Environmental Effects of Hydrological Alterations
Global–Scale Environmental Effects of Hydrological Alterations: Introduction
by David M. Rosenberg, Patrick McCully, and Catherine M. Pringle*
The magnitude and extent of dam construction and associated water diversion, exploitation of groundwater aquifers, stream channelization, and interbasin water transfer in the world today are so large that these hydrological alterations are having global–scale environmental effects. The articles in this issue highlight the cumulative effects of hydrological alterations associated with dam and reservoir development. Such information is critical for deciding whether, when, and where the next major hydrological project will be built; it can also warn us about impending environmental impacts.
Attempts to study the cumulative effects of hydrological alterations are relatively recent compared to the study of individual dam and reservoir developments (e.g. Hall 1971, Hecky et al. 1984). For example, the issue of greenhouse gas emissions from reservoirs is less than a decade old (Rudd et al. 1993). The global significance of reservoirs as sources of greenhouse gases depends on the total surface area of reservoirs and the flux rates from the major types of reservoirs in different geographical locations (Rosenberg et al. 1997). Neither of these quantities is very well known, but flux rates have now been measured in 21 locations, enabling the first reasonable estimate of global greenhouse gas emissions from reservoirs (St. Louis et al. 2000).
There are other recent examples of attempts to determine cumulative environmental effects at global or hemispheric scales. Chao (1991, 1995) reported that worldwide impoundment of water has reduced sea levels. Also, the concentration of water in reservoirs at high latitudes has actually increased, albeit minutely, the speed of the earth’s rotation and changed the planet’s axis. Vörösmarty et al. (1997) demonstrated a dramatic ageing in river runoff caused by the global population of large dams. Dynesius and Nilsson (1994) determined that 77% of the total discharge of the 139 largest river systems in the northern third of the world is affected by river channel fragmentation caused by dams, reservoirs, interbasin diversions, and irrigation. Moreover, this fragmentation could profoundly affect biological populations over a substantial area of the world.
Even smaller–scale studies examining the collective effects of more than one hydroelectric development on a regional basis are relatively recent (summarized in Rosenberg et al. 1997). The oldest of these assessed the effects of the W.A.C. Bennett Dam and Williston Reservoir (Peace River, British Columbia, Canada) on the downstream Peace–Athabasca Delta in Alberta (Townsend 1975), and the effects of multiple dams on the River Don on the downstream Azov Sea, Russian Federation (Tolmazin 1979). Most of the remaining examples date from the late 1980s and the 1990s.
The emerging area of the study of global–scale, cumulative, environmental impacts was the focus of a special symposium held in 1998, entitled "Global–scale effects of hydrological alterations: What we know and what we need to know". The symposium was held at a national conference (The land–water interface: Science for a sustainable biosphere) co–sponsored by the Ecological Society of America and the American Society of Limnology and Oceanography. Participants in the symposium were asked to synthesize information in their area of interest, working at the largest possible spatial and temporal scales, and to identify knowledge gaps that inhibit work at global scales.
The articles in this issue are based on presentations at the 1998 workshop. The series has the following objectives: to synthesize as much information as possible on large–scale environmental effects of dams and reservoirs, to identify knowledge gaps and research needs to improve our understanding of global–scale effects, and to produce currently available information in a form that is readily accessible to policy makers.
What do we mean?
Hydrological alteration can be defined as any anthropogenic disruption in the magnitude or timing of natural river flows. The articles in this issue focus on a major cause of these disruptions on a global scale – dams (and associated impoundments). These structures are built to store water to compensate for fluctuations in river flow, thereby providing a measure of human control of water resources, or to raise the level of water upstream to either increase hydraulic head or enable diversion of water into a canal. The creation of storage and head allows dams to generate electricity; to supply water for agriculture, industries, and municipalities; to mitigate flooding; and to assist river navigation. However, the effectiveness of dam technology in delivering these services is currently being hotly debated.
The International Commission on Large Dams (ICOLD), an industry association, defines large dams as being "15 m high from foundation to crest (Fig. 1). Major dams are defined as those meeting at least one of the following criteria: height >150 m, volume >15,000,000 m³, reservoir storage >25 km³, electrical generation capacity >1000 MW. ICOLD has published incomplete statistics on the world’s large dams; a similar dataset is not available for small dams (St. Louis et al. 2000, Vörösmarty and Sahagian 2000), arbitrarily defined here as being <15 m high from foundation to crest (but see footnote 3 below).
Extent of development
Hydrological alterations are one of many environmental problems affecting the world today, and it is often difficult to disentangle the biological effects of hydrological alterations from other environmental perturbations in heavily developed catchments (Rosenberg et al. 1997). However, along with persistent synthetic chemicals and global warming, dams produce global effects that will continue well into the future (see also Rosenberg et al. 1997).
