consequent changes not only in the igure 2: Alteration of Freshwater Systems Worldwide temperature and chemistry of river water, but in the living functions of he riverine ecosystem. Water falls Alteration Pre-190019001950-6019851996-98 rapids, riparian vegetation, and , wetlands are some of the habitats Navigation 3,125km8,750km that disappear when dams impor riversDynesius and Nilsson 8,750km21,250km 63,125km 994:759). These habitats are Large Reservoirs* essential feeding and breeding area Number 2,836 for many aquatic and terrestrial Volume(km) ,68658796,385 pecies, and also help to remove 533 pollutants and maintain water quality Large Dams(>15m 5,749 41,413 How Threatened Are Fres water Installed Hydro Capacity <290,000542,000~660,000 Hydro Capacity Unde of freshwater biodiversity s the tion One measure of the actual condit Construction(MW) 126,000 extent to which species are 578 200~3,800 threatened with extinction. Globally Water Withdrawals km /year km/year km/year km/year scientists estimate that more than 20 nt of the worlds 10.00 Wetlands Drainage** ecorded freshwater fish species have become extinct. threatened *Large reservoirs are those with a total volume of 0. 1 km or more. This is only a endangered in recent decades Moyle subset of the world's reservoirs nd Leidy 1992: 140). This number, **Includes available information for the drainage of natural bogs and low-lying however, may well be an grasslands as well as disposal of excess water from irrigated fields. There is no underestimate Brautigam 1999: 4) comprehensive data for wetland loss for the world According to the 1996 IUCN Red List of t Sources: Based on Naiman et al. 1995, as adapted from Lvovich and white 1990 of fish are classified as threatened. of Data on dams are from ICOLD 1998. Reservoir data are from Avakyan and which 84 percent are freshwater Iakovleva 1998. Hydro capacity data are from IJHD 1998 and L'vovich and White pecies(IUCN 1996:intro p 37; 1990. Water withdrawal data are from shiklomakov 1997 McAllister et al. 1997: 38). For some countries and regions more detailed Of the many ways in which under construction around the world, information is available. In South humans alter freshwater ecosystems, mostly in developing countries(HD Africa, 63 percent of freshwater fish dams are probably the most 1998:12-14 are threatened or e widespread and significant in their Dams provide unquestionable Europe, 42 percent; and in Iran, 22 impact. Today, the world's rivers are benefits-from water supply to percent Moyle and Leidy 1992: 138) dotted with more than 45,000 large power generation-but they disrupt Unfortunately, global data on the dams-dams higher than 15 meters ne hydrological cycle profoundly status of the range of different or impounding more than three suppressing natural flood cycles, freshwater species-plant and million cubic meters of water (WCD disconnecting rivers from their 2000:8, 11). Most of these dams were wetlands and floodplains, disrupting difficult to quantify the orca ery limal-is sparse, making it v built in the last 50 years (ICOLD fish migrations, and altering the ion of the worlds freshwater 1998:13) ion of sediments downstream Dam building has slowed in many For example, the World Commission biodiversity. Where we do have ommission however trends look bleak. In the countries, particularly in the n Dams found that more than 60 United States, which has developed world. In the United percent of the large dams it surveyed comparatively detailed data on States, for example, since 1998, more report significant problems with freshwater species, 37 percent of dams have been decommissioned disrupted fish migrations (WCD freshwater fish species, 67 percent of than built(WCD 2000: 10). However, 2000: 82) dam construction is still robust in Rivers with multiple dams can mussels, 51 percent of crayfish, and other countries. As of 1998. there 40 percent of amphibians are become little more than chains of hreatened or have become extinct were 349 dams over 60 meters high connected reservoirs, with (Master et al. 1998: 6). Indeed, studies OEarthTrends 2001 World Resources Institute. All rights reserved. Fair use is permitted on a limited scale and for educational purposes©EarthTrends 2001 World Resources Institute. All rights reserved. Fair use is permitted on a limited scale and for educational purposes. 