Geoscience Reference
In-Depth Information
VISUAL CONCEPT CHECK 16.3
This satellite image shows the deltas
of the Krishna (bottom) and Godarvi
(top) rivers in India. The whitish ar-
eas in the Indian Ocean are plumes of
sediment from these rivers. Why are
they present?
you live. These features have the effect of sealing the ground
so that water runs directly off the surface rather than soaking
into the ground. In many cases, entire drainage basins now lie
within urban areas where the ground cover is no longer veg-
etation and soil, but mostly asphalt and concrete.
The impact of urbanization on stream behavior is dra-
matic, especially following a heavy rainfall. Figure 16.32
shows a hydrograph that illustrates the theoretical response
of a stream to a heavy storm prior to urbanization (green line)
and after urbanization (blue line). Before the city was con-
structed, the lag time between the storm peak (when it was
raining hardest) and peak stream discharge was slow. This
long lag time occurred because the soil-water belt first had to
be saturated before the stream could be supplied with runoff
or water from a recharged aquifer.
After urbanization, however, the response of stream dis-
charge is markedly different in two specific ways. Can you see
them in the stream hydrograph? First, the stream response is
much faster; in other words, the lag time between the storm
peak and peak discharge is less. This decreased lag time oc-
curs because rainwater is no longer absorbed first into the soil,
but runs off the urban parking lots and other concrete surfaces
directly into the stream. Second, the peak discharge in an ur-
banized setting is greater after a given storm event than it was
following a storm of similar magnitude during pre-urban times.
This increased discharge occurs in the urbanized stream be-
cause more water runs off into the stream from the various as-
phalt and concrete areas rather than being absorbed by the soil.
Given these relationships, urban streams are often described as
being flashy because they react more quickly than their rural
counterparts.
These conditions make flash floods much more likely in
A delta is a low-lying plain that forms at the mouth
of rivers where they meet the ocean. They develop
because stream power is lost when the stream flows
into the relatively still ocean water. This change in
velocity causes massive sedimentation. The sedi-
ment contained within a delta is well sorted, with
coarse deposits accumulating at the river mouth
and progressively finer sediments transported to the
delta front.
Human Interactions with Streams
As indicated previously, rivers and streams have long been
important for human societies. They function as transporta-
tion networks, sources of water for direct human consump-
tion and irrigation, and power that drives factories and even
cities. Although this section does not describe every way that
people interact with rivers and streams, it provides a good
feel for the way we impact streams and how they can impact
us. The need to understand these relationships was tragically
demonstrated in August 2010 when severe monsoon flooding
in Pakistan killed over 1500 people and affected another 10
million due to the low-lying nature of the country.
Urbanization
People have a huge impact on fluvial systems through the pro-
cess of urbanization. As cities expand, former vegetated soil
surfaces are converted into parking lots and other impervious
structures. Just consider for a moment the size of the park-
 
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