Flow of a River - Hydrology
Water or hydrologic cycle

Most rivers are like perpetually flowing fountains. Where does the water come from? While many ancients understood that precipitation fed streams, many thinkers during the Middle Ages believed that water from the ocean traveled through the ground and fed the headwaters of the world's streams.

Now, we understand that precipitation through the hydrologic cycle feeds the world's streams. Water evaporates from the world's oceans (and to a lesser extent, lakes and streams) and via transpiration of plants. It falls from the atmosphere as rain or snow. Flowing over ground as runoff or underground as groundwater, water finds its way to a stream and then eventually to the sea.

Why do rivers continue to flow, even when little or no rain has fallen? Much of the water feeding a stream runs slowly underground through shallow aquifers. These sediments are saturated like natural sponges and respond slowly to rainfall and drought.