Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 404

important downstream manifestation of river impoundment is the loss of pulse-stimulated responses at
the water-land interface of the riverine system (Gill, 1971). The species adapted to pulse-stimulated habitats
are adversely affected by flow-regulation. The Eucalyptus forests (primarily eucalyptus camaldulensis)
of the Murray floodplain, Australia, depend upon periodic flooding for seed germination, and regeneration
has been curtailed by headwater impoundment (Walker, 1979). Artificial pulses generated by dam releases
at the wrong time—in ecological terms—have also been recognized as a cause of forest destruction. Acacia
xanthoploea, for example, is disappearing from the Pongolo system below Pongolapoort Dam, South
Africa, as a result of mistimed high-flows (Furness, 1978).
For floodplain habitats, river impoundment and flow-regulation can have disastrous consequences.
Periodic inundation is a vital component of wetland ecosystems, and the removal or altered timing of the
annual flooding has changed, in many cases, highly productive habitats into unproductive scrub or, with
further intervention by man, into vast areas of monoculture or industrial or urban uses. Within channels,
the increased growth of macrophytes can support substantial numbers of invertebrates, but on floodplains,
the elimination of the annual flood has devastated the often unique faunal assemblage. Moreover, the
direct loss of annual silt and nutrient replenishment, consequent upon upstream impoundment, is thought
to have contributed to the gradual loss of fertility of formerly productive floodplain soils (Attwell, 1970).
However, the Aswan High Dam has increased productivity of the floodplain as a result of the provision
of a reliable, all-year-round water-supply (Kinawy et al., 1973).
7.1.5.3 Macroinvertebrate Response to Upstream Impoundment
Within most natural rivers the pattern of flow, the temperature variations, the substrates and the bed
stability are the dominant factors controlling macroinvertebrate distributions (Ward and Stanford, 1979).
The life-cycles of many lotic species are related to the natural seasonal variations of discharge, and the biotic
communities of streams are significantly affected by flow-velocity because of respiratory, physiological,
and feeding requirements (Petts, 1984). In some of their habitats, most lotic species are limited in their
preference of water-depth and the short-term magnitude and frequency of flow-variations can have a
marked influence upon organisms of narrow depth-tolerance.
Many life-cycle phenomena, such as hatching, growth, and emergence, depend on thermal cues
(Lehmkuhl, 1972) and the alteration of the thermal regime has been credited as a primary factor influencing
community change (Gore, 1980). Substrate heterogeneity is a necessary requirement for the maintenance
of a diverse number of macroinvertebrate species. Most aquatic insect adults are rheophilic and select
upstream areas to colonize, reproduce, and deposit eggs (Hynes, 1955). A dam will act as a barrier to
aerially colonizing adults as well as to the passive downstream drift of nymphs and larvae (Hynes, 1955;
Minckley, 1964).
Most studies of impounded rivers utilize comparative data derived either from upstream or from
adjacent or tributary rivers, because of the lack of reliable pre-dam data. Several responses can be
recognized which are common to many impounded rivers in a review of twenty-three studies from USA,
Europe, and South Africa. Stanford and Ward (1979) found that in all but three cases, species-diversity
was reduced by river impoundment. However, even if the relative abundance does not change, the
composition of each taxonomic group can be modified considerably. Table 7.5 lists some reported
responces of macro-invertebrates to the impoundment (Petts, 1984).
Reservoir operations for hydroelectric power-generation, irrigation supply, or recreational or fishery
demands, produce artificial discharge variations. These often involve extreme fluctuations of water depth
and flow velocity, having unnatural rates of change, unnatural durations, and unnatural frequencies.
Within natural rivers experiencing flows of high variability, a high level of production can be attained,
provided that the community present is adapted to the frequency and magnitude of flow fluctuation