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.
23.3 Water Shapes the Land
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.
23.3 Water Shapes the Land
Running Water Erodes the Land
What is the most important factor
influencing the ability of a stream to cause
erosion?
A stream’s ability to erode depends mainly
on its speed.
23.3 Water Shapes the Land
Running Water Erodes the Land
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.
23.3 Water Shapes the Land
Running Water Erodes the Land
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.
23.3 Water Shapes the Land
Running Water Erodes the Land
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.
23.3 Water Shapes the Land
Running Water Erodes the Land
Streams continue the process of erosion by
transporting sediment.
Direction of flow
23.3 Water Shapes the Land
Features Formed by Water Erosion
What features are formed by surface water
erosion?
Water erosion forms V-shaped valleys,
waterfalls, meanders, and oxbow lakes.
23.3 Water Shapes the Land
Features Formed by Water Erosion
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.
23.3 Water Shapes the Land
Features Formed by Water Erosion
As a river winds its way from the
mountains to the ocean, it changes
the surrounding landscape through
erosion and deposition.
Waterfall
V-shaped
valley
Meander
Oxbow lake
Sediment
River mouth
23.3 Water Shapes the Land
Features Formed by Water Erosion
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.
23.3 Water Shapes the Land
Features Formed by Water Erosion
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.
23.3 Water Shapes the Land
Features Formed by Water Erosion
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.
23.3 Water Shapes the Land
Features Formed by Water Erosion
Rivers often form winding meanders and
oxbow lakes across their flood plain.
23.3 Water Shapes the Land
Features Formed by Water Deposition
What features are deposited by running
water?
Features deposited by flowing water include
alluvial fans and deltas.
23.3 Water Shapes the Land
Features Formed by Water Deposition
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.
23.3 Water Shapes the Land
Features Formed by Water Deposition
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.
23.3 Water Shapes the Land
Groundwater Erosion
What causes groundwater erosion?
The processes of chemical weathering
causes much groundwater erosion,
including the formation of caves and
sinkholes.
23.3 Water Shapes the Land
Groundwater Erosion
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.
23.3 Water Shapes the Land
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.
23.3 Water Shapes the Land
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.
23.3 Water Shapes the Land
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.
23.3 Water Shapes the Land
Groundwater Erosion
Carlsbad Caverns in New
Mexico contain spectacular
formations, such as the
stalactites and stalagmites
shown here.
23.3 Water Shapes the Land
Assessment Questions
1. What is the most important factor in determining a
stream’s ability to cause erosion?
a.
b.
c.
d.
width of the stream
depth of the stream
composition of the water
speed of the stream
23.3 Water Shapes the Land
Assessment Questions
1. What is the most important factor in determining a
stream’s ability to cause erosion?
a.
b.
c.
d.
width of the stream
depth of the stream
composition of the water
speed of the stream
ANS: D
23.3 Water Shapes the Land
Assessment Questions
2. Which feature is most likely to form near the
source of a river?
a.
b.
c.
d.
meander
flood plain
V-shaped valley
alluvial fan
23.3 Water Shapes the Land
Assessment Questions
2. Which feature is most likely to form near the
source of a river?
a.
b.
c.
d.
meander
flood plain
V-shaped valley
alluvial fan
ANS: C
23.3 Water Shapes the Land
Assessment Questions
3. Caves and sinkholes are geological features that
are generally found where the bedrock consists of
a.
b.
c.
d.
granite.
quartzite.
sandstone.
limestone.
23.3 Water Shapes the Land
Assessment Questions
3. Caves and sinkholes are geological features that
are generally found where the bedrock consists of
a.
b.
c.
d.
granite.
quartzite.
sandstone.
limestone.
ANS: D