ICOLD has estimated that there were ~42,000 large dams in the world as of 1996 (ICOLD 1998). The associated reservoirs are estimated to have a combined storage capacity of ~10,000 km³, which is equivalent to five times the volume of water in all the world’s rivers (Chao 1995). The present storage capacity of large dams amounts to 5500 km³, of which 3500 km³ are actively used in regulating river runoff (Postel et al. 1996). Shiklomanov (1996, as cited in Postel 1998) estimated that in 1995, 2500 km³ of water was withdrawn from rivers, lakes, and aquifers for irrigation. Humans have appropriated ~50% of the accessible global freshwater runoff, and conservative estimates indicate that this appropriation could reach 70% by the year 2025 (Postel et al. 1996).
Various estimates exist for the total surface area of reservoirs in the world: 400,000 km² (0.3% of global land surface, Shiklomanov 1993), 500,000 km² (Kelly et al. 1994), 600,000 km² (Pearce 1996), and 1,500,000 km² (St. Louis et al. 2000). The global surface area of lakes is estimated to be 1,500,000 km² (Shiklomanov 1993). China has the most large dams (24,671), followed by the United States (6375), and then by India (4010) (ICOLD 1998). The United States has the most major dams (50), followed by the Russian Federation (34), and Canada (26) (McCully 1996).
According to ICOLD (1998) figures, <1000 large dams were built each decade between 1900 and 1949. The rate then soared, reaching a peak of 5415 large dams completed during the 1970s. The rate has recently fallen sharply, to a projected figure of 1963 in the 1990s (Fig. 2, ICOLD 1998). Future trends in the building rates of dams will depend on many factors, including the changing economic viability of hydropower and dam and canal irrigation schemes compared with other power generation and irrigation options, the strength of public opposition, the availability of public funds and political support for dam building, and the availability of suitable sites.
The aggregate extent of small dams should not be underestimated. McCully (1996) used the ratio of large:small dams in the United States to estimate that there are 800,000 small dams in existence globally. The ratio used was 5500 large:96,000 small dams. McCully’s number of large dams came from the ICOLD 1988 World Register of Dams. The estimate of small dams came from USCOLD (1995). Whereas the ICOLD 1998 World Register of Dams lists the 6375 large dams in the United States as having a total surface area of ~60,500 km², the US Army Corps of Engineers National Inventory of Dams, which lists small dams as well as large dams, records a total of ~75,200 dams having ~260,000 km² of reservoir surface area. The Army Corps of Engineers inventory thus suggests approximately four times more reservoir area behind small than large dams (St. Louis et al. 2000).
The extent of dam construction on a single catchment can be massive. For example, there are 19 large dams on the mainstem of the 2000 km long Columbia River in the United States and Canada; only 70 km of river remain free flowing (McCully 1996). The Columbia catchment as a whole contains 194 large dams (Revenga et al. 1998). Almost 200 reservoirs occupy the Danube River catchment (Horvath et al. 1997; see also Pringle et al. 1993); 11 large hydropower stations and 200 small and large reservoirs (inundating 26,000 km² of land) have been built on the Volga–Kama River catchment; and >130 reservoirs have been built on the River Don catchment (holding 37 km³ of water and covering 5500 km²; summarized in Rosenberg et al. 1997).
Environmental effects
"Large dams and river diversions have proven to be primary destroyers of aquatic habitat, contributing substantially to the destruction of fisheries, the extinction of species, and the overall loss of the ecosystem services on which the human economy depends. Their social and economic costs have also risen markedly over the past two decades" (Postel 1998, p. 636).
This statement is not surprising in view of the extent of hydrological development in the world today. The environmental implications of the human appropriation of huge amounts of water on a global scale are profound: decreasing amounts of fresh water are available to maintain ecological values and related ecosystem services (e.g. Pringle, in press). Future water needs (e.g. Postel 1998) will compound the problem.
The conspicuous impacts of large–scale hydrological alteration include: habitat fragmentation within dammed rivers (e.g. Dynesius and Nilsson 1994); downstream habitat effects caused by altered flows, such as loss of floodplains, riparian zones, and adjacent wetlands (Fig. 3), and deterioration and loss of river deltas and ocean estuaries (e.g. Rosenberg et al. 1997); deterioration of irrigated terrestrial environments and associated surface waters (e.g. McCully 1996); and dewatering of rivers, leading to reduced water quality because of dilution problems for point and non–point sources of pollution (NRC 1992, Gillilan and Brown 1997). A number of major rivers are so overexploited that no water reaches the sea for much of the year (Gillilan and Brown 1997, Brown et al. 1998, Postel 1998). Both the Nile and the Colorado rivers seldom discharge fresh water into the sea (Postel 1998). Water diverted from Central Asian rivers for irrigation has caused the Aral Sea to lose 80% of its volume since 1960 (Stone 1999).