2 Of the many ways in which humans alter freshwater ecosystems, dams are probably the most widespread and significant in their impact. Today, the world’s rivers are dotted with more than 45,000 large dams—dams higher than 15 meters or impounding more than three million cubic meters of water (WCD 2000:8,11). Most of these dams were built in the last 50 years (ICOLD 1998:13). Dam building has slowed in many countries, particularly in the developed world. In the United States, for example, since 1998, more dams have been decommissioned than built (WCD 2000:10). However, dam construction is still robust in other countries. As of 1998, there were 349 dams over 60 meters high under construction around the world, mostly in developing countries (IJHD 1998:12-14). Dams provide unquestionable benefits—from water supply to power generation—but they disrupt the hydrological cycle profoundly, suppressing natural flood cycles, disconnecting rivers from their wetlands and floodplains, disrupting fish migrations, and altering the deposition of sediments downstream. For example, the World Commission on Dams found that more than 60 percent of the large dams it surveyed report significant problems with disrupted fish migrations (WCD 2000:82). Rivers with multiple dams can become little more than chains of connected reservoirs, with consequent changes not only in the temperature and chemistry of river water, but in the living functions of the riverine ecosystem. Waterfalls, rapids, riparian vegetation, and wetlands are some of the habitats that disappear when dams impound rivers (Dynesius and Nilsson 1994:759). These habitats are essential feeding and breeding areas for many aquatic and terrestrial species, and also help to remove pollutants and maintain water quality. How Threatened Are Freshwater Species? One measure of the actual condition of freshwater biodiversity is the extent to which species are threatened with extinction. Globally, scientists estimate that more than 20 percent of the world's 10,000 recorded freshwater fish species have become extinct, threatened, or endangered in recent decades (Moyle and Leidy 1992:140). This number, however, may well be an underestimate (Bräutigam 1999:4). According to the 1996 IUCN Red List of Threatened Animals, 734 species of fish are classified as threatened, of which 84 percent are freshwater species (IUCN 1996:intro p. 37; McAllister et al. 1997:38). For some countries and regions more detailed information is available. In South Africa, 63 percent of freshwater fish are threatened or endangered; in Europe, 42 percent; and in Iran, 22 percent (Moyle and Leidy 1992:138). Unfortunately, global data on the status of the range of different freshwater species—plant and animal—is sparse, making it very difficult to quantify the overall condition of the world’s freshwater biodiversity. Where we do have data, however, trends look bleak. In the United States, which has comparatively detailed data on freshwater species, 37 percent of freshwater fish species, 67 percent of mussels, 51 percent of crayfish, and 40 percent of amphibians are threatened or have become extinct (Master et al.1998:6). Indeed, studies Endangering Freshwater Biodiversity Figure 2: Alteration of Freshwater Systems Worldwide Pre-1900 1900 1950-60 1985 1996-98 3,125 km 8,750 km - >500,000 km - Canals 8,750 km 21,250 km - 63,125 km - Large Reservoirs* Number 41 581 1,105 2,768 2,836 Volume (km3 ) 14 533 1,686 5,879 6,385 - - 5,749 - 41,413 - - <290,000 542,000 ~660,000 - - - - ~126,000 Water Withdrawals - 578 km3 /year 1,984 km3 /year ~3,200 km3 /year ~3,800 km3 /year Wetlands Drainage** - - - - - *Large reservoirs are those with a total volume of 0.1 km3 or more. This is only a subset of the world's reservoirs. **Includes available information for the drainage of natural bogs and low-lying grasslands as well as disposal of excess water from irrigated fields. There is no comprehensive data for wetland loss for the world. Sources: Based on Naiman et al. 1995, as adapted from L'vovich and White 1990. Data on dams are from ICOLD 1998. Reservoir data are from Avakyan and Iakovleva 1998. Hydro capacity data are from IJHD 1998 and L'vovich and White 1990. Water withdrawal data are from Shiklomakov 1997. Alteration Hydro Capacity Under Construction (MW) Waterways Altered for Navigation Installed Hydro Capacity (MW) Large Dams (>15m high)