Floods and Droughts
12.4
Floods usually result from heavy rainfall or rapid snowmelt. Droughts
are caused by dry weather conditions where water evaporates faster
than it is replenished. The media often report these events because
of the loss of life and property damage that are associated with them
(Figure 1). Floods are important to the natural environment because
they add moisture to dry land, top up aquifers, and provide nutrients
to the soil. Droughts often result in forest fires that clear out old dead
trees and smaller plants, making room for new life. Some tree seeds
only sprout and grow after being heated by a forest fire.
Figure 1 Both floods and droughts can be devastating to people living and farming in an area.
Floods
Floods are the most costly natural disasters in Canada in terms of
property damage. There are different kinds of floods. Each is caused
by a variety of factors.
Rapid snowmelt and many days of heavy rain cause regional floods.
These floods are especially bad if the ground is frozen or already
saturated with water. The water that causes a regional flood tends to
move slowly and covers a wide area. Flooding is an annual event in
many regions, such as the Red River in Manitoba and the Nile River
in Egypt.
Flash floods are the most deadly and damaging kind of flood.
They develop quickly and often without warning. Huge volumes of
fast-moving water rush along river valleys, or even along streets. Flash
floods are generally produced by a sudden storm that stalls over one
location.
Storm surges are floods that occur along coastlines when hurricanes
come ashore. They are particularly dangerous if the coastal land
is below—or only slightly above—sea level. The storm surge that
accompanied Hurricane Katrina left a path of death and destruction
when it came ashore near New Orleans in 2006.
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Ice-jam floods usually occur in the spring along
Ontario’s rivers. These floods are caused by river ice
breaking up and becoming jammed in a narrow part
of the river or behind a bridge (Figure 2). The water
backs up and begins to flow around the ice, flooding
over the banks. Suddenly, the ice breaks, releasing a
surge of cold water that extends beyond the banks of
the river.
Preventing Floods
There are things that people can do to protect
Figure 2 The ice jam under this bridge is blocking the flow of
water, which may lead to flooding of the surrounding area.
themselves and their property from floods.
A flood plain is a low area of land alongside a river.
flood plain: an area of flat land that
New homes should not be built on, or too close to, flood plains.
is flooded when a river overflows its
banks
Federal and provincial governments, often working with
Conservation Authorities, dam rivers to retain water. Dams control
the release of water during potential flood conditions. Holding back
the natural flow of a river alters the local water system by forming a
reservoir or lake above the dam. Water that is stored in a reservoir is
often diverted to other areas. Some areas may now receive less water,
or no water at all.
Droughts
To learn more about floods
and droughts,
Go to Nelson Science
There are also different types of droughts. An area with a permanent
drought (less than 2.5 cm of precipitation annually) is called a desert.
A period of six months or more with no rain or snow is referred to
as seasonal drought. Drought can also occur when expected rains do
not fall for a long period of time. People who get their water from
wells are particularly affected by droughts. Wells can dry up, leaving
households and farmers with no access to fresh water. Unlike floods,
not much can be done to prevent droughts.
Figure 3 Trees help to hold water in
an area. When the trees are removed,
the water leaves, often taking some of
the soil with it.
Human Activities
Water shortages are often caused by human activities. These are not
natural droughts, but they have similar effects on local populations.
Nearly all of the world’s river systems are now altered by human
activities. People altered water systems by building dams. Large
populations of people withdraw water from river systems for drinking
water and for irrigation. Companies extract and bottle water to sell.
Together, these activities reduce the supply of surface water and
groundwater.
Trees and other vegetation naturally absorb and store water during
wet periods. They then clean the water and release it slowly during
dry periods. The loss of trees in a watershed and the destruction of
wetlands remove this “sponge” effect (Figure 3). Turning natural areas
into farms and cities often reduces water quality.
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Soil
Soil is made up of tiny pieces of rock and organic material. These
components generally pack together loosely, leaving open spaces that
fill with water and air (Figure 4). If soil does not receive rain and
decaying vegetation regularly, it can lose its ability to support life.
The soil suffers when humans manage the land poorly. Poor land-use
practices include clear-cutting of forests, overgrazing by farm animals,
and poor construction methods. The soil’s nutrients are used up, and
the soil becomes less productive and cannot support life.
TRY THIS: Water Absorption by Soil
tiny piece of rock
layer of water
air space
Figure 4 When soil is loosely packed
and has large air spaces, water can
pass through easily.
SKILLS HANDBOOK
SKILLS MENU: performing, observing, analyzing
Does water sink into soil in some areas better than in other
areas? This activity will help you to find out.
Equipment and Materials: 3 open-ended containers; ruler;
marker; beaker (600 mL); timing device; masking tape; water
2.B.7.
3. Using the beaker, pour 500 mL of water into each container.
Record the time it takes for all the water in each container
to sink into the soil.
A. Which spot absorbed the water fastest? Slowest?
1. Mark the outside of each container 3 cm from one end with
a piece of masking tape.
B. Compare the soil in the areas around the three containers.
Are they different in any way?
2. Identify three spots in your schoolyard, one for each
container. At each spot, use your foot to press the container
into the soil until the soil level reaches the 3 cm mark.
C. Explain the different rates at which the water was absorbed.
Soil Compaction
Some soils, such as clay, have very small air spaces. Soil scientists call
these “compacted soils” (Figure 5). Water will often collect in pools
on the surface of compacted soils, or run off to lower areas. What will
happen to an area with loosely packed soil if people start to walk on
it regularly? If one of the areas that you tested in the Try This activity
was an area where people frequently walk, you may have already
observed the effects on the soil.
Unit Task
tiny piece of rock
layer of water
air space
Figure 5 Compacted soil
Does your water issue for the Unit Task relate to floods and droughts?
If so, how?
CHECK YOUR LEARNING
1. How are floods and droughts sometimes good for the natural
environment?
3. Does damming a river make the water source more
sustainable? Explain your answer.
2. Briefly describe two strategies that would reduce the chances
of homes being damaged by floods.
4. What human activities worsen the effects of drought?
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5. How does soil compaction affect the water system?
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