Threats to Rivers

Modern-day threats to rivers have been reviewed by a number of authors (Allan and Flecker 1993, Pringle 2000a, Malmqvist and Rundle 2002, Dudgeon et al. 2006, Strayer 2006). Causes of the imperilment of river ecosystems and their biota are diverse, and are discussed below under the headings of habitat alteration, invasive species, pollution, overexploitation, and climate change (Table 13 2). Ultimate causes can be found in the conflicting demands on fresh water, changing land use, and the many unsustainable practices that characterize growing populations and expanding economies throughout the world. Most human activities that harm river ecosystems show an upward trend throughout the 20th century, and although some aspects of pollution have ameliorated in recent years, other pressures, including species invasions and climate change, are expected to worsen.

13.2.1 Habitat alteration

Alteration of physical habitat is the most significant threat to biodiversity and ecosystem function in the majority of human-impacted river systems. The consequence of many different

TABLE 13.2 The primary threats to streams and rivers. (Modified from Malmqvist and Rundle 2002.)

Proximate causes

Abiotic effects

Biotic effects

Habitat alteration

Invasive species



Damming, water abstractions and diversions


Land-use change including deforestation, intensive agriculture, urban development

Aquaculture, sports fishing, pet trade, ornamental plants

Nutrient enrichment from agriculture, municipal wastes, atmospheric deposition Acidification from fossil fuels (SO2, NOx), mines Toxic metals from mining, industrial gaseous emissions, waste disposal Organic toxins

Loss of natural flow variability, altered habitat, severing of upstream-downstream linkages Reduced habitat and substrate complexity, lower base flows

Altered energy inputs, increased delivery of sediments and contaminants, flashy flows

Some invasive species modify habitat, otherwise minor

Increased N and P, altered nutrient ratios

Reduced pH, increased Al+, metals

Increased trace metal concentrations (e.g., Hg, Cu, Zn, Pb, Cd) Increased levels of PCB, endocrine disruptors, some pesticides

Commercial harvest for food, Usually none pet trade recreational fisheries

Climate change Temperature changes

Precipitation changes

Milder winters, altered evapotranspiration patterns and flow regimes Altered flow regimes, greater flashiness

Reduced dispersal and migration, changes to water quality and assemblage composition

Reduction in biological diversity favoring highly tolerant species Changes in assemblage composition, altered trophic dynamics, can facilitate invasions

Declines in native biota, biotic homogenization, can result in strong ecosystem-level effects

Increased productivity, algal blooms, altered assemblage composition

Physiological and food chain effects

Toxic effects through biomagnification

Physiological and toxic effects

Changes in assemblage composition, altered trophic dynamics, can facilitate invasions

Range shifts in accord with physiological tolerances, increased productivity Greater role for disturbance human activities, for the purpose of discussion human impacts on habitat can be grouped under altered flows, altered channels, and altered land use. Flows are affected by impoundments, water abstraction, and changing land use. The effects range from imposed constancy, to enhancement of floods and droughts, and even to dewatering.

River channels are straightened, widened, realigned, and stabilized for flow conveyance, habitat-forming wood is removed, and levees are imposed to reduce flooding, all of which result in simplification and homogenization of habitat. Changing land use includes all aspects of forest harvest, agricultural intensification, and the spread of urban areas, and so has indirect and diffuse effects on flow, habitat, and contaminant levels. Altered hydrology

The extent of alteration of river flow and loss of river connectivity due to dams and impoundments is staggering. Worldwide, it is estimated that there are >45,000 large dams >15 m in height or of large reservoir capacity (Figure 2.1) and perhaps a million smaller dams. The United States has >75,000 dams >2 m in height and approximately 2.5 million smaller water control structures (Hart and Poff 2002). As a consequence, most river systems are impacted and few free-flowing sections of any size remain. Within the 48 contiguous US states, only 42 high-quality rivers contain free-flowing sections >200 km in length (Benke 1990). Most of the largest river systems of North America, Europe, and former USSR are highly or moderately affected by fragmentation of their main channels (Dynesius and Nilsson 1994). Only tundra rivers in the northern hemisphere and some large tropical rivers, particularly in South America, remain predominantly undammed (Nilsson et al. 2005). The pace of dam construction increased steadily throughout the 20th century, reached a peak between 1970 and 1975 during which roughly 5,000 large dams were constructed, and now has slowed to approximately 2,000 large dams per decade (Gleick 2000). This slowing is due partly to a declining number of suitable sites, and partly to growing appreciation of the resultant social and ecological disruptions.

Dams vary widely in their size, purpose, and mode of operation and these differences influence their impact upon river ecosystems (Petts 1984). Dams also differ in whether water is released from the surface of the dam, near the bottom, or both. Water supply impoundments require a large storage volume to meet projected needs and outlast droughts. Dams constructed for irrigation must store as much water as possible during the rainy season for release during the growing season. Flood control reservoirs maintain only a small permanent pool in order to maximize storage capacity, and draw down as soon as possible after a flood event to restore their capacity. Navigation requires water storage in upper reaches to offset seasonal low flow conditions, and may be complemented by a system of locks and dams. Hydroelectric dams store water for release to meet regional energy demands, which can vary seasonally or over the course of 24 h. "Run-of-the-river" dams release water at the rate it enters the reservoir, usually are of low height, and are thought to have few adverse effects on hydrology, although they may still impair longitudinal connectivity. "Peaking" hydropower dams meet daily fluctuations in energy demand by allowing water to flow through turbines only at certain times, usually from mid-morning through early evening. Changing water levels create unstable habitat conditions that can be especially disruptive to juvenile fishes and limit spawning opportunities for adults (Freeman et al. 2001). Finally, reservoirs may also serve recreational purposes including fisheries, but typically, this is a secondary function of a multipurpose facility.

The river environment below an impoundment is affected by changes in flow, sediment load, temperature, and water quality of the outflow (Stanford and Ward 1979). Effects on channel shape and substrate conditions are varied and can be especially serious. Dams that release very high discharges may cause scouring of fine materials and armoring of the streambed, a process in which the surface substrate becomes tightly compacted. Because of the loss of the river's normal sediment load, the result of deposition in the slow waters of the impoundment, the discharge immediately below a dam is "sediment-starved." This can lead to substantial channel and bank erosion and down-cutting of the streambed as the river adjusts to the altered balance between the amount of water and sediments that it is transporting. Where fine sediments are available, the absence of flushing flows can result in their accumulation within the streambed, reducing habitat space for invertebrates.

A river's temperature regime is altered to varying degrees by impoundments, strongly so in the case of large reservoirs with deep-release dams located on temperate rivers, which release water of cool and relatively constant temperature throughout the year (Figure 13 3). The new thermal regime facilitates the replacement of warm-water by cool-water fish species, which often are introduced trout in the western and southeastern United States. A moderated thermal regime has been shown to cause dramatic reductions in macroinvertebrate diversity at high latitudes, where exposure to near-freezing temperatures followed by a spring temperature rise is necessary to break egg diapause. In the Saskatchewan River, Canada, a fauna that originally included 12 orders, 30 families, and 75 species was reduced to only the midge family Chironomidae following construction of a deep-release dam (Lehmkuhl 1974).

Because impoundments trap sediments, water clarity typically increases below dams, resulting in a greater abundance of periphyton or higher plants than is found elsewhere in the river. For example, a dense growth of the aquatic moss Fontinalis neo-mexicana developed in riffle habitats of a regulated reach of an Idaho river (Munn and Brusven 1991), and this in turn

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