River Systems
If you were hiking in the San Juan Mountains, you could observe the path of the runoff from melting snow. As you followed one small stream downhill, you would notice that the stream reached a larger stream and joined it. You could then continue along this stream until it flowed into a small river. Eventually, this path would lead you to the Rio Grande itself.
Tributaries are the smaller streams and rivers that feed into a main river. A river and all its tributaries together make up a river system. The tributaries flow toward the main river following a downhill path due to the pull of gravity. Even a land area that appears flat can have small differences in height that affect how water flows.
Watersheds
Just as all the water in a bathtub flows toward the drain, all the water in a river system drains into the main river. The land area that supplies water to a river system is called a watershed. Watersheds are also called drainage basins.
A river can flow into another, larger river. When rivers join another river system, the areas they drain become part of the largest river's watershed. You can identify a river's watershed on a map by drawing an imaginary line around the region drained by all its tributaries. Some watersheds are very small. The watershed of a stream that flows down a hill into a river is just that hillside - maybe a square kilometer or two. By contrast, the watershed of the Mississippi River covers more than 3 million square kilometers!
Divides
One watershed is separated from another by a ridge of land called a divide. Streams on each side of the divide flow in different directions. The continental Divide, the longest divide in North America, follows the line of the Rocky Mountains. Between the Rocky Mountains and the Appalachian Mountains, water flows toward the Mississippi River or directly into the Gulf of Mexico.
Rivers shape the land
The next time it rains, watch the rainwater flow along the side of a road. Notice how the water picks up leaves and twigs and carries them away. Bits of paper and small pebbles bounce and swirl along in the flow. Even the tiny stream has the power to move objects.
Picture a stream ten times larger, and you will start to get an idea of how running water can cause erosion. Erosion is the process by which fragments of soil and rock are broken off from the ground surface and carried away. These fragments are carried along by the moving water until they are eventually dropped, or deposited, in a new location. Deposition is the process by which soil and rock are left behind. Rivers wear away landforms through erosion and build new landforms through deposition. The particles of rock and soil that are picked up and moved by erosion and deposition are called sediments.
A river's speed affects its ability to wear away, or erode, the land. The faster the water flows, the more energy it has. A river traveling at a speed of 1 kilometer an hour has enough energy to move pebbles along. At 18 kilometers an hour, it can move a boulder the size of an armchair! When a river slows down, its energy decreases. It can no longer move heavy objects. The river deposits heavier sediment particles first, then lighter ones.
One factor that affects how fast a river flows is the steepness of its slope. Water flows faster down a mountainside than over a flat plain. A second factor that affects a river's speed is the volume of water in the river. An increase in the amount of water in a river Causes the river to flow faster. A third factor is the shape of the channel through which the river flows. As the water in the river rubs against the sides and buttons of its channel, it creates friction. This friction slows the water's movements. In a shallow, narrow channel, almost all the water is in contact with the sides or bottom, and it moves slowly. In a broad, deep channel, however, most of the water can flow without any friction, so the river flows faster.
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