Page 415 - Environment: The Science Behind the Stories

Despite the vital roles played by wetlands, people have have been strongly or moderately affected by artificial dams,
drained and filled them extensively for agriculture. Many wet- dikes, and diversions.
lands are lost to these and other human activities. Southern We have seen that dams and channelization in the Mis-
Canada and the United States, for example, have lost well over sissippi River basin have led to adverse impacts at the river’s
half their wetlands since European colonization. mouth, showing clearly that what we do in one part of the
Wetlands, like other aquatic systems, are affected by peo- interconnected aquatic system affects other portions, some-
ple when we withdraw water for human use, build dams and times in significant ways. Let’s now take a closer look at the
levees, and introduce pollutants that alter water’s chemical, many ways human activities affect freshwater systems.
biological, and physical properties. Let’s now take a closer
look at such impacts on freshwater ecosystems.
Fresh water and human populations
are unevenly distributed across Earth
Human Activities Affect
Waterways The availability of fresh water varies widely around the
world because different regions possess varying amounts of
groundwater, surface water, and precipitation. People are not
Fresh water is one of the world’s most precious resources. Not distributed across the globe in accordance with water avail-
only do we need it to keep our bodies hydrated and healthy, ability (Figure 15.9). For example, Asia possesses the most
but we also require huge quantities of water for our homes, water of any continent but has the least water available per
farms, and factories. Although water is a limited resource, it person, whereas Australia, with the least amount of water,
is also a renewable resource as long as we manage our use boasts the most water available per person. Many densely
sustainably. Unfortunately, people are withdrawing water at populated nations, such as Pakistan, Iran, and Egypt, face
unsustainable levels and are depleting many sources of sur- serious water shortages. Because of the mismatched distribu-
face water and groundwater. Already, one-third of the world’s tion of water and population, human societies have always
people are affected by water shortages. struggled to transport fresh water from its source to where
Additionally, people have intensively engineered fresh- people need it.
water waterways with dams, levees, and diversion canals to Fresh water is distributed unevenly in time as well as
satisfy demands for water supplies, transportation, and flood space. India’s monsoon storms can dump half of a region’s
control. An estimated 60% of the world’s largest 227 rivers annual rain in just a few hours, for example. Seasonal rains
(and 77% of those in North America and Europe), for example, lead to differences in flow throughout the year in many places.

Available freshwater
(cubic meters per capita
per year)
Less than 1000
1000–2000
2000–5000
5000–10,000
10,000–20,000
20,000–100,000
More than 100,000
Insufficient data
Major inland waterway

Figure 15.9 Nations vary tremendously in the amount of fresh water per capita available to their citi-
zens. For example, Iceland, Papua New Guinea, Gabon, and Guyana (dark blue in this map) each have over 100
times more water per person than do many Middle Eastern and North African countries. Data from Harrison, P., and
F. Pearce, 2000. AAAS atlas of population and the environment, edited by the American Association for the Advancement of Science, ©
414 2000 by the American Association for the Advancement of Science.

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