RIVER VALUES AND THREATS

THE FRESHWATER BIODIVERSITY CRISIS

Rivers and their associated floodplains, groundwater, and wetlands are in crisis. Globally they are the world's most damaged ecosystems, losing species at a rate that far outstrips the decline of biodiversity in terrestrial and marine systems (Dudgeon et al. 2006). A new synthesis of threats to the world's rivers (Vörösmarty et al. 2010) has found that over 83% of the land surface surrounding aquatic systems has been significantly influenced by the "human footprint." The stamp of human activities is manifest as widespread catchment disturbance, deforestation, water pollution, river corridor engineering, impoundments and water diversions, irrigation, extensive wetland drainage, groundwater depletion, habitat loss, and introduced species. Impoundments and depletion of river flows are the clearest sources of biodiversity threat in that they directly degrade and reduce river and floodplain habitat, with 65% of global river discharge and aquatic habitat under moderate to high threat. This threat level exceeds past estimates of human appropriation of accessible freshwater runoff and is approaching the 70% level anticipated by 2050 (Postel et al. 1996).

The worldwide pattern of anthropogenic threats to rivers documented by Vörösmarty et al. (2010) offers the most comprehensive accounting ever undertaken to explain why freshwater biodiversity is in a state of crisis. Estimates suggest that at least 10,000–20,000 freshwater species are extinct or at risk, largely from anthropogenic factors. Loss rates for freshwater biodiversity are believed to rival those of the Pleistocene-to-Holocene transition.

In 2002, Paul Crutzen suggested that the world has entered a new epoch—the Anthropocene—because humans dominate the biosphere and largely determine environmental quality (Zalasiewicz et al. 2008). Numerous correlative studies, experiments, and meta-analysis all point to human activities as the common factor in freshwater biodiversity decline. Given escalating trends in species extinction, human population growth, water use, development pressures, and the additional stresses associated with climate change, the new global synthesis predicts that freshwater systems will remain under threat well into the future.

RIVER VALUES

River degradation and loss of freshwater biodiversity have major implications for human water security, prosperity, health, and well-being because they threaten the provision of ecosystem services—the tangible benefits people gain from ecosystems. The Millennium Ecosystem Assessment, a global synthesis and analysis of the state of the world's major ecosystems, grouped ecosystem services into four main categories: provisioning, regulating, cultural, and supporting (MEA 2005).

Provisioning services (Table 1) are the products obtained from ecosystems. Rivers and lakes hold about 100,000 km3 of freshwater globally, amounting to less than 0.01% of all water on Earth (Schwarz et al. 1990). Yet this tiny fraction of global water is absolutely vital for human life support and the provision of most other ecosystem services, including those dependent on diverse biological systems. These biologically based services include food (shrimp, fish, plants), fuel products (peat), fibers and building materials (timbers and thatch), and pharmacological products. Freshwater ecosystems underpin global food production based on artisanal and commercial fisheries, aquaculture, flood-recession agriculture, and pastoral animal husbandry (MEA 2005). Clean freshwater of low salinity is essential to grow most food and fiber crops and to drive the industries that produce food products, cooking utensils, clothing, housing, infrastructure, transport, recreation, and entertainment. Some estimates suggest that global food production must double by 2050 to meet human needs, and this must be achieved using less water; already, 70% of the world's freshwater is used in agriculture (Molden 2007).

Regulating services (Table 2) are equally vital, being the benefits obtained from the regulation of ecosystem processes such as the climate regime, hydrologic cycle, nutrient processing, and natural hazards. Rivers, lakes, wetlands, and aquifers store freshwater or slow the passage of water. Floodplains and wetlands absorb large pulses of catchment runoff and help mitigate the damaging effects of floods on landscapes and the built environment (Freitag et al. 2009). Water returning off floodplains is rich in nutrients and energy sources that fuel the food webs supporting riverine biota and dependent terrestrial species such as waterbirds and amphibians (Douglas et al. 2005). Rivers also convey freshwater to estuaries, coastal wetlands, and the nearshore environment, where flow pulses contribute to maintaining habitats of tolerable salinity for plants and animals directly used by humans. Estuarine inflows carry nutrients that stimulate primary and secondary production and support the recruitment of fish and crustaceans (Gillanders and Kingsford 2002). Mangroves, salt marshes, and seagrasses that are partly dependent on freshwater help to stabilize sediments, alter water-flow patterns, produce large quantities of organic carbon, and influence nutrient cycling and food web structure (Hemminga and Duarte 2000).

Cultural services (Table 3) are the benefits people obtain through recreation, education, and aesthetics; celebrations revolving around water and its goods and services to humans; and spiritual enrichment (MEA 2005). For many societies, rivers and lakes have profound cultural and religious significance; they are the sites for important ceremonies, the burial places of beloved family members, and the dwelling places of gods and guardian spirits. The Australian Aboriginals who lived as nomads in a very dry continent with extremely patchy freshwater resources appreciated...