How a Stream Becomes a River
The journey from stream to ocean begins in our backyards and farms.
Precipitation comes down, literally, everywhere—in various forms. It may rain, hail, snow or sleet. Whatever the form, once it reaches the ground some water is absorbed by trees and other plants. 
Stream Running Through Wooded Area
Water not absorbed by roots enters the ground water supply by infiltration. Most water runs off the surface, down a hill and eventually into a river. Ditches and small streams, which may be dry when it does not rain, can quickly fill up with water when it does rains. 
On a rainy day look out the window and see if you can determine where the water goes during a rain shower.

Does it flow down the street?

Does it flow down your backyard?

Are there man-made devices to capture or navigate rain water?

What natural features are present that move water into the banks of streams and rivers?

Once the water finds its way into a stream or ditch, where does it go?
If you have ever studied a map in detail, you will notice how rivers and streams form a network of waterways across the countryside. 
Little streams come together to form small rivers. Small rivers join together and become medium-sized rivers. All these rivers may be tributaries of a large river. such as the Mississippi. Collectively, the network of rivers and streams form a watershed which drains the land of excess water. 
LungsThis type of patern is repeated many places in nature. In the human circulatory system, for example, or in human lungs, like those represented to the left. When you exhale (breath out), your lungs behave just like a watershed. The air in your lungs (which is like the water in a watershed) starts out in the smallest vessels (which are like small streams), passing into larger vessels (that are like medium rivers) and finally into one large central air passage (which is like a large river). Ultimately, all the air in the lungs (which are like a large watershed) pass through this one large air passage, out your mouth, and to the atmosphere beyond, just as water in a watershed flows out the mouth of a large river and into the ocean! (Of course rivers don't breath in!)
Meramec River, MissouriIt comes as no surprise that the widest rivers have the biggest watersheds. The Mississippi watershed drains about half the Midwest and eastern United States. 

Think about rivers as excess water disposal machines. Rivers transport water from wherever it is to the ocean. In places where it rains more, such as in the temperate deciduous forest and the rainforest, there are more rivers and streams to deal with the large and consistent annual rainfall. 

In the desert, however, there is significantly less rain. Logically, fewer rivers exist in the desert. When it does rain, the rains are very short and intense. Since there is no vegetation to absorb the water, almost all the water drains immediately into dry river basins that for most of the year look like flat plains. These rivers swell up very quickly and produce swift torrents. 

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