dioxide capture and storage
4.2 ECOLOGICAL IMPACTS OF HYDROPOWER PLANTS
This section provides an overview of ecological impacts that can be caused by dams and reservoirs. As previously mentioned, the actual impacts depend very much on the specific project and biogeography. Dams can serve many purposes such as flood control, irrigation and navigation; hydropower generation is often only one of several purposes. The construction of hydropower plants modifies creeks and streams and often leads to the flooding of land areas; they are hence associated with changes in both aquatic and terrestrial habitat (Alho, 2011). The ecological consequences of such habitat change are site-specific and difficult to generalize.
Habitat change leads to a change in species that populate these habitats, with potential consequences for larger regions, including the areas downstream of dams. The ecological impacts of hydropower dams are subject to controversy, and there are efforts to mitigate adverse consequences. Hydropower plants are massive civil engineering projects that may involve substantial earth movement, dam construction, tunnelling, and weir, pipe, turbine and electrical equipment installation. Although some environmental impacts associated with construction and machinery have been assessed with LCA, these assessments have gaps. A full review of the environmental impacts of hydropower was not possible as part of this work; we hence limit ourselves to a short description.
In the following section, we will first discuss the upstream impacts from the dam resulting from reservoir formation, those caused by the dam through the blocking of migration pathways, and downstream impacts resulting from changes in the flow regime and water properties. Finally, we address macroecological effects caused by hydropower projects over a larger region, as they are often placed several in series. In the subsequent section, we briefly address opportunities to mitigate these impacts.
4.2.1 RESERVOIR
The creation of a reservoir transforms terrestrial and riparian ecosystems into aquatic lake ecosystems.
The inundation of land and embankments can affect both terrestrial and aquatic species. Shallow water habitat represents important breeding grounds and depends on the interaction of terrestrial and hydrologic processes. It is thus vulnerable to environmental change (Alho, 2011).
4.2.1.1 Sedimentation
Sediment carrying capacity is directly related to the current velocity and slop of the water body. As a result, the reduction of stream velocity leads to the sediment deposition in the reservoir. Kumar et al. (2011) point to earlier work which indicates that 0.5-1.0 per cent of the global freshwater storage capacity of reservoirs is lost annually as a result of sedimentation. The filling of reservoirs by sediments can raise the riverbed and increase flood risks, as was the case in the lower reaches of the Yellow River (Xu, 2002). The extent of sedimentation depends on the sediment flow of the drainage basin which again is a function of geological and climatic conditions. Human activities such as agriculture, mining, urbanization, river regulation, and infrastructure projects influence both the amount and composition of sediments.
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4.2.1.2 Water quality
Sediments consist not only of minerals but also of organic matter, which contains nutrients. This nutrient input improves growing conditions for phytoplankton and algae, further increasing the organic content of the reservoir. Reduced turbulent mixing through reduced velocity and increased depth results in thermal stratification. The oxidation of organic matter in a stratified reservoir leads to the depletion of oxygen and may thus result in the formation of an anoxic zone (Kumar et al., 2011). Under anoxic conditions, the degradation of organic matter leads to the formation of CH4. Water quality of the tributaries, especially the input of nutrients and organic matter, are important factors in determining the water quality and health of a reservoir.
4.2.1.3 Public health
Increased still water and poor water quality provide good habitats for disease vectors for malaria, river blindness, dengue or yellow fever, amongst others (Kumar et al., 2011; Ziegler et al., 2013). This is especially a problem in tropical and subtropical regions and in cases where dam construction or population displacement leads to higher concentrations of humans. In other cases, the deforestation associated with dam construction also contributes to the spread of disease vectors. For example, Vilela et al. (2011) investigate the spread of leishmaniasis as a result of the construction of the Luís Eduardo Magalhães Hydroelectric Plant in Brazil.
The creation of anoxic conditions can lead to the release of the mercury bound in soil and accumulated biomass, and the subsequent methylation of this mercury by sulfate and iron reducing bacteria (Driscoll et al., 2013). The resulting methylmercury can then enter aquatic food chains and lead to toxic effects in humans (Gump et al., 2012). Reservoirs also reduce the transport of mercury from other natural or anthropogenic sources, such as mining and fossil fuel power plants, to the oceans, thus leading to the mercury accumulation in freshwater bodies (Kumar et al., 2011).
Hydropower can also contributed to improved public health, in most cases through the development of the local economy that goes hand in hand with hydropower development. Dam operators sometimes finance public health programs to protect and improve the health of the local population.
