How Rivers Form
Imagine yourself on vacation this summer, floating down a "lazy" river on an inner tube. The reason you float "down" a river is the same reason you speed down a snowy hill—gravity. Water in a river flows downhill, even if the difference in elevation is small and you do not notice it just by looking at it. Likewise, the runoff water that forms a river got there by running downhill. All streams, including rivers, begin when runoff water flows downhill, collects in channels, grows, and continues its downhill journey.
In the case of rivers, the water comes from precipitation that falls over mountains or hills. It flows downhill and forms tiny streams called tributaries. As more and more water is added, the tributaries grow bigger and may join with others to begin the formation of a large river. Along the way, these streams may also be fed by groundwater that seeps upward.
As rivers make their journey out of the mountains, they take on different characteristics along the way. On its journey, three main parts become evident: the collecting system, the transporting system, and the dispersing system. Click each area of the river to learn more about these three parts.
