The USGS Water Science School
Rivers and streams begin their lives as smaller creeks, often called "the headwaters". These small tributaries (the small Drippies in the picture) run downhill until they merge to form bigger tributaries (the medium-sized Drippies), which continue merging to form larger rivers (the biggest Drippy). Rivers keep flowing to lower altitudes, towards the oceans. River systems are similar to the blood vessels in your body. Tiny capillaries that carry blood keep merging together until all of the blood empties into large veins, which deliver the blood to your heart.
All rivers are surrounded by a certain amount of land that is higher in altitude (upgradient) than the actual river. Precipitation that falls in this area eventually flows downhill towards the river. At any particular point on a river, the land upgradient of the point is the river's watershed, or drainage basin. This example of a watershed gives a rough idea of how precipitation flows downhill into rivers (and lakes).
What separates two watersheds from each other are ridges of higher land. You might have heard of the Continental Divide, which runs along the highest ridges of the Rocky Mountains. Precipitation falling on the western side of the Divide will flow towards the Pacific Ocean and precipitation falling on the eastern slopes will flow towards the Atlantic Ocean, via the Gulf of Mexico. The United States has many watersheds of many sizes, and many, many ridges that define these drainage basins. Here in Atlanta, Ga, the main street through the city is Peachtree Street. It is actually built on top of a ridge that is a drainage-basin divide. Rain falling on the eastern side of Peachtree Street will flow towards the Atlantic Ocean, while runoff on the western side heads to the Gulf of Mexico.
Rivers and streams have a great impact on both the local landscape and our own lives. Flowing water continually erodes the land it runs through, and over millions of years the topography of the land can be greatly changed (notice how deep the Grand Canyon gorge is). River water is used by humans for irrigation; rivers deposit mineral-rich soil in their flood plains where man can grow crops; rivers are used to transport people and their manufactured products; rivers can produce hydroelectric power; and, if you notice where towns and cities are located, people build their communities next to rivers (don't miss our riveting water tale, the Story of Dryville).
Rivers also transport soil and sediment from one place to another, which has a great impact on the landscape. This is important to our stomachs, as silt that is deposited in flood plains of rivers makes excellent farmland (just ask the ancient Egyptians who lived along the Nile River and depended on the annual flooding of the river for their livelihood).
While you're here .. care to vote for your favorite water body?
Investigate the water cycle: streamflow (in many languages!)
Rivers and streams Rivers and sediment River lengths Surf your watershed (EPA)