Profile of a river
Imagine taking a rafting trip along the entire length of a river to observe how it changes firsthand. you can follow the journey by exploring a River.
The headwaters
Your trip starts near the river's beginning, or source, in the mountains. The many small streams that come together at the source of the river are called the headwaters. Your ride through the headwaters is quite bumpy as your raft bounces through rapids, dropping suddenly over a small waterfall. You notice how the fast-flowing water breaks off clumps of soil from the riverbanks and carries them along. As the river continues this erosion, it wears away the sides and cuts into the bottom of its channel. The channel gradually becomes wider and deeper.
Downriver
As you continue downriver, your ride becomes smoother. The land around the river is less steep than it was near the headwaters. Some smaller streams have joined the river, increasing the volume of water. Since less of the water is in contact with the channel, there is less friction slowing it down. As a result, although the slope is less steep, the river continues to flow fairly swiftly.
The flood plain
Next, your raft travels through the middle of a wide valley. The river created this valley over time by eroding the land along its banks. The broad, flat valley through which the river flows is called the flood plain.
In places, small obstacles in the river's channel cause the water to flow slightly to one side or the other. This movement creates a bend in the river.
The mouth
Your raft trip is nearly over as you approach the river's mouth. The mouth is the point where a river flows into another body of water - a larger river, a lake, or an ocean. When the fast-moving water of a river hit the slower waters of a lake or ocean, the river suddenly slows down. As it slows, the river deposits most of its sediment. These deposits at the river's mouth build up, forming an area called a delta. The sediment deposits are rich in nutrients and minerals. As a result, the soil in delta areas is very fertile for farming.
Habitats along a river
Recall that an organism's habitat provides the things that the organism needs to live. As you saw in Exploring a River. a river provides habitats for many living things. Some organisms live in the river and obtain nutrients and dissolved gases from the water. Others find shelter and food along its banks.
Rivers and Floods
Spring floods occur frequently on rivers in the Midwest, but the floods of 1997 were far worse than usual. The residents of Fargo, North Dakota, had already used a million sandbags, and the Red River of the North was still rising! As the flood waters rose, people piled the sandbags higher around their houses, hoping no water would break through. People moved their homes.
The Red River floods went on for weeks, fed by rain and melting snow. A spring blizzard added more snow. Other nearby rivers also flooded. Parts of North Dakota, South Dakota, and Minnesota were declared a disaster area. Weary residents just waited for the waters to recede so they could start to repair the damage.
What caused the Red River to flood so badly? A flood occurs when the volume of water in a river increases so much that the river overflows its channel. As rain and melting snow added more and more water, the river gained in speed and strength. Recall that as the speed of a river increases, so does the amount of energy it has. A flooding river can uproot trees and pluck boulders from the ground. As it overflows onto its floodplain, the powerful water can even wash away bridges and houses.
Throughout History, people have both feared and welcomed floods. Ancient Egyptians, for instance, called their fertile cropland " the gift of the Nile." The deposition of regular floods left a layer of rich soil on each side of the river, creating a green strip of good land in the middle of the desert. But floods can also destroy farms, towns, and crops. In the United States, 20 million people live in places where flooding is likely. Even in the last century, floods have killed millions of people around the world, many of them in the heavily populated flood plains of China, Bangladesh, and India.
Can floods be controlled?
As long as people have lived on flood plains, they have tried to control floods. Building dams is one method of flood control. A dam is a barrier across a river that may redirect the flow of a river to other channels or store the water in an artificial lake. Engineers can open the dam's floodgates to release water for dry reasons. Dams work fairly well to control small floods. During severe floods, however, powerful flood waters can wash over the top of a dam or break through it.
Sediment deposits actually build a natural defense against floods. As a river overflows onto its flood plain, it slows down, depositing heavier sediments alongside the channel. Over time, these deposits build up into long ridges called levees. These natural levees help keep the river inside its banks. People sometimes build up natural levees with sandbags or stone and concrete to provide further protection against floods.
But building up levees can sometimes backfire. These walls prevent the natural channel-widening process that rivers normally undergo as their volume increases. As a result, during a flood, the water has nowhere to go except downstream. Although built-up levees can work well to prevent small floods, they often make heavy flooding worse for areas farther downstream. The full power of the surge of flood water is passed on to flood the downstream areas.
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