Handbook on Water Management - page 16

16
2.2.2. Hydromorphology: Mechanical action in ecosystems
Hydromorphological pressures on rivers and aquatic ecosystems are numerous, take several
forms and can be originated from various human activities or water uses.
They comprise all physical alterations of water bodiesmodifying their shores, riparian/littoral
zones, water level and flow (except water abstraction). Examples of such pressures are
damming, embankment, channelization, non-natural water level fluctuations.
Hydromorphological pressures are the consequence of human activities in the catchment area
including hydropower production, flood defence structures, navigation, agriculture, land
drainage, urban development, sand extraction and fisheries. Hydromorphological changes
may also result frommore than one activity (e.g. amulti-purpose dam for hydropower
generation, water supply and flood protection).
Thosemechanical actions, like dredging rivers and channelling bankswith rocks or cement
have an influence on the aquatic environment functioning, as theymodify the equilibrium of
river dynamics. It impactswater velocity, sediment transport and biological habitats, thus
disturbing the structure of the biotope and consequently the organisms participating in the
ecosystem.
Dredging awatercourse is generally a response to hydraulic considerations. For example,
silting and clogging of the bed can cause floods so that the contractor is obliged to dredge.
The primary cause is generally at a different level, i.e. urbanism, sediment erosion accelerated
by deforestation, elimination of ditches, badly channelled drainage, etc.
A rivermay need to be dredgedwhen navigation is hampered ormade impossible by silt.
Mechanical works on thewatercoursemay also be necessarywhen the bed is invaded by
plants from the banks, aquatic plants have proliferated, the bed is clogged by diverse deposits
(floating, blocking, etc.), or if thewatercourse has shifted or the banks have collapsed.
Dragging rivers to extract gravel or sand can also perturb aquatic ecosystems.
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THEHANDBOOK FORMANAGEMENT
ANDRESTORATIONOF AQUATIC ECOSYSTEMS INRIVERAND LAKE BASINS
2FUNCTIONSANDBENEFITSOFAQUATICECOSYSTEMS
Example 1: Invasive plants and neophytes in Lake Geneva
Japanese knotweed, summer lilac, cherry laurel and the black locust; these names have become
familiar because they now appear in the vegetation around Lake Geneva. However, it was not
always so.
These species are not indigenous; they were imported during recent centuries for trade, their
ornamental qualities, or by accident, and have gone on to colonize natural environments. On the
banks of Lake Geneva, some of these plants can be seen in gardens, but some are outside and
have taken over unplanted areas or those previously occupied by indigenous vegetation. They
can now be found in natural environments, like along watercourses, the lake or in forests, but
also in urban environments, along roads or railways, or in rocks on the banks of Lake Geneva.
The Convention on Biological Diversity, which Switzerland and France have ratified, advocates
preventing new introductions and requiresmonitoring or eradicating species already in place,
which shows the benefit of the CIPEL for this problem and the need to act.
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