The most simplistic answer is that all the water in a river comes from the sky—and that is certainly true, as streamflow is one part of the water cycle. It is also true that most of the water flowing in rivers comes from precipitation runoff from the surrounding landscape (watershed).

The water in a river originates from several sources integrated into the hydrological cycle, beginning with precipitation in the form of rain or snow. In highland or mountainous areas, the "source" or "headwaters" of a river are often formed by glacial melt or the seasonal melting of snowpacks. Additionally, a significant portion of a river's flow comes from groundwater discharge, where water stored in underground aquifers seeps through the streambed—a process that keeps "gaining streams" flowing even during dry periods without rain. In 2026, hydrologists also emphasize the role of surface runoff, where water from heavy storms flows over saturated ground into small channels called rills and gullies, eventually merging into larger tributaries. Finally, some modern river systems are supplemented by anthropogenic sources, such as treated wastewater discharge or agricultural runoff. Regardless of the specific geography, all river water is part of a continuous loop of evaporation from the oceans, condensation into clouds, and eventual return to the land surface to begin the journey back to the sea.