Page 412 - Environment: The Science Behind the Stories

the most bountiful grain-producing region in the world. How-
ever, unsustainable water withdrawals are threatening the Wyoming South Dakota
long-term use of the aquifer for agriculture (Figure 15.5).

FaQ Is groundwater found in huge Nebraska
underground caverns?
As one of the “out of sight” elements of the water cycle, it’s
sometimes difficult for people to visualize how water exists
underground. Many incorrectly assume that groundwa- Ogallala
ter is found in large, underground caves—essentially lakes Aquifer
beneath Earth’s surface. The reality is far less glamorous. Kansas
If you look at soil under a microscope, you’ll see there are
small pores between the particles of minerals and organic
matter that compose the soil. In the soil near the surface, Colorado
these pores are filled with air. Beneath the water table, how-
ever, the pores are filled with water. Groundwater can even
be found within rock. There are many types of rock, such as
limestone and sandstone, which have relatively large pores
between the particles of minerals that compose the rock.
Just as in soil, these pores can be filled with air or, beneath New Mexico Oklahoma
the water table, by water. So when people extract ground-
water with wells, we are simply sucking water out of the
pores that exist within soil or rock under the surface of the
Earth. Saturated thickness,
in feet
Miles 0–100
0 100 Texas 100–400
Surface water converges in river 0 160 400–800
and stream ecosystems Kilometers 800–1200

Surface water accounts for just 1% of fresh water, but it is vital for Figure 15.5 The Ogallala Aquifer is the world’s largest
3
3
our survival and for the planet’s ecological systems. Once water aquifer, and it held 3700 km (881 mi ) of water before pump-
ing began. This aquifer underlies 453,000 km (175,000 mi ) of the
2
2
falls from the sky as rain, emerges from springs, or melts from Great Plains beneath eight U.S. states. Overpumping for irrigation
snow or glaciers, it may soak into the ground or may flow down- is currently reducing the volume and extent of this aquifer.
hill over land. Water that flows over land is called runoff (p. 126).
As it flows downhill, runoff converges where the land
dips lowest, forming streams, creeks, or brooks. These small rivers are meandering rivers (Figure 15.6b). In a meandering
watercourses may merge into rivers, whose water eventually river, the force of water rounding a bend gradually eats away
reaches a lake or ocean. A smaller river flowing into a larger at the outer shore, eroding soil from the bank. Meanwhile,
one is a tributary. The area of land drained by a river sys- sediment is deposited along the inside of the bend, where
tem—a river and all its tributaries—is that river’s watershed water currents are weaker. Over time, river bends become
(p. 123), also called a drainage basin. If you could stand at exaggerated in shape, forming oxbows (Figure 15.6c). If water
the mouth of the Mississippi River and trace every drop of erodes a shortcut from one end of the loop to the other, pur-
water in it back to the spot it first fell as precipitation, then you suing a direct course, the oxbow is cut off and remains as an
would have delineated the Mississippi River’s watershed, the isolated, U-shaped water body called an oxbow lake.
very area shaded in Figure 15.1. Over thousands or millions of years, a meandering river CHAPTER 15 • Fr E shwat E r s yst E m s and rE sour CE s
Groundwater and surface water interact extensively. Sur- may shift from one course to another, back and forth over a
face water becomes groundwater by infiltration. Groundwa- large area, carving out a flat valley. Areas nearest a river’s
ter becomes surface water through springs (and from wells course that are flooded periodically are said to be within the
drilled by people), often keeping streams flowing or wetlands river’s floodplain. Frequent deposition of silt from flooding
moist when surface conditions are otherwise dry. Each day in makes floodplain soils especially fertile. As a result, agriculture
the United States, 1.9 trillion L (492 billion gal) of groundwa- thrives in floodplains, and riparian (riverside) forests are pro-
ter are released into bodies of surface water—nearly as much ductive and species-rich. A river’s meandering is often driven
as the daily flow of water in the entire Mississippi River. by large-scale flooding events that scour new channels during
Landscapes determine where rivers flow, but rivers shape periods of high flow. However, extensive damming on the Mis-
the landscapes through which they run. A river that runs sissippi and other rivers in its watershed has reduced the rate
through a steeply sloped region and carries a great deal of sed- of river meandering 66–83% from its historic rate. This occurs
iment may flow as an interconnected series of watercourses because floodwaters are trapped by dams and held in reservoirs
called a braided river (Figure 15.6a). In flatter regions, most rather than coursing down the river and creating meanders. 411

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