Human impacts

Although the human footprint is appreciable in all but the most inaccessible regions of the planet, freshwater ecosystems are especially subject to multiple pressures: water abstraction, industrial and domestic effluents, the spread of invasive species, altered hydrology, habitat degradation, and overharvest of resources. Over half of the accessible freshwater runoff is already appropriated for human use, more than one billion people lack access to safe drinking water, and approximately half of the world's six billion people lack basic sanitation services (Jackson et al. 2001, Postel et al. 1996). The demand for freshwater resources creates an urgent need to ensure sufficient freshwater for human well-being, while minimizing declines in biological diversity, the deterioration of freshwater ecosystems, and loss of ecosystem services.

The biological diversity of freshwater ecosystems is experiencing much greater declines than is seen in the majority of terrestrial ecosystems (Sala et al. 2000, Dudgeon et al. 2006), and if human pressures continue to rise and biodiversity continues its downward trend, the prospects for freshwater ecosystems are alarming and perhaps catastrophic. The freshwater biota is experiencing a biodiversity crisis (Revenga and Kura 2003), brought about by multiple interacting threats. Habitat degradation is pervasive, the result of instream alterations including dams, dredging, and channelization, harmful activities along the water's edge that destabilize banks, and changes in land use that affect hydrology with secondary consequences for physical processes and the biota. Pollution from diffuse and end-of-pipe sources is widespread, and in many areas of the world the water is not safe for humans or for aquatic life. Although significant advances have been made in treating wastewater and reducing point source pollution in more developed nations, sewage spills continue to occur because of aging infrastructure and the rapid pace of development. Invasive species, spread by accident and design, often have devastating effects on native species via predation, competition, habitat alteration, and as conveyers of diseases. Overexploitation does not affect all species or areas, but turtles, mollusks, some crayfishes, and many fishes have been or are being forced into decline by capture rates that are unsustainable. Finally, climate change has direct impacts by changing temperature and runoff patterns, and has indirect effects on many aspects of lotic ecosystem function. Individually and through their interactions, these five categories of threats demand urgent action to reverse the course of biodiversity and ecosystem decline.

Rivers provide numerous benefits to humankind, some of them irreplaceable. These ecosystem services include water supply for domestic, industrial, and agricultural uses, harvestable organisms, hydropower, waste disposal, navigation, recreational enjoyment, and spiritual fulfillment. Impaired lotic ecosystems may fail to provide these services, and instead of contributing to human well-being become the source of water-borne diseases including diarrhea, river blindness, schistosomiasis, and malaria. The great utility of rivers results in conflicts between types of uses, especially between those uses for which an economic value can easily be assigned, and other uses that have historically been excluded from any explicit valuation.

This chapter reviews the imperiled state of river ecosystems and their biota, examines the threats, and explores the science-based knowledge that can help us manage, restore, and conserve streams and rivers. It is all too easy to catalog the harm and perhaps take away too bleak a message. Scientific knowledge, citizen awareness, and new policies are growing rapidly. There is hope that ecosystem-based management within a framework that recognizes both human and environmental needs for freshwater will underpin a new era of improving ecological conditions for rivers and their biota.

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