Stream and River Deposits

What You Will Learn

• / Describe the four different types of stream deposits.
• / Describe how the deposition of sediment affects the land.

If your job were to carry millions of tons of soil across the United States, how would you do it? You might use a bulldozer or a dump truck, but it would still take you a long time. Did you know that rivers do this job every day?

Rivers erode and move enormous amounts of material, such as soil and rock. Acting as liquid conveyor belts, rivers often carry fertile soil to farmland and wetlands. Although erosion is a serious problem, rivers also renew soils and form new land. As you will see in this section, rivers create some of the most impressive landforms on Earth.

Deposition in Water

You have learned how flowing water erodes the Earth’s surface. After rivers erode rock and soil, they drop, or deposit, their load downstream. Deposition is the process in which material is laid down or dropped. Rock and soil deposited by streams are called sediment. Rivers and streams deposit sediment where the speed of the water current decreases. Figure 1 shows this type of deposition.

Figure 1 This photo shows erosion and deposition at a bend, or meander, of a river in Alaska.

Placer Deposits

Heavy minerals are sometimes deposited at places in a river where the current slows down. This kind of sediment is called a placer deposit (PLAS uhr dee PAHZ it). Some placer deposits contain gold. During the California gold rush, which began in 1849, many miners panned for gold in the placer deposits of rivers, as shown in Figure 2.

Figure 2 Miners rushed to California in the 1850s to find gold. They often found it in the bends of rivers in placer deposits.

Delta

A river’s current slows when a river empties into a large body of water, such as a lake or an ocean. As its current slows, a river often deposits its load in a fan-shaped pattern called a delta. In Figure 3, you can see an astronaut’s view of the Nile Delta. A delta usually forms on a flat surface and is made mostly of mud. These mud deposits form new land and cause the coastline to grow. The world’s deltas are home to a rich diversity of plant and animal life.

Figure 3 As sediment is dropped at the mouth of the Nile River, in Egypt, a delta forms.

If you look back at the map of the Mississippi River watershed, you can see where the Mississippi Delta has formed. It has formed where the Mississippi River flows into the Gulf of Mexico. Each of the fine mud particles in the delta began its journey far upstream. Parts of Louisiana are made up of particles that were transported from places as far away as Montana, Minnesota, Ohio, and Illinois!

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What are deltas made of?

Deposition on Land

When a fast-moving mountain stream flows onto a flat plain, the stream slows down very quickly. As the stream slows down, it deposits sediment. The sediment forms an alluvial fan, such as the one shown in Figure 4. Alluvial fans are fan-shaped deposits that, unlike deltas, form on dry land.

Figure 4 An alluvial fan, like this one at Death Valley in California, forms when an eroding stream changes rapidly into a depositing stream.

Floodplains

During periods of high rainfall or rapid snow melt, a sudden increase in the volume of water flowing into a stream can cause the stream to overflow its banks. The area along a river that forms from sediment deposited when a river overflows its banks is called a floodplain. When a stream floods, a layer of sediment is deposited across the flood plain. Each flood adds another layer of sediment.

Flood plains are rich farming areas because periodic flooding brings new soil to the land. However, flooding can cause damage, too. When the Mississippi River flooded in 1993, farms were destroyed, and entire towns were evacuated. Figure 5 shows an area north of St. Louis, Missouri, that was flooded

Figure 5 The normal flow of the Mississippi River and Missouri River is shown in black. The area that was flooded when both rivers spilled over their banks in 1993 is shaded red.

Flooding Dangers

The flooding of the Mississippi River in 1993 caused damage in nine states. But floods can damage more than property. Many people have lost their lives to powerful floods. As shown in

Figure 6, flash flooding can take a driver by surprise. However, there are ways that floods can be controlled.

Figure 6 Cars driven on flooded roads can easily be carried down to deeper, more dangerous water.

One type of barrier that can be built to help control flooding is called a dam. A dam is a barrier that can redirect the flow of water. A dam can prevent flooding in one area and create an artificial lake in another area. The water stored in the artificial lake can be used to irrigate farmland during droughts and provide drinking water to local towns and cities. The stored water can also be used to generate electricity.

Overflow from a river can also be controlled by a barrier called a levee. A levee is the buildup of sediment deposited along the channel of a river. This buildup helps keep the river inside its banks. People often use sandbags to build artificial levees to control water during serious flooding.

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List two ways that the flow of water can be controlled.

Section Summary

/ • / Sediment forms several types of deposits.
/ • / Sediments deposited where a river’s current slows are called placer deposits.
/ • / A delta is a fan-shaped deposit of sediment where a river meets a large body of water.
/ • / Alluvial fans can form when a river deposits sediment on land.
/ • / Flooding brings rich soil to farmland but can also lead to property damage and death.

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