4.2.1.4 Habitat change
Impoundment leads to a substantial change of habitat for fish and other aquatic species such as amphibians and crustaceans. Species adapted to fast flowing rivers are replaced by species adapted to lake-type
environments. From an anthropocentric perspective, this can be positive or negative. Although the total biomass often increases with dam construction, it tends to favour species of lower the commercial value. Habitat created by dams can be ecologically valuable for birds, as these habitats replace wetland lost to agriculture in nearby areas (Kumar et al., 2011). However, from a biodiversity perspective, the reservoir is often of lower value than wetlands as it contains fewer ecological niches and often becomes the habitat of alien introduced species, which displace native species. Globally, there is a loss of riparian habitat and associated biodiversity.
4.2.1.5 Social impacts
The creation of a reservoir can also lead to the displacement of populations (Bao, 2010; Heming et al., 2001;
Nakayama et al., 1999) and the flooding of cultural heritage sites (Kumar et al., 2011). Scudder (Scudder, 2002, 2005) surveyed 50 cases of dam construction involving the resettlement of a total of 1.5 million people.
About half of the affected population was classified as tribal or indigenous, and the majority consisted of smallholder farmers. Scudder found that in 82 per cent of the investigated cases, resettlement lead to a deterioration of living conditions for the affected population; living conditions improved in only 7 per cent of cases. The results are complicated by the fact that in many cases, the resettlement process had not been completed at the time of the survey. The affected populations were found to suffer from unemployment and landlessness, implying a loss of livelihood resulting from the construction project. The survey found that displaced native populations were unable to compete with migrants attracted by the construction project.
This was an important contributing factor to the overall negative outcome.
4.2.2 DAM
Dams present large, physical barriers to passage up and down rivers. This obstruction leads to habitat fragmentation, decrease of in-stream habitat and blockage of migrating fish (Finer and Jenkins 2012;
Renöfält et al., 2010; Wollebæk et al., 2011b; Ziv et al., 2012; Sheaves et al., 2008; McLellan et al., 2008;
Saunders et al., 1991).
4.2.2.1 Obstruction of fish migration
Dams obstruct the migration of migratory fish species, thus interfering with their life cycles. Blocking migrating species is a serious problem caused by damming the river. Many fish populations have been, or are expected to become, extinct because of dam blocking. Diadromous fish that live in salt water and spawn in freshwater or vice versa are, in many cases, entirely unable to reach their spawning grounds. Salmon and shad have become locally extinct due to dam construction at several sites (Mann and Plummer, 2000; Larinier, 2001;
Thorstad et al., 2008). The dams in Elwha river have obstructed the upstream migration of salmonidae to over 90 per cent of the watershed for over 90 years in Washington State (Pess et al., 2008). In some cases, these impacts can be mitigated through fish ladders, e.g., for salmon.
4.2.2.2 Habitat fragmentation
Dams also isolate local fish, insects and larval clam populations (Wollebæk et al., 2011b). Reduced genetic exchange between populations can lead to decreased survivability. Biological interactions also play a part.
For example, reductions in insects and larval clam populations, which serve as food for organisms higher up on the food chain, can have indirect effects on fish populations (Finer and Jenkins 2012). Dams can also compromise the dispersal of seeds (Nilsson and Berggren 2000).
4.2.3 DOWNSTREAM IMPACTS
Dams affect the natural fluctuations in water flow. Although reservoirs prevent seasonal flooding, this can reduce the deposition of nutrients on flood plains and affect species and ecosystems that are dependent on regular flooding (Kunz et al., 2011). We discuss below some of the concerns that can arise from hydropower projects.
4.2.3.1 Volume and timing of water release
Such changes affect many downstream species and habitats. Fish and amphibians require specific
conditions on banks and in flood pools to spawn and rear. These conditions may be affected by the timing, volume, ramping and pulsing of water flow from the dam, leading to reproductive failure (Yarnell et al., 2012; Young et al., 2011; Guo et al., 2011; Arias et al., 2014). For example, Yarnell et al., investigate the effect of seasonal water pulses of regulated waterways in Northern California on the reproduction of foothill yellow-legged frog (Rana boylii) populations and find that there is a disconnect between suitable sites that protect egg masses and tadpole habitats. In addition, the timing of pulses e.g., spring floods can be the cue for species to begin migrating, and a hydropower driven modification can disturb these signals (Young et al., 2011). In extreme cases, hydropower reservoir operation can cause rivers to temporarily run dry (Fu et al., 2008). Regulating the river flow to avoid or mitigate spring floods and other seasonal flow variations may be included in the purpose of the dam and can have both positive and negative effects on local wildlife and economy (Arias et al., 2014).
