Geoscience Reference
In-Depth Information
Table 22.4
Changes in some hydrologic parameters caused by selected dams in the USA (percent difference in the means between
pre-impact and post-impact conditions).
1-day
maximum
Q
1-day
minimum
Q
Hydrograph
rise rate
Hydrograph
fall rate
River (state)
Dam(s)
Bill Williams (Arizona)
Alamo
−
73
19
−
81
−
55
Colorado (Arizona)
Glen Canyon
−
64
31
22
62
Colorado (Texas)
EV Spence
−
79
156
−
92
−
90
Coyote (California)
Coyote and Anderson
−
79
0
−
91
−
79
−
72
−
73
−
78
−
54
Leon (Texas)
Belton
Source:
after Magilligan and Nislow (2005).
the number of dams located in drylands is undoubtedly
considerable. The largest of these are situated on exotic
rivers (with perennial flows) that run through deserts.
Geomophologically, the effects of damming a river and
creating a reservoir are numerous. Most dams reduce flow
downstream and hence reduce the power and sediment-
carrying capacity of the river. Peak discharges are also
commonly reduced. Table 22.4 shows changes in this and
some other hydrologic parameters for a number of US
dams. Any resultant changes in fluvial geomorphology
will depend on many different aspects of the dam, includ-
ing the size and shape of the reservoir and the flow release
policy. The number of dams on a river may also have an
important influence.
Dam construction and reservoir impoundment raises
the local base-level and can lead to sedimentation in the
reservoir and channel aggradation upstream. A typology
of the effects of dams on downstream geomorphology was
devised by Brandt (2000) based on the balance between
water discharge, sediment load, grain size and river slope.
Nine cases were identified, each involving different com-
binations of changes in slope, cross-section, bedform and
planform.
Graf (2005) notes that channel degradation and armour-
ing were two downstream issues that received consider-
able early attention from geomorphologists working on
dam effects in North America. Both processes result from
the clear-water discharge from dams that have trapped
sediments in their reservoirs.The loss of sediment due
to reservoir impoundment can be considerable. On the
Colorado River in the USA the amount of sediment trans-
ported declined from 1500 parts per million before the
Glen Canyon Dam was closed in 1963 to just 7 ppm there-
after (Graf, 1988). In other deserts, fluvial sediment lost
due to reservoir impoundment may be replaced, at least
in part, by inputs of windblown sand. This is the situation
on the reach of the Yellow River that flows through the
reservoirs in northern China, where Ta, Xiao and Dong
(2008) demonstrate that the river channel has experienced
considerable aggradation.
Such channel changes usually begin in the initial stages
of dam construction and continue throughout the life of
the dam thanks to fluctuations of the water level in the
reservoir. At the end of a dam's useful life it may be de-
commissioned and removed. The removal of large dams
is a recent phenomenon and there is little geomorpholog-
ical research on the effects published in formal, refereed
outlets. In consequence, the development of mature sci-
entific generalisations about fluvial processes related to
dam removal is in its infancy (Graf, 2005).
22.4.2
Urbanisation
Urban development can have numerous effects on hydro-
logical systems and fluvial geomorphology, both through
deliberate interventions for hazard mitigation (e.g. from
flash flooding) and resource exploitation (e.g. river sand
and gravel) as well as via inadvertent effects (e.g. through
large-scale introduction of impervious surfaces). How-
ever, our understanding of such effects is at a relatively
early stage because the volume of research conducted
in and around dryland cities lags some way behind that
carried out in other environments (Cooke
et al
., 1982;
Chin, 2006). This is important because the particular dy-
namics of many dryland rivers may mean that they re-
act to the effects of urbanisation in significantly different
ways from those in temperate and tropical regions. The
ephemeral nature of many streams (which can neverthe-
less typically carry high loads when water does flow), the
extreme spatial and temporal variability in precipitation
input and response mechanisms, along with rapid and
irregular changes in channel morphology, all prompted
Chin (2006, p. 475) to suggest that the impacts of ur-
