Page 431 - Environment: The Science Behind the Stories

distances by river currents (Figure 15.24). Clear-cutting, min-
ing, clearing land for development, and cultivating farm fields
all expose soil to wind and water erosion (pp. 240–241). Some
water bodies, such as the Colorado River and China’s Yellow
River, are naturally sediment-rich, but many others are not.
When a clear-water river receives a heavy influx of eroded
sediment, aquatic habitat changes dramatically, and fish
adapted to clear water may be killed. We can reduce sediment
pollution by better managing farms and forests and avoiding
large-scale disturbance of vegetation.

Thermal pollution Water’s ability to hold dissolved
oxygen decreases as temperature rises, so some aquatic organ-
(a) Oligotrophic water body isms may not survive when human activities raise water tem-
peratures. When we withdraw water from a river and use it
to cool an industrial facility, we transfer heat from the facil-
ity back into the river where the water is returned. People also
raise water temperatures by removing streamside vegetation
that shades water.
Too little heat can also cause problems. On the Mississippi
and many other dammed rivers, water at the bottoms of reser-
voirs is colder than water at the surface. When dam operators
release water from the depths of a reservoir, downstream water
temperatures drop suddenly. In some river systems, these
pulses of cold water have favored cold-loving invasive fish spe-
cies over native species adapted to normal river temperatures.

Groundwater pollution is a difficult problem

Most pollution control efforts focus on surface water. Yet
groundwater sources once assumed to be pristine are regularly
polluted by industry and agriculture. Groundwater pollution
(b) Eutrophic water body is hidden from view and difficult to monitor; it can be out-of-
Figure 15.23 Pollution of freshwater bodies by excess nutri- sight, out-of-mind for decades until widespread contamina-
ents accelerates the process of eutrophication. An oligotrophic tion of drinking supplies is discovered.
water body (a) with clear water and low nutrient content may Groundwater pollution is also more difficult to address
eventually become a eutrophic water body (b) with abundant algae than surface water pollution. Rivers flush their pollutants
and high nutrient content. fairly quickly, but groundwater retains its contaminants until
they decompose, which in the case of persistent pollutants can
Biodegradable Wastes Introducing large quantities of
biodegradable materials into waters decreases dissolved oxy-
gen levels, too. When human wastes, animal manure, paper Figure 15.24 Sediments wash into the Pacific Ocean from
pulp from paper mills, or yard wastes (grass clippings and a river in Panama. Farming, construction, and other human
leaves) enter waterways, bacterial decomposition escalates as activities can cause elevated levels of soil to enter waterways,
organic material is metabolized. This lowers dissolved oxygen affecting water quality and aquatic wildlife.
levels in the water, just as in waters receiving elevated inputs
of plant nutrients. Wastewater is water affected by human
activities and can be a source of biodegradable wastes. It
includes water from toilets, showers, sinks, dishwashers, and
washing machines; water used in manufacturing or industrial
cleaning processes; and stormwater runoff. The widespread
practice of treating wastewater to remove organic matter has
greatly reduced impacts from biodegradable wastes in rivers
in developed nations. Oxygen depletion remains a major prob-
lem in some developing nations, however, where wastewater
treatment is less common.

Sediment As we saw in the Central Case Study, eroded
430 soils are carried to rivers by runoff and transported long

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