Running waters are dynamic systems, ecologically and geomorphologically. Variation in the earth's geology and climate profoundly affect the biological functioning of rivers through their influence on valley vegetation and also on slope, hydrologic flowpaths, and channel characteristics. A principal goal of this book is to explain, as best we can, the enormous diversity among streams and rivers that is associated with stream size, landscape, and longitudinal position. Unfortunately, few rivers today are truly pristine, and many have been highly altered by various human activities. Habitat alteration, invasive species, contaminants, overexploitation, and climate change are major threats facing rivers today.

Freshwater ecosystems occupy <1% of the earth's surface but support at least 100,000 known animal species, including over 10,000 freshwater fishes and 90,000 invertebrates. There is no reliable estimate of the fraction that is imperiled, but data for major North American plant and animal groups provide evidence that a high proportion of the freshwater biota is at risk. Even when species persist, recent surveys often reveal reduced abundances and restricted distributions compared with historical occurrences, pointing to profound functional alteration of the remaining assemblage.

Alteration of physical habitat is likely the single most significant threat, and is due to many different human activities that affect flows, channel morphology, and land use. Due to construction of dams around the world, most river systems are impacted and few free-flowing sections of any size remain. Most of the largest river systems of North America, Europe, and former

USSR are highly or moderately affected by dams, and dam construction on the rivers of Asia is of growing concern. Effects on rivers include changes to flow, sediment load, temperature, and water quality, as well as loss of upstream-downstream and lateral connectivity. Small streams are widened and straightened for water conveyance, whereas larger rivers commonly are modified for navigation, flood control, and utilization of floodplain land; both result in significant degradation and loss of habitat. Changing land use, including forest harvest and the spread of agriculture and urban areas, has pervasive but diffuse effects on river habitat by altering flows, increasing sediment inputs, and influencing energy supplies, temperature, and bank stability through changes to the riparian vegetation.

Invasive species pose a significant risk by dominating biomass and space, resulting in declines in native species and changes in trophic structure and ecosystem processes. Although a great many species have been introduced by accident or design, only a modest number become established as assemblage dominants, and so qualify as invasive or nuisance species. At the other extreme, nine species have been introduced into more than 30 countries, including three sports fish, a species used in mosquito control, and two tila-pias and three species of carp popular for aquaculture and weed control. The successful establishment of an alien species can be viewed as a succession of probabilistic events, beginning with transport or dispersal and followed by establishment and further spread, until finally becoming fully integrated into the community. Invasion success is best predicted by a past history of successful invasions, broad environmental tolerance, ability to thrive in human-altered environments, similarity of source and recipient environments, and large propagule size. Although many nonindigenous species appear to have little impact on the recipient community, trout competitively displace native species and the zebra mussel has substantially modified ecosystem processes where it has become abundant in North America.

Declines in water quality result from contaminants that reach surface waters at specific locations, which is often true of industrial and municipal wastes, or from diffuse sources, as runoff from agricultural and urban land or atmospheric deposition. Harmful water quality conditions are widespread in developed countries, where nutrients and sediments from agriculture affect many streams and rivers and toxic contaminants vary in importance depending on present-day and legacy sources. In many developing countries, with less effective use of pollution control technologies, problems can be much worse. Acidification of surface waters from atmospheric deposition of SO2 and NOx has affected rivers and lakes in Scandinavia, Northern Europe, the northeastern and some areas of the western United States.

Overexploitation is of primary concern for fishes, other vertebrates including turtles and crocodilians, and for some mollusks and decapod crustaceans. Its importance relative to other causes of imperilment can be difficult to assess because exploited stocks commonly face multiple threats including dams, habitat degradation, and pollution. Overexploitation of freshwater turtles is the single most important threat to the turtles of Asia, and many tropical fisheries are harvested at such high levels that their sus-tainability is in question. The overfishing of inland waters is a concern not only because it threatens the loss of apex predators and influential species, but also the well-being of tens of millions of fishers who rely on this resource for income and food.

Climate change undoubtedly will have important consequences for aquatic ecosystems, having proximate effects due to changes in temperatures and flow regimes and more subtle effects due to changes in riparian vegetation, disturbance intensity and frequency, water chemistry, and species interactions. Whether species will be able to shift to locations within their physiological tolerances depends on chance and terrain, as dispersal routes and suitable habitat may not always be available. Because precipitation and runoff are expected to become more extreme under future climates, the role of disturbance in structuring biological assemblages may increase in importance.

Despite the seriousness of these threats, rivers have great restorative powers, advances in science are beginning to point the way toward holistic, ecosystem-based management, and public awareness and concern are growing rapidly. Advances in bioassessment provide the tools to monitor freshwater biota and ecosystems in order to quantify human impacts and evaluate the effectiveness of management actions. The practice of river restoration is growing rapidly, and although inadequately assessed and documented at present, this field holds great promise for the future. It is also important to identify areas that are relatively unaltered or harbor a distinctive biota and thus are most worthy of protection, and devise appropriate strategies tailored to the distinctive challenges of freshwater conservation. Most importantly, water development and conservation planning need to be integrated so that sustaining ecosystem function and providing for human livelihoods and well-being are pursued as linked goals.

Chapter fourteen

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