Less conspicuous, but still significant, impacts of hydrological alterations can affect genes to ecosystems: genetic isolation as a result of habitat fragmentation (e.g. Pringle 1997); changes in ecosystem–level processes such as nutrient cycling and primary productivity (e.g. Pringle 1997, Rosenberg et al. 1997); impacts on biodiversity (e.g. Rosenberg et al. 1977, Master et al. 1998, Richter et al. 1998, Wilcove et al. 1998); methylmercury contamination of food webs (e.g. Verdon et al. 1991, Kelly et al. 1997, Rosenberg et al. 1997); and greenhouse gas emissions from reservoirs (e.g. Duchemin et al. 1995, Kelly et al. 1997, Rosenberg et al. 1997).
Measuring cumulative global–scale environmental effects
Large–scale hydrological alteration leads to a suite of interrelated environmental impacts. The articles in this series yield insight into individual parts of this continuum of effects, but also yield an even better overall view when read together as a package.
The environmental chain–of–effects is set in motion by impeding natural flows of water and sediments, and by altering natural seasonal patterns of river discharge (Vörösmarty and Sahagian 2000). The river channel and riparian zone are immediately affected because riparian areas are particularly sensitive to variations in the hydrological cycle (Nilsson and Berggren 2000). Nutrient delivery, especially of silicates, to offshore marine areas is also disrupted by upstream damming activities, with implications for the biogeochemistry and algal ecology of these downstream areas (Ittekkot et al. 2000). Dam construction is especially inimical to the biodiversity of aquatic fauna because of the alteration of natural seasonal flow patterns to which the fauna has become adapted over time, the blockage of normal seasonal migrations, and the resultant fragmentation of populations (Dudgeon 2000, Pringle et al. 2000). Last, the normal source/sink characteristics of natural terrestrial and aquatic habitats for greenhouse gases (CO2 and CH4) are disrupted by inundation, leading to the production and emission of large quantities of these gases and contributing to global warming potential (St. Louis et al. 2000).
How successfully do the articles in the series measure global–scale environmental effects of hydrological alterations? Current states of knowledge are best for physical disruptions of river discharge and sea–level changes (Vörösmarty and Sahagian 2000), the production of greenhouse gases (St. Louis et al. 2000), and biogeochemical alterations in offshore areas (Ittekkot et al. 2000). Cumulative, global estimates of effects are produced by Vörösmarty and Sahagian (2000) and St. Louis et al. (2000). Determination of the global effects on downstream areas of silicate retention behind dams will require expanded measurements (Ittekkot et al. 2000).
Current states of knowledge are less well developed for the effects of hydrological alterations on global–scale biodiversity. Effects of dam construction on riparian zones are described for a number of catchments all over the world, but more comparative work is needed between catchments and over long time periods before the global implications of changes to riparian ecosystems are known (Nilsson and Berggren 2000). An expansion of spatial and temporal scales is also required to develop global estimates of the effects of hydrological alterations on faunal biodiversity; the Dudgeon (2000) and Pringle et al. (2000) articles are regional treatments. However, more complete knowledge of the effects of hydrological alterations on regional fauna is required before global–scale effects can be identified.
Last, what is needed to either improve the measurement of known global–scale environmental effects of hydrological alterations, or to improve the potential for making these measurements? The authors of articles in the series identify a number of needs. Some of them are unique to the subject area examined (e.g. St. Louis et al. 2000: more CO2 and CH4 flux measurements from reservoirs all over the world; Ittekkot et al. 2000: better understanding of the silica cycle and its interaction with other life–supporting elements). However, some needs are common to more than one article: better documentation of the location, physical features, and ways of operating reservoirs the world over; more attention to the spatial and temporal scales at which studies are done; and much more basic knowledge – from global water supply and use (Vörösmarty and Sahagian 2000), to detailed description of riparian processes before reservoir construction (Nilsson and Berggren 2000), to the need for better knowledge on the (seasonal) distribution, abundance, and productivity of riverine species (Dudgeon 2000; Pringle et al. 2000). The whole story has not yet been told, but the authors in the series have served notice that human interference in the natural hydrological cycle is producing environmental effects detectable at very large ––– and sometimes ––– global scales.
Acknowledgments
We thank C.A. Kelly for fruitful discussions on the genesis of the idea of reservoirs as sources of greenhouse gases, and for her thoughtful review of an earlier draft of the paper. R. Chasan, C.J. Vörösmarty, and 3 anonymous reviewers also provided critical comments on the manuscript. V. L. St. Louis kindly provided unpublished data on the surface areas of large and small dams.
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*David M. Rosenberg (e–mail: rosenbergd@dfo–mpo.gc.ca) is a research scientist at the Freshwater Institute, 501 University Crescent, Winnipeg, MB R3T 2N6. Patrick McCully (e–mail: patrick@internationalrivers.org') is campaigns coordinator for the International Rivers, 1847 Berkeley Way, Berkeley, CA 94703. Catherine M. Pringle (e–mail: pringle@sparc.ecology.uga.edu) is an associate professor in the Institute of Ecology, University of Georgia, Athens, GA 30602–2602.
