Living with dams: managing the environmental impacts Matthew McCartney International Water Management Institute, PO Box 5689, Addis Ababa, Ethiopia. Fax: �� 251 11 617 2100. E-mail: m.mccartney@cgiar.org Abstract Dams, through disruption of physiochemical and biological processes, have water and associated environmental impacts that have far reaching social and economic consequences. The impact of each dam is unique. It depends not only on the dam structure and the attributes of local biota but also climatic and geomorphic conditions. Given the number of existing dams (over 45,000 large dams) and the large number that may be built in the near future, it is clear that humankind must live with the environmental and social consequences for many decades to come. This paper provides a review of the consequences for ecosystems and biodiversity resulting directly from the presence of dams on rivers, and of constraints and opportunities for environmental protection. It illustrates that a wide range of both technical and non-technical measures has been developed to ameliorate the negative impacts of dams. It argues that relatively few studies have been conducted to evaluate the success of these measures and that it is widely perceived that many interventions fail, either for technical reasons or as a consequence of a variety of socio- economic constraints. It discusses the constraints to successful implementation and mechanisms for promoting, funding and ensuring compliance. Finally, it contends that there is a need to improve environmental practices in the operation of both existing and new dams. Keywords: Biodiversity Ecosystems Environmental Protection Large dams 1. Introduction Freshwater habitats comprise less than 0.01% of the Earth���s water, but they contain exceptional concentrations of biodiversity. Although occupying a smaller area than land and oceans, freshwater ecosystems are home to a relatively high proportion of species, with more per unit area than other environments: 10% more than land and 150% more than the oceans (Millennium Ecosystem Assessment, 2005). About 45,000 species of freshwater animals, plants and micro-organisms have been scientifically described and named. However, scientists estimate that at least a million more species remain to be identified (McAllister et al., 1997). doi: 10.2166/wp.2009.108 Water Policy 11 Supplement 1 (2009) 121���139 q IWA Publishing 2009

Dams represent one of the most significant human interventions in the hydrological cycle. Through provision of water for drinking, irrigation and electricity, they have supported human socio-economic development, but simultaneously they have had a considerable impact on freshwater ecosystems. It is estimated that inter-basin transfers and water withdrawals for supply and irrigation have fragmented 60% of the world���s rivers (Revenga et al., 2000). For most of the world���s existing stock of dams, environmental issues played little part in their design and operation. However, in the last three decades, an increase in environmental awareness has led to the recognition that the management of water resources includes a responsibility to protect the users of water (and the natural resources that depend on water) from over- utilisation or impacts that cause degradation. As a result, considerable effort has been invested in developing approaches to lessen the most damaging affects of dams. However, experience indicates that the success of these measures is extremely variable and far from assured (Bergkamp et al., 2000). This paper provides a review of the consequences for ecosystems and biodiversity resulting directly from the presence of dams on rivers, and of constraints and opportunities for environmental protection. It illustrates that a wide range of both technical and non-technical measures has been developed to ameliorate the negative impacts of dams. It argues that relatively few studies have been conducted to evaluate the success of these measures and that it is widely perceived that many interventions fail, either for technical reasons or as a consequence of a variety of socio-economic constraints. It discusses the constraints to successful implementation and mechanisms for promoting, funding and ensuring compliance. Finally, it contends that there is a need to improve environmental practices in the operation of both existing and new dams. 2. Impacts of dams on river flow, water quality and sedimentation Rivers exist as a continuum of linked surface and groundwater flow paths and are important natural corridors for the flows of energy, matter and species. The spatial and temporal heterogeneity of river systems is responsible for a diverse array of dynamic aquatic habitats and hence biological diversity, all of which are maintained by the constantly changing flow regime. Upstream and downstream linkages as well as lateral connections between the river and floodplain are encompassed within the ���river continuum concept��� (Vannote et al., 1980) and the ���flood pulse��� concept (Junk et al., 