Name
CHAPTER 15
Class
Date
River Systems
2 Stream Erosion
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
• How does a river develop?
• What factors affect a river’s ability to erode its channel?
• How do erosive factors affect the development of a
river channel?
• What are the parts of a river system?
What Are the Parts of a Stream?
A stream is a body of water that flows over the land. A
stream has three main parts: a channel, banks, and a bed.
Part
Description
Channel
the narrow groove in the ground in which a stream flows
Banks
the edges of the channel that rise above the water level
Bed
the part of the channel below the water level
There are three main characteristics of streams that
affect the stream’s ability to erode land. These three factors are stream load, stream discharge, and gradient.
As it flows downhill, a stream carries soil, pieces of
rocks, and dissolved minerals. These materials that a
stream carries are called the stream load. The larger a
stream’s load is, the greater the stream’s ability to erode
its channel is. Stream load has three parts: suspended
load, bed load, and dissolved load.
Suspended load is made up of fine sand and silt. These
fine particles do not sink because the water is moving so
quickly. Bed load is made up of heavier materials, such
as coarse sand, gravel, and pebbles. Dissolved load is the
minerals that are carried in liquid solution.
The volume of water that moves along the stream bed
in a given time period is called the stream’s discharge.
The faster a stream flows, the higher its discharge is. The
greater a stream’s discharge, the heavier a load it can
carry. A fast-moving stream carries more sediment and
larger particles than a slow-moving stream does.
The speed of the stream also affects how the stream
widens its channel. Fast-moving streams erode their
channels more quickly than slow-moving streams do.
READING TOOLBOX
Outline Before you read this
section, write each of the
headings in your notebook.
As you read, use the headings to create an outline of
the section.
READING CHECK
1. List What are the three
main stream erosive factors?
READING CHECK
2. Compare How is
dissolved load different
from suspended load?
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Name
SECTION 2
Class
Date
Stream Erosion continued
Critical Thinking
3. Predict A particular
stream flows at a very high
speed. What kind of gradient
does the stream most likely
have?
Math Skills
4. Calculate You can use
the following equation to
calculate the discharge of a
stream or river:
discharge =
speed of the water ×
cross-sectional area of the
channel
Suppose a river moves
1.5 m/s through a crosssectional area of 520 m2.
What is the discharge of
water that the river carries?
Express your answer in cubic
meters per second (m3/s).
STREAM GRADIENT
The speed of a stream affects the stream load and
how quickly the stream erodes its channel. The speed of
a stream depends mainly on gradient. Gradient is the
steepness of the stream’s slope.
Near the headwaters, or beginning of a stream, the
gradient is generally steep. Near the mouth, or where
the stream enters a larger body of water, the gradient
is flatter. A flatter gradient makes the stream slower.
A flatter gradient also decreases the stream’s ability to
erode its channel.
How Does a River Channel Develop?
Over time, a stream channel erodes and becomes
wider and deeper. When a stream becomes longer and
wider, it is called a river.
MEANDERING CHANNELS
A river that has a low gradient typically has more
bends than a river with a steep gradient has. As gradient
decreases, the speed of the water decreases. When water
speed decreases, the river is less able to erode into its
bed. Instead, the energy of the water flowing through the
channel erodes the banks. Erosion of the banks produces
a winding pattern of wide curves called meanders. The
figure below shows how meanders form.
Direction of water flow
When a river flows around a bend, the
speed of the water on the outside of the
bend increases. The fast moving water
erodes the bank.
LOOKING CLOSER
5. Analyze Diagrams On
the diagram, label the area
where water is moving
fastest and the area where
water is moving slowest.
The speed of the water on the inside of the bend
decreases. The decrease in speed causes the river to
deposit sediment on the inside of the bend.
The deposited sediment is called a bar.
Over time, erosion and deposition make the bends in the river sharper.
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River Systems
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SECTION 2
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Date
Stream Erosion continued
BRAIDED STREAMS
Most rivers have a single channel. However, under
certain conditions, sediment bars between a river’s banks
can divide the river into several channels. The water
in the separate channels may join around the sediment
bars. A stream or river made up of multiple channels that
divide and rejoin is called a braided stream. In general,
a high gradient and a large sediment load can cause a
braided stream to form.
What Are the Parts of a River System?
A river system is made up of a main stream and
smaller streams that feed into it. The figure below shows
a general river system.
Water runs off land and into the tributaries.
The land from which water runs off into a stream
is a watershed.
READING CHECK
6. Identify What are the
main factors that cause a
braided stream to form?
Ridges or elevated areas that separate
watersheds are called divides.
Watershed
LOOKING CLOSER
7. Describe Relationships
What is the relationship
between a tributary and a
stream?
A small stream that
feeds into the main
stream is called a
tributary.
River
River systems change continuously because of
erosion. In a process called headward erosion, channels
become longer at their upper ends. They branch out
where runoff enters the streams. Erosion of slopes can
make a watershed larger. A stream in a watershed with a
high rate of erosion may capture a stream from another
watershed. This process is called stream piracy. The
stream that has been captured drains into its new river
system.
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Section 2 Review
SECTION VOCABULARY
braided stream a stream or river that is composed of multiple channels that divide and
rejoin around sediment bars
discharge the volume of water that flows within
a given time
gradient the change in elevation over a given
distance
meander one of the bends, twists, or curves in a
low-gradient stream or river
stream load the materials other than the water
that are carried by a stream
tributary a stream that flows into a lake or into a
larger stream
watershed the area of land that is drained by a
river system
1. Summarize Relationships Complete the concept map below to describe streams
and river systems. Use the following terms in your concept map: braided stream,
stream load, suspended load, dissolved load, bed load, meander, stream
gradient, tributaries, and watershed.
A stream is fed by
runoff from the land in
a stream’s
and water from
At the stream’s headwaters,
is generally steep, and water
speed is high.
Under these
conditions, a
The greater the water speed,
the larger the
which is
made up of
that the stream can carry.
Where gradient
flattens, a
may form
2. Explain What are the effects of stream discharge and gradient on the ability of a
river to erode its channel?
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River Systems