4.2.3.2 Flood plains
Dam construction reduces seasonal flooding and associated nutrient deposition, affecting the extent and fertility of flood plains (Zeilhofer and de Moura 2009). Flood plains tend to have a high biodiversity and high productivity. On the Zambezi river, the completion of the Itezhi-Tezhi Reservoir in 1978 has led to a reduction of nitrogen and phosphorus transport to the floodplains of the Kafue flood plains by 50 per cent and
60 per cent, respectively (Kunz et al., 2011). The regulation of flow can hence have an important impact on downstream terrestrial habitat.
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4.2.3.3 Reduced sediment flow
Sedimentation in reservoirs reduces the sediment load in rivers, changes their morphology, and can lead to a deepening of rivers, a reduction of water tables, and the erosion of river deltas, subsequently affecting these downstream ecosystems. Sediment starvation attributed to retention by dams can alter the substrate composition downstream, which is important for spawning and rearing habitat formation. In coastal areas, the erosion caused by waves is no longer counteracted by deposition of sediment; the WCD reports that the coastline of Togo and Benin has decreased by 10-15 meters per year after the Akosombo Dam on the Volta River was completed (World Commission on Dams, 2000). For the Nile River, the Aswan High Dam has stopped the flow of sediment, resulting in a significant erosion of the riverbed and banks and a retreat of its estuary. As a result of lowering the river bed by 2-3 m, irrigation intakes were left dry and bridges undermined (Kumar et al., 2011).
There are indications that erosion may also contribute to floodplain fertility (Arias et al., 2014). Downstream, changes in flows of freshwater and in nutrient levels can influence the estuarine habitats where many marine fish come to spawn. Lowered nutrient levels can result in lowered overall productivity from a diminished primary food source, i.e. less primary production, as occurred with the Aswan High Dam in Egypt.
Furthermore, increases in salinity resulting from reduced freshwater flows can allow marine predators to invade, lowering recruitment rates (WCD, 2000).
A reduction of the sediment flow can have a substantial impact on marine ecosystems through reduced input of silica and other nutrients, which affect algal ecology (Ittekkot et al., 2000). The reduction of freshwater input to estuaries also affects the composition of fish found in these habitats (Vorwerk et al., 2008).
4.2.3.4 Changes in water quality of downstream waterways Stratification effects
A reservoir affects a number of variables, including the water temperature through thermal stratification and the content of dissolved gases through the hydrostatic pressure and creation of potentially anoxic conditions.
Hydropower plants sometimes draw water from deeper layers of the reservoir, where the temperature and dissolved gas content can be substantially different from the natural conditions of the river. Lower water temperatures in a river can have an impact on sensitive native fish species and life history processes of invertebrates. In the long term, susceptible species may be eliminated altogether from the downstream habitat. Coldwater releases have been found to delay spawning by up to 30 days in some fish species (Sherman et al., 2007; Miles and West, 2011). However, human activities generally tend to increase water temperatures, e.g., by cooling systems for thermal power plants (Hester and Doyle, 2011), and dams are sometimes used to reduce water heating effects and thus maintain more natural water temperatures.
Dissolved gas supersaturation
Spills over dams may cause supersaturation of waters downstream, which influence the physiological
processes in aquatic fauna. For example, supersaturated waters absorbed by fish during respiration cause the formation of gas bubbles in the bloodstream (Johnson et al., 2007; Li et al., 2009). This is called gas bubble disease. The physiological effects are similar to decompression-induced supersaturation occurring when divers emerge too quickly from deep dives (Beyer et al., 1976). Gas bubble disease damages the fish’s tissue.
If extensive, it can even lead to the fish’s death (Weitkamp et al., 2003).
4.2.4 MACROECOLOGICAL IMPACTS
In the previous sections, we listed a range of individual impacts that can occur as a result of the construction of hydropower stations with the associated infrastructure of reservoirs and dams. Many of these impacts are strongly influenced by other anthropogenic activities, such as erosion resulting from agriculture, forestry activity and infrastructure, water pollution, or the regulation of waterways for navigation and flood protection.
Dams can have a substantial influence on biodiversity and ecosystems. The extent of these consequences can be identified only when looking at the macro level, i.e., at entire river basins. Some impacts only
become apparent at this level because dam construction affects migratory species and because dams are often built in series rather than in isolation (Dudgeon, 2000, 2011; Van Looy et al., 2014; Carrara et al., 2014). The effects from the different dams interact with each other and other human development effects, and the total impact can only be understood taking into consideration the interaction between all of these factors (Xu, 2013). The impact is strongest where dam construction induces development in previously undeveloped areas and therefore necessitates road construction, deforestation, and the construction of settlements (Finer and Jenkins, 2012).
