The Active River
What You Will Learn
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Describe how moving water shapes the surface of the Earth by the process of erosion.
Explain how water moves through the water cycle.
Describe a watershed.
Explain three factors that affect the rate of stream erosion.
Identify four ways that rivers are described.
If you had fallen asleep with your toes dangling in the Colorado River 6 million
years ago and you had woken up today, your toes would be hanging about 1.6
km (about 1 mi) above the river!
The Colorado River carved the Grand Canyon, shown in Figure 1, by washing
billions of tons of soil and rock from its riverbed. The Colorado River made the
Grand Canyon by a process that can take millions of years.
The Grand Canyon is located in northwestern Arizona. The canyon formed
over millions of years as running water eroded the rock layers. (In some places,
the canyon is now 29 km wide.)
Figure 1
Rivers: Agents of Erosion
Six million years ago, the area now known as the Grand Canyon was nearly as
flat as a pancake. The Colorado River cut down into the rock and formed the
Grand Canyon over millions of years through a process called erosion. Erosion is
the process by which soil and sediment are transported from one location to
another. Rivers are not the only agents of erosion. Wind, rain, ice, and snow can
also cause erosion.
Because of erosion caused by water, the Grand Canyon is now about 1.6 km deep
and 446 km long. In this section, you will learn about stream development, river
systems, and the factors that affect the rate of stream erosion.
Describe the process that created the Grand
Canyon.
The Water Cycle
Have you ever wondered how rivers keep flowing? Where do rivers get their
water? Learning about the water cycle, shown in Figure 2, will help you answer
these questions. The water cycle is the continuous movement of Earth’s water
from the ocean to the atmosphere to the land and back to the ocean. The water
cycle is driven by energy from the sun.
Figure 2
The Water Cycle
River Systems
The next time you take a shower, notice that individual drops of water join
together to become small streams. These streams join other small streams and
form larger ones. Eventually, all of the water flows down the drain. Every time
you shower, you create a model river system—a network of streams and rivers
that drains an area of its runoff. Just as the shower forms a network of flowing
water, streams and rivers form a network of flowing water on land. A stream that
flows into a lake or into a larger stream is called a tributary.
Watersheds
River systems are divided into regions called watersheds.
A watershed,or drainage basin, is the area of land that is drained by a water
system. The largest watershed in the United States is the Mississippi River
watershed. The Mississippi River watershed has hundreds of tributaries that
extend from the Rocky Mountains, in the West, to the Appalachian Mountains, in
the East.
The satellite image in Figure 3 shows that the Mississippi River watershed covers
more than one-third of the United States. Other major watersheds in the United
States are the Columbia River, Rio Grande, and Colorado River watersheds.
Watersheds are separated from each other by an area of higher ground called
a divide.
The Continental Divide runs through the Rocky Mountains. It separates
the watersheds that flow into the Atlantic Ocean and the Gulf of Mexico from
those that flow into the Pacific Ocean.
Figure 3
Describe the difference between a watershed and
a divide.
Stream Erosion
As a stream forms, it erodes soil and rock to make a channel. A channel is the
path that a stream follows. When a stream first forms, its channel is usually
narrow and steep. Over time, the stream transports rock and soil downstream
and makes the channel wider and deeper. When streams become longer and
wider, they are called rivers. A stream’s ability to erode is influenced by three
factors: gradient, discharge, and load.
Gradient
Figure 4 shows two photos of rivers with very different gradients.Gradient is the
measure of the change in elevation over a certain distance. A high gradient gives
a stream or river more erosive energy to erode rock and soil. A river or stream
that has a low gradient has less energy for erosion.
A mountain stream, such as the one at left, at Kenai Peninsula in Alaska,
flows rapidly and has more erosive energy. A river on a flat plain, such as the
Kuskokwim River in Alaska, shown below, flows slowly and has less erosive
energy.
Figure 4
Discharge
The amount of water that a stream or river carries in a given amount of time is
called discharge. The discharge of a stream increases when a major storm occurs
or when warm weather rapidly melts snow. As the stream’s discharge increases,
its erosive energy and speed and the amount of materials that the stream can
carry also increase.
What factors cause a stream to flow faster?
Load
The materials carried by a stream are called the stream’s load. The size of a
stream’s load is affected by the stream’s speed. Fast-moving streams can carry
large particles. Rocks and pebbles bounce and scrape along the bottom and sides
of the stream bed. Thus, the size of a stream’s load also affects its rate of
erosion. The illustration below shows the three ways that a stream can carry its
load.
The Stages of a River
In the early 1900s, William Morris Davis developed a model for the stages of river
development. According to his model, rivers evolve from a youthful stage to an
old-age stage. He thought that all rivers erode in the same way and at the same
rate.
Today, scientists support a different model that considers factors of stream
development that differ from those considered in Davis’s model. For example,
because different materials erode at different rates, one river may develop more
quickly than another river. Many factors, including climate, gradient, and load,
influence the development of a river. Scientists no longer use Davis’s model to
explain river development, but they still use many of his terms to describe a
river. These terms describe a river’s general features, not a river’s actual age.
Youthful Rivers
A youthful river, such as the one shown in Figure 5, erodes its channel deeper
rather than wider. The river flows quickly because of its steep gradient. Its
channel is narrow and straight. The river tumbles over rocks in rapids and
waterfalls. Youthful rivers have very few tributaries.
This youthful river is located in Yellowstone National Park in Wyoming.
Rapids and falls are found where the river flows over hard, resistant rock.
Figure 5
Mature Rivers
A mature river, as shown in Figure 6, erodes its channel wider rather than deeper.
The gradient of a mature river is not as steep as that of a youthful river. Also, a
mature river has fewer falls and rapids. A mature river is fed by many tributaries.
Because of its good drainage, a mature river has more discharge than a youthful
river.
A mature river, such as this one in the Amazon basin of Peru, curves back
and forth. The bends in the river’s channel are called meanders.
Figure 6
What are the characteristics of a mature river?
Old Rivers
An old river has a low gradient and little erosive energy. Instead of widening and
deepening its banks, the river deposits rock and soil in and along its channel. Old
rivers, such as the one in Figure 7, are characterized by wide, flat flood plains, or
valleys, and many bends. Also, an old river has fewer tributaries than a mature
river because the smaller tributaries have joined together.
Figure 7
This old river is located in New Zealand.
Rejuvenated Rivers
Rejuvenated (ri JOO vuh NAYT ed) rivers are found where the land is raised by
tectonic activity. When land rises, the river’s gradient becomes steeper, and the
river flows more quickly. The increased gradient of a rejuvenated river allows the
river to cut more deeply into the valley floor. Step like formations
called terraces often form on both sides of a stream valley as a result of reju
venation. Can you find the terraces in Figure 8?
Figure 8
This rejuvenated river is located in Canyonlands National Park in Utah.
How do rejuvenated rivers form?
Section Summary
• Rivers cause erosion by removing and transporting soil and rock from the riverbed.
• The water cycle is the movement of Earth’s water from the ocean to the atmosphere to the land and
back to the ocean.
• A river system is made up of a network of streams and rivers.
• A watershed is a region that collects runoff water that then becomes part of a river or a lake.
• A stream with a high gradient has more energy for eroding soil and rock.
• When a stream’s discharge increases, its erosive energy also increases.
• A stream with a load of large particles has a higher rate of erosion than a stream with a dissolved
load.
• A developing river can be described as youthful, mature, old, or rejuvenated.