The regulation of rivers has brought many benefits, but the adverse impacts have often failed to receive adequate and transparent consideration
Yellow (He, Cheng, and Luo 2005), Aral Sea tributaries, Chao Phraya, Ganges, Inco- mati, Indus, Murray-Darling, Nile, and Rio Grande (Falkenmark and Lannerstad 2005). Smakhtin, Revenga, and Döll (2004) suggest that the streamflow required for aquatic ecosystem health (environmental flow) has already been overappropriated in many rivers.
In the United States the construction of dams and water diversions for irrigation and other purposes in the Colorado Basin, together with large-scale interbasin transfers, has greatly reduced the flow of the river to the delta. A considerable portion of the delta has been transformed into mudflats, saltflats, and exposed sand. With the loss of the delta habitats, wetlands now exist mainly in areas where agricultural drainage has occurred (Pos- tel 1996). e Ganges is among the major rivers of South Asia that no longer discharge year round to the sea. As a result there is a rapid upstream advance of a saline front, with consequent changes in mangrove communities, fish habitat, cropping, and human liveli- hoods (Postel 1996; Mirza 1998; Rahman and others 2000). On the Zambezi River in Southern Africa damming for electricity and agriculture has reduced flows to the coast and led to a decline in shrimp production that could have been worth as much as $10 million a year (Gammelsrod 1992).
e regulation of rivers has brought many benefits to people, but the adverse impacts,
especially those related to reduced downstream flows, have often failed to receive adequate and transparent consideration (WCD 2000; Revenga and others 2000; MEA 2005b).
Drainage of wetlands. Water regulation and drainage for agricultural development are the main causes of wetland habitat loss and degradation (Revenga and others 2000; Finlayson and D’Cruz 2005) and consequent loss of ecosystem services. By 1985 drainage and conversion of wetlands, mainly for agriculture, had affected an estimated 56%–65% of inland and coastal marshes in Europe and North America and 27% in Asia (OECD 1996). Drainage of wetlands often reduces important regulating ecosystem services, with such outcomes as increased vulnerability to storms and flooding and further eutrophica- tion of lakes and coastal waters.
Harder to demonstrate is the cumulative effect of the loss of smaller sites, both indi- vidual sites and networks of sites, such as those used by migratory waterbirds (Davidson and Stroud forthcoming). e adverse effects are often assumed, but the evidence is in- complete. Still, there are many lessons, such as those from the drainage and subsequent burning of forested peat swamps in Southeast Asia (box 6.6), a case that has had dramatic health effects on many people across the region (see box 6.5). e loss of small wetlands (referred to as potholes) on the prairies of Canada and the United States through drain- age and infilling has led to the loss of habitat for large numbers of migratory waterbirds (North American Waterfowl Management Plan 2004). e loss of forested riparian wet- lands adjacent to the Mississippi River in the United States was seen as an important factor contributing to the severity and damage of the 1993 flood in the Mississippi Basin (Daily and others 1997).
Wetlands are often thought to act as “sponges” that soak up water during wet periods and release it during dry periods. While there are numerous examples of wetlands, notably floodplains, where this does occur, there is increasing evidence that such generalizations are not applicable for all hydrological contexts or wetland types (Bullock and Acreman