1989). These contend that natural changes in river flows, water quality and species, both within the river and on the floodplain, are all interlinked. Nutrients and sediment generated in the headwaters are recycled downstream driving plant growth and biotic productivity. Regular inundation of floodplains increases organic matter decomposition and nutrient cycling and has led to the evolution of adaptive strategies that are tightly coupled to the flood regime. In some places nutrients are returned upstream through the passage of migratory fish (e.g. salmon). When spawning is completed and the fish die, the carcasses floating downstream decompose releasing nutrients. This release leads to significant increases in primary and secondary production (i.e. phytoplankton and zooplankton). In some North American rivers, primary productivity and bacterial activity reach their annual peaks during carcass decomposition, even when spawning runs occur during the winter (Helfman et al., 1997). Dams constitute obstacles for longitudinal exchanges along fluvial systems. The construction of a dam results in ���discontinuities��� in the river continuum (Ward & Stanford, 1995). Post impoundment phenomena directly and indirectly influence a myriad of factors that affect natural processes and so, ultimately, alter the ecological structure of ecosystems, sometimes tens or even hundreds of kilometres downstream. M. McCartney / Water Policy 11 Supplement 1 (2009) 121���139 122

2.1. Impacts on flow regime The most obvious impact of storage reservoirs is the upstream inundation of terrestrial ecosystems and, in the river channel, the conversion of lotic to lentic systems. Dams also alter the downstream flow regime. The effect of a dam and its reservoir on flow regimes depends on both the storage capacity of the reservoir relative to the volume of river flow and the way the dam is operated. The most common attribute of flow regulation is a decrease in the magnitude of flood peaks and an increase in low flows (Table 1). A consequence of reduced flood peaks is reduction in the frequency and extent of overbank flooding (McCartney & Acreman, 2001). For example, in the Hadejia���Nguru wetlands in Nigeria, annual flooding of about 3,000 km2 prior to the building of dams was reduced to less than 1,000 km2 after construction (Hollis et al., 1993). In some circumstances, operational procedures can result in rapid flow fluctuations that occur at non-natural rates. Hydroelectric power and irrigation demands are the most usual causes, but short-duration high discharges are also utilised for navigational purposes and for recreation. For many purposes, so called ���pulse releases��� are made regularly. For example, daily releases through power turbines often reflect diurnal variation in power demand (Table 1). 2.2. Impacts on thermal regime Water temperature influences many important ecological processes. Temperature is an important factor affecting growth in freshwater fish, both directly and indirectly, through feeding behaviour, food assimilation, and the production of food organisms. Under natural conditions the relatively small volume of water in a river section and turbulent mixing ensure that river water responds rapidly to changes in the prevailing meteorological conditions. In contrast, the relatively large mass of still water in reservoirs allows heat storage and produces a characteristic seasonal pattern of thermal behaviour. Depending on geographical location, water retained in deep reservoirs may become stratified. Releases of cold water from the hypolimnion (i.e. the deep cold layer) of a reservoir, is the greatest ���non-natural��� consequence of stratification. Even without thermal stratification, water released from reservoirs is often thermally out of phase with the natural regime of the river (Table 1). 2.3. Impacts on water chemistry Water storage in reservoirs induces physical, chemical and biological changes, all of which affect water chemistry. Consequently, the water discharged often has a very different composition to that of inflowing rivers (Table 1). Nutrients, particularly phosphorous, are released biologically and leached from flooded vegetation and soil. Oxygen demand and nutrient levels generally decrease as the organic matter decays, but some reservoirs require many years for the development of stable water-quality regimes (Petts, 1984). After maturation, reservoirs can, like natural lakes, act as nutrient sinks. For example, in comparison to the inflows, mean concentrations of orthophosphate in the outflows from the Callahan Reservoir, Missouri, USA, were reduced by 50% (Schreiber & Rausch, 1979). Eutrophication of reservoirs may occur as a consequence of large influxes of organic material and nutrients, often arising as a consequence of anthropogenic activity in the catchment (Chapman, 1996). The quality of water released from a stratified reservoir is determined by the elevation of the outflow structure(s) relative to the different M. McCartney / Water Policy 11 Supplement 1 (2009) 121���139 123