2.4 Landforms from Stream Erosion and Deposition

 ^[1]

What on Earth are ‘goosenecks’?

In Southeastern Utah, stream meanders have been immortalized by erosion into the Goosenecks of the San Juan River. This satellite image shows the amazing path the river has cut. Even better is to stand at the edge and look into one of the meanders. Goosenecks State Park is in the southeastern corner of Utah.

Erosion by Streams

Flowing streams pick up and transport weathered materials by eroding sediments from their banks. Streams also carry ions and ionic compounds that dissolve easily in the water.

Sediment Transport

Sediments are carried as:

• Dissolved load: Dissolved load is composed of ions in solution. These ions are usually carried in the water all the way to the ocean.
• Suspended load: Sediments carried as solids as the stream flows are suspended load. The size of particles that can be carried is determined by the stream’s velocity (Figure below). Faster streams can carry larger particles. Slower streams can only carry smaller particles. Streams with a steep gradient (slope) have a faster velocity and can carry larger particles.

The Amazon River appears brown when carrying a large sediment load. ^[2]

• Bed load: Some particles are too large to be carried as suspended load. These particles bumped and pushed along the stream bed as bed load. Bed load sediments do not move continuously. This intermittent movement is called saltation. Streams with high velocities and steep gradients cut down into the stream bed. This type of erosion is primarily by movement of particles that make up the bed load.

• An animation of saltation is found here:http://www.weru.ksu.edu/new_weru/multimedia/movies/dust003.mpg.
• A video of bedload transport is found here:http://faculty.gg.uwyo.edu/heller/SedMovs/Sed%20Movie%20files/bdld.mov.

Stream Deposition

A stream is at its base level where it meets a large body of water. As a stream gets closer to base level, its gradient lowers. The stream deposits more material than it erodes. On flatter ground, streams deposit material on the inside of meanders. Meanders are bends in the stream’s path. Placer mineral deposits are often deposited on the inside of meanders.

A stream’s floodplain is much broader and shallower than its channel. When a stream flows onto its floodplain, its velocity slows. The stream deposits much of its load. Stream sediments are rich in nutrients and make excellent farmland. The Mississippi River floodplain is heavily farmed. Flooding can wipe out farms and towns, but the stream also deposits nutrient-rich sediments that enrich the floodplain (Figure below).

The Mississippi River floodplain at normal flow and during flood. ^[3]

A stream at flood stage carries lots of sediments. When its gradient decreases, the stream overflows its banks and broadens its channel. The decrease in gradient causes the stream to deposit its sediments. The largest sediments are deposited first. These large sediments build a higher area around the edges of the stream channel. This creates a natural levee.

When a river enters standing water, its velocity slows to a stop. The stream moves back and forth across the region. The stream drops its sediments in a wide triangular-shaped deposit called a delta (Figure below).

The Ganges River forms an enormous delta in Bangladesh. ^[4]

If a stream falls down a steep slope onto a broad flat valley, an alluvial fandevelops (Figure below). Alluvial fans generally form in arid regions.

A series of alluvial fans spread out from mountains along the Badwater Basin in Death Valley, California. ^[5]

Summary

• Streams carry dissolved ions and sediments. The sizes of the sediments a stream can carry depend on the stream’s velocity.
• Particles that are too large to be suspended move along the stream bed by saltation.
• Rivers deposit sediments on levees, floodplains, and in deltas and alluvial fans.

Practice

Use this resource to answer the questions that follow.

1. What is laminar flow?

2. What is turbulent flow?

3. What is jet flow?

4. Where does jet flow occur?

5. What is water velocity?

6. What factors can influence the stream velocity?

Review

1. If flood waters decrease, what will happen to the size of particle the stream can carry? What will be deposited and where?

2. Under what conditions do streams cut down into their beds? Under what conditions do they erode their banks?

3. Deserts are extremely dry, yet alluvial fans are said to be deposited by stream flow. Describe how this occurs.

Image Attributions

1. ^ License: CC BY-NC 3.0
2. ^ Credit: Image copyright Wigi Photography, 2012; License: CC BY-NC 3.0
3. ^ Credit: Image courtesy of Jesse Allen/NASA’s Earth Observatory;License: CC BY-NC 3.0
4. ^ Credit: Courtesy of NASA; License: CC BY-NC 3.0
5. ^ Credit: Courtesy of National Park Service; License: CC BY-NC 3.0