Environmental Engineering Reference
In-Depth Information
10
3
10
2
10
1
10
0
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
Drainage area (km
2
)
FIGURE 5.12
Relationship between drainage area and meander length (redrawn from
Leopold
et al.,
1964).
Floodplains are often inundated seasonally and provide numerous wet-
lands and habitat for many species of plants and animals. Over geological
time, a river moves back and forth across its valley. Any traveler on an air-
plane can observe the related landscape heterogeneity.
The aggregate effect of these different aspects of channel structure, in
addition to the effects of fallen trees and large rocks, leads to a highly het-
erogeneous system on the scale of tens to hundreds of meters (Fig. 5.13B).
The degree of heterogeneity in the floodplain includes raised levees that are
deposited naturally along stream channels and old scoured oxbows. In
large rivers, depressions caused by river actions may form important wet-
lands, ponds, or lakes. This heterogeneity alters response to floods and
greatly influences the ecology of rivers and the riparian zone's function as
an interface between terrestrial and aquatic habitats (Naiman and Dé-
camps, 1997).
Few relatively pristine river systems remain. Humans have had a ma-
jor impact on the geomorphology of rivers throughout the world via
damming, channelization, and excessive water usage (Sidebar 5.1). Other
organisms, such as beaver, hippopotamus, crocodile, and elephant, alter
channel morphology and riparian areas (Naiman and Rogers, 1997), but
none does so as strongly as humans.
Most large rivers have been altered significantly from their natural
state. For example, 70% of the discharge from the 139 largest rivers in
North America, Europe, and the former Soviet Union is affected by irriga-
tion, diversion, or reservoirs. Most of the unaffected river systems that re-
main in these regions are in the far north (Dynesius and Nilsson, 1994). In
the United States, it is estimated that there are 2.5 million dams (National
Research Council, 1992). Reservoirs cause sediments to settle from rivers,
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