4.2.5 MITIGATION OF IMPACTS OF HYDROELECTRIC DAMS
The ecological and health impacts of hydroelectric dams can be reduced in a number of ways (Liu et al., 2013). Mitigation initiatives can be categorised by their goal, or effect: measures to ensure the continued migration of fish, controlled flooding to simulate conditions in natural river habitats, upstream water quality improvements and erosion control, mitigation measures related to sediment transport, and the compensation of habitat loss through the construction of new shallow-water habitat. Some measures, such as the
maintenance of a minimum “environmental flow” can fulfil several of these services at once. The success of mitigation measures must be monitored and verified.
4.2.5.1 Measures to allow fish migration
A number of measures have been developed to allow migratory fish to pass dams. Upstream passage is ensured through gateways such as fish ladders (Wollebæk et al., 2011b, 2011a), while downstream passage turbines for run-of-the-river plants have been developed to allow fish to pass through the turbine on the way downstream (Deng et al., 2010). A range of other, sometimes species-specific devices is under development (Hassinger 2011). The overall success of such mitigation strategies, however, has been questioned (Brown et al., 2013).
4.2.5.2 Environmental flow
As emphasized above, the water flow in rivers is an important parameter defining the habitat of species. Many hydropower dams alter the flow regime, reduce floods and shift the timing of water flow variations. It has been found that in many cases, adjusting the operation of hydropower dams can substantially reduce ecological impacts while having only a small impact on power production (Guo et al., 2011; Esselman and Opperman, 2010). Such “environmental flow” regimes include a minimum flow requirement and the simulation of seasonal floods to allow for sediment transport and trigger life cycle processes of specific species (McCartney et al., 2009; Kang et al., 2010; Poff and Matthews 2013). Issues of flow management also include avoiding undesirable pulses to meet peak demand, for example, in order to avoid the stranding of fish amongst other consequences (Young et al., 2011).
4.2.5.3 Habitat enhancement and offsets
It is possible to design reservoirs such that they offer more habitat for endemic aquatic species, or to
construct or enhance adequate habitat in nearby areas, such as tributaries, dead arms etc. The focus is often on shallow water and wetland habitats that may otherwise be lost due to reservoir construction. The objective is to offer a diversity of habitats to ensure a diversity of species (Wen et al., 2008).
Existing literature pays significant attention to the development of mitigation measures, but few encompass a systematic, comparative study of the effectiveness of such measures. The focus is often on individual species, but sometimes, appropriate attention is given to landscape level issues relating to interactions between ecosystems. The multitude of relevant effects and species concerned requires comprehensive knowledge for optimal design and operation of dams and power plants. Awareness and competence issues or a lack of data often leads to inadequate project design or inappropriate environmental flow management (Renöfält et al., 2010; Esselman and Opperman, 2010). Minimizing the ecological impacts of hydropower projects requires further knowledge about both design and operational issues and their influence on biodiversity and threatened freshwater species. Available guidelines are process-oriented and require adequate attention and competent
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execution. There is a need for more research, for the appropriate training of responsible personnel, and for follow-up control and evaluation of the measures taken to ensure a learning cycle.
4.2.6 SUMMARY OF ECOLOGICAL IMPACTS
In this section, we briefly reviewed ecological impacts of hydropower. We have not found a general, systematic basis for a summary evaluation and synthesis of what type of project causes which impacts, or the success of potential mitigation measures. The hydropower industry points to environmental benefits of flow regulation, but we have found few peer-reviewed studies documenting such benefits. Where summary evaluations of individual projects or regions have been undertaken, the net ecological impacts of hydropower tend to be negative (Fu et al., 2014; Reed et al., 2013; Bai et al., 2013).
Some people argue that that ecological impacts of hydropower can be reduced by pursuing small
hydropower projects rather than large ones (Abbasi and Abbasi, 2011). Current evidence, however, suggests that small dams may have disproportionately large impacts on ecosystems (Kibler and Tullos, 2013; Lehner et al., 2011; Kareiva, 2012). In a study of the multiple effects of hydropower stations on the Nu River in China, Kibler and Tullos (2013) find that smaller dams impact longer stretches of the river channel and have larger
hydropower projects rather than large ones (Abbasi and Abbasi, 2011). Current evidence, however, suggests that small dams may have disproportionately large impacts on ecosystems (Kibler and Tullos, 2013; Lehner et al., 2011; Kareiva, 2012). In a study of the multiple effects of hydropower stations on the Nu River in China, Kibler and Tullos (2013) find that smaller dams impact longer stretches of the river channel and have larger