144 CHAPTER 6 Ecosystems and Evolution
warmer in winter than in adjacent parts of the stream or
river. The kinds of organisms found in flowing water vary
greatly from one stream to another, depending primarily
on the strength of the current. In streams with fast cur-
rents, some inhabitants have adaptations such as suckers,
with which they attach themselves to rocks to prevent be-
ing swept away. Some stream inhabitants have flattened
bodies to slip under or between rocks. Other inhabitants
such as fish are streamlined and muscular enough to
swim in the current.
Human activities such as pollution and dam construc-
tion have adverse impacts on rivers and streams. These
activities damage wildlife habitat and threaten water sup-
plies and fisheries. (See Chapter 10 for more discussion
of the environmental effects of dams.)nitrates and phosphates. Although eutrophication is
a natural process, human activities often accelerate it,
such as the runoff of agricultural fertilizers and dis-
charge of treated or untreated sewage. Eutrophication
of lakes is discussed in detail in Chapter 10.
Flowing-water ecosystems are highly variable. The
surrounding environment changes greatly between a
river’s source and its mouth (Figure 6.13). Certain parts
of the stream’s course are shaded
by forest, while other parts are ex-
posed to direct sunlight. Ground-
water may well up through
sediments on the bottom in one
particular area, making the water
temperature cooler in summer or
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Headwater streams
flow downstream
rapidly, often over
rocks (as rapids) or
bluffs (as waterfalls).The river begins at
the source, often
high in the mountains
and fed by melting
snows or glaciers.Along the way,
tributaries feed into
the river, adding to
the flow.
The floodplain is the
relatively flat area on
either side of the
river that is subject to
flooding.
Near the ocean, the
river may form a salt
marsh where fresh water
from the river and salt
water from the ocean
mix.MouthThe delta is a fertile,
low-lying plain at the
river’s mouth that forms
from sediments that the
slow-moving river deposits
as it empties into the
Ocean ocean.WaterfallRapidsBends called
meanders form
as the river’s course
levels out, so that
the river flows more
slowly and winds
from side to side.
MeandersFloodplainFrans Lanting/Corbisb. Aerial view of meanders in the Tambopata River, Peru.a. A river flows from its source to the ocean.
Many of the world’s busiest cities are
located at river mouths, or along rivers, on
the floodplain. Can you name examples in
your region, or anywhere else in the world?GLOBALLOCALflowing-water
ecosystem A
freshwater ecosystem
such as a river or
stream in which water
flows in a current.