1.2.3 LANDFORMS FROM EROSION/DEPOSITION

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, but it’s also breathtaking 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. Streams that carry larger particles have greater competence. Streams with a steep gradient (slope) have a faster velocity and greater competence.

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The Amazon River appears brown when carrying a large sediment load.

Bed load: Particles that are too large to be carried as suspended load are 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 do a great deal of down cutting into the stream bed, which is primarily accomplished 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

As a stream gets closer to its base level, where it meets a large body of water, its gradient lowers and it deposits more material than it erodes. On flatter ground, streams deposit material on the inside of meanders. Placer mineral deposits are often deposited there.

A stream’s floodplain is much broader and shallower than the stream’s channel. When a stream flows onto its floodplain, its velocity slows and it deposits much of its load. These sediments are rich in nutrients and make excellent farmland (Figure below).

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The Mississippi floodplain is heavily farmed. Flooding can wipe out farms and towns, but the stream also deposits nutrient-rich sediments that enrich the floodplain.

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 first. These large sediments build a higher area around the edges of the stream channel, creating natural levees.

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

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The Ganges River forms an enormous delta in Bangladesh.

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

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

Vocabulary

alluvial fan: Curved, fan-shaped, coarse-sediment deposit that forms when a stream meets flat ground.
base level: Where a stream meets a large body of standing water, usually the ocean.
bed load: Sediments moved by rolling or bumping along the stream bed.
competence: A measure of the largest particle a stream can carry.
delta: A triangular-shaped deposit of sediments that forms where a river meets standing water.
dissolved load: The elements carried in solution by a stream.
floodplain: The flat area around a stream where water flows when the stream is in flood.
gradient: The slope of a stream.
meander: A bend or curve in a stream channel.
saltation: The intermittent movement of bed load particles.
suspended load: Solid particles that are carried in the main stream flow.

Summary

Streams carry dissolved ions and sediments. The sizes of the sediments a stream can carry, its competence, 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.