin feed the main river.
- Human inputs: treated wastewater, irrigation return flows, reservoir releases and diversions can add to or reduce river flow.

What makes rivers flow
- Gravity: water has potential energy at higher elevation; gravity pulls it downhill. The steeper the slope (greater gradient), the faster the water tends to flow.
- Hydraulic gradient / head difference: flow is driven by differences in water surface elevation or pressure along the channel and in the subsurface.
- Channel characteristics: channel shape, roughness (rocks, vegetation), width and depth influence velocity and discharge.
- Inputs and timing: storms and snowmelt rapidly increase flow (floods); groundwater inputs provide steady baseflow during dry periods.
- Base level: rivers flow toward a lower end point (often the sea or a lake). Local changes (dams, sediment buildup, erosion) also affect flow patterns.

In short, rivers are fed by the water cycle (precipitation, storage, and transport) and move because gravity makes water flow from higher to lower elevations, with the rate controlled by slope, inputs, and channel properties.