Use of decision support systems to improve dam planning and dam ...

2. The environmental impacts of dams
Key message:
River ecosystems are complex interlinked systems supporting immense biodiversity all of directly or indirectly
maintained by constantly changing river flow, including floods. If the natural flow regime is disturbed by a
dam much of the biodiversity and many of the ecosystem services are placed at risk.
2.1 River ecosystems
Rivers are central elements in most landscapes. They are important natural corridors for flows of energy,
matter and species. Very often, they are key elements in the maintenance of biodiversity. Despite occupying
a smaller area than land and oceans, freshwater habitats are home to a relatively high proportion of species.
Almost half (42%) of all known fish species and a quarter of all mollusc species (25%) occur in freshwater
(Darwall et al. 2005). These figures reflect not only the diversity of habitats that can support organisms
within individual rivers but also the great diversity of river systems themselves. Rivers are also essential for
many terrestrial species. Through the production of natural resources and provision of ecosystem services,
rivers and the biodiversity they support, are essential for the livelihoods of many people (Chapter 3).
River ecosystems are complex, comprising all the physical, chemical and biological components of the
environment linked to the river. A river ecosystem extends from the headwaters to the sea and includes
riparian areas, associated groundwater in the channel/banks and floodplains, wetlands, the estuary and any
near-shore environment dependent on freshwater inputs (Figure 2.1). Flow is one of the principal factors
influencing the character of river ecosystems. The frequency of flow extremes (i.e., floods and droughts)
exerts important controls on every physical, chemical and biological attribute of the river ecosystem.
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Evaporation
Groundwater
Surface runoff
Precipitation
Estuary
Coast
Evaporation
Figure 2.1. River ecosystems depend on lateral, longitudinal and vertical fluxes of water. Within a catchment
all environments are dependent to a greater or lesser degree on connectivity with the active river
channel. The ecological character of the river depends on the interactions with those environments.