the primary force of erosion is gravity, which pulls sediment and water downhill

23.3 Water Shapes the Land

Streams play an important role in erosion. The stream banks collapse in mass movement. The soil is then carried away by the stream as sediment.


The primary force of erosion is gravity, which pulls sediment and water downhill.

• Deposition is the process in which sediment is laid down in new locations.

• Most sediment is moved and deposited by flowing water.


What is the most important factor influencing the ability of a stream to cause erosion?

Running Water Erodes the Land

A stream’s ability to erode depends mainly on its speed.


Erosion begins when runoff carries small particles of soil downhill.

• Runoff forms small channels in the soil.• Channels join together to form gullies. • Gullies flow together to form streams.



Most of the sediment in streams is carried in suspension, in which tiny sediment grains move along with the water.

Larger particles of sediment slide, roll, or bounce along the bottom of a stream. The process of particles bouncing along a stream bottom is called saltation.

A large amount of sediment is also carried in solution.



The water in a mountain stream moves quickly because it is flowing down a steep slope.

• Fast streams carry more sediment, which causes more abrasion.

• Fast streams can also carry large sediment grains.

As a stream flows toward the sea, its slope decreases, and larger sediments settle on the stream bottom.



Streams continue the process of erosion by transporting sediment.


Direction of flow


What features are formed by surface water erosion?

Features Formed by Water Erosion

Water erosion forms V-shaped valleys, waterfalls, meanders, and oxbow lakes.


V-Shaped Valleys

Near a stream’s source, the stream flows fast down steep slopes.

Mass movement on the stream slopes causes a V-shaped valley with sharply angled sides to form.

A waterfall may develop where a stream crosses rock layers that differ in hardness.



As a river winds its way from the mountains to the ocean, it changes the surrounding landscape through erosion and deposition.


Waterfall

V-shapedvalley

Oxbow lake

River mouth

Sediment

Meander


Flood Plains

Where a river or stream crosses gently sloping land, it forms a flood plain, a flat area along a stream that is entirely covered only during times of flood.

Sediment builds up into long, low ridges called natural levees, which help prevent a river from spilling over its banks.



At a curve in a river, the water on the outside of the curve moves more rapidly than the water on the inside and causes more erosion.

Sediment is deposited on the inside of the curve, where water moves more slowly.

This process forms a looplike bend in the river called a meander.



Sometimes during a flood, the river erodes through a narrow neck of land at the base of a meander and forms a new path.

Sediments build up and cut the old meander off from the rest of the river.

The result is a separate, curved lake, called an oxbow lake.



Rivers often form winding meanders and oxbow lakes across their flood plain.



What features are deposited by running water?

Features Formed by Water Deposition

Features deposited by flowing water include alluvial fans and deltas.


Deposition is the process in which sediment is laid down in new locations. As a stream flows out of the mountains and onto the plains, it slows down and sediment settles out.

• An alluvial fan is a fan-shaped deposit of sediment on land. Alluvial fans often grow into thick deposits of sediment.

• A delta is a mass of sediment deposited where a river enters a large body of water.



A. This alluvial fan in Death Valley, California, formed from sediment deposited when a mountain stream reached flat land.

B. When a large river empties into the ocean, the deposited sediment often forms a delta.



What causes groundwater erosion?

Groundwater Erosion

The processes of chemical weathering causes much groundwater erosion, including the formation of caves and sinkholes.


Erosion and deposition occur below ground as well as at the surface.

Carbon dioxide in the air combines with rainwater to form carbonic acid, which reacts with some rocks.

Groundwater Erosion


Limestone easily erodes away through this process, forming caves, or caverns.

Caves usually form in the saturated zone, below the water table. When the water table drops, it leaves dry caves.

Groundwater Erosion


Sometimes water drips into the cavern from the rock layers above, carrying dissolved minerals that are left behind when carbon dioxide escapes from the water.

• Water dripping from the ceiling forms a stalactite.

• Water dripping to the floor forms a stalagmite.

Groundwater Erosion


If erosion weakens a layer of limestone, entire portions of the ground can suddenly collapse.

The resulting hole is called a sinkhole. Sinkholes can appear suddenly, swallowing buildings and roads.

Groundwater Erosion


Carlsbad Caverns in New Mexico contain spectacular formations, such as the stalactites and stalagmites shown here.

Groundwater Erosion


Assessment Questions

1. What is the most important factor in determining a stream’s ability to cause erosion? a. width of the stream

b. depth of the stream

c. composition of the water

d. speed of the stream



1. What is the most important factor in determining a stream’s ability to cause erosion? a. width of the stream

b. depth of the stream

c. composition of the water

d. speed of the stream

ANS: D



2. Which feature is most likely to form near the source of a river? a. meanderb. flood plainc. V-shaped valleyd. alluvial fan



2. Which feature is most likely to form near the source of a river? a. meanderb. flood plainc. V-shaped valleyd. alluvial fan

ANS: C



3. Caves and sinkholes are geological features that are generally found where the bedrock consists of a. granite.

b. quartzite.

c. sandstone.

d. limestone.



3. Caves and sinkholes are geological features that are generally found where the bedrock consists of a. granite.

b. quartzite.

c. sandstone.

d. limestone.

ANS: D

the primary force of erosion is gravity, which pulls sediment and water downhill

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