Hydroelectric Power Generation - Renewable Energy: Technology, Economics and Environment - page 379

habitats for fish and other small biota are covered by this fine matter. The usual
habitats are taken away from the organisms that live in the hyporheic zone and at
the river bottom; but that is exactly where the majority of biological processes oc-
cur in running waters in medium and high altitudes. This leads to a loss in the di-
versity of local habitat conditions.
Further consequences of the damming of rivers frequently are an increase in
water temperature and thus a reduction in the oxygen content in deep reservoirs.
Furthermore, every reservoir interrupts the continuous flow, i.e. it is a barrier that
can only be overcome with great difficulty, or not at all, by the many organisms in
running waters that migrate or move around for various reasons. In more or less
stagnant reservoirs they cannot find the living conditions they need. Therefore, the
natural running waters species composition changes; due to the different food
available for predators, this can, for example, lead to a total shift in the composi-
tion of species that also includes mammals, birds and amphibians. In the worst
case methane can occur because of the anaerobic decomposition of organic sedi-
ments (i.e. biogas).
Furthermore, the agglomeration of pollutants (especially heavy metals) in these
fine-grained sediments can lead to an increase in the pollutant concentration in the
reservoir. Apart from the eco-toxic dangers, the disposal of dredging material
from the reservoir causes further problems.
Through flushing of the reservoirs, within very short periods large amounts of
mainly small-grained sediments might be released. In the past, this has often led to
a strong impairment of the ecosystems in the lower course of the rivers. Reservoir
flushing during natural floods with a slowly increasing discharge, a high enough
oxygen concentration in the water, an acceptable maximum concentration of sus-
pended matter and pollutants, plus the adjustment of the flushing time to the de-
velopment stage of the fish fauna, can help minimise these negative effects.
Apart from fine sediments, the coarser bed load is also held back in the reser-
voir. Due to the lack of sediment downstream of the dam, erosion, and thus an in-
cision of the riverbed, can occur. This leads to a lowering of the groundwater ta-
ble linked to the stream, which can lead to the draining of wetlands and to a
change in the riparian vegetation.
The effects are most serious when linking up several power stations in series
(i.e. power station chains). A longer river section then loses its flow characteris-
tics and the end of backwater of one dam often stretches to the next power station
upstream.
If reservoirs are designed naturally, valuable new habitats for flora and fauna
can be created. They will not be the typical river biotopes; they are rather living
communities similar to the ones in still waters /8-14/.
Barrier effect of dam and power house. A large number of aquatic animals mi-
grate up or downriver; e.g. spawning migrations, migrations of invertebrates or
fish into sections that are scarcely populated after flooding, or migrations looking
for better feeding grounds. Reservoirs, dams and powerhouses are barriers for all