Hydroelectric Power Generation - Renewable Energy: Technology, Economics and Environment - page 380

fauna and sometimes also flora migrations and dispersion. The passive drift with
the flow is no longer possible in reservoirs and only occurs during floods when
organisms are washed across the dam or the spillway.
Such installations therefore interrupt the connectivity of the running waters.
This leads to a split and a reduction in the size of the water habitat and impairs or
even prevents spawning, feeding, expansion and compensation migrations. It can
particularly lead to the following consequences of interrupted fish migration (see
/8-13/).
− The potential for reproduction of certain types of fish is limited.
− The species diversity upstream of the dam is being reduced.
− The isolation of various fish populations increases.
− The recolonisation in regions impoverished by a catastrophic event such as
flood or pollution is slower.
For migratory fish that move to small tributaries for spawning, dams are the most
massive barriers; for small dams natural bypass channels or fish ladders might be
a possible countermeasure. To enable the full functionality of fish ladders, they
have to be allocated and dimensioned properly. As they have often not worked
properly in the past, natural bypass channels are increasingly built that can offer
habitats similar to those of the main waters in spite of lower flows.
If the running waters lose interconnectivity because of a dam or reservoir, this
also has consequences for the downstream migration of fish. For example, fish
that live in fast-running waters can only orient themselves with difficulty in the
reservoirs with their lower flow velocities. Fish can also be injured if the water
after an overflow weir crest is not deep enough. Furthermore, there is a danger for
fish when moving through the turbine because of the prevailing pressure and flow
conditions; additionally, the machines can cause mechanical injuries on the fish
bodies. This can be partly prevented by tight gaps in the screening before the tur-
bine inflow or correspondingly allocated devices that keep fish from entering into
the turbines.
Diverted reaches. In diversion type power stations the water is diverted from the
original bed into a headrace canal or pipe. This can lead to the following envi-
ronmental effects /8-14/, /8-18/:
− reduction of flow in the riverbed,
− loss of natural annual and diurnal periodic flow variations,
− extension of the low flow periods,
− change in the water supply for riparian areas,
− change in the temperature regime in the diverted river reach,
− surge and sink effects in times of flood,
− increase in sedimentation,
− lower water quality,
− drop of numbers of species in flora and fauna,
− decrease in spawning sites,
− increase in algae growth.