In this section, we will consider other impacts of dams.
In addition to the increased nutrient concentrations from agricultural return flow downstream of dams, the reduction in flow velocity in slackwater behind dams leads to reduced flushing of pollutants that enter the river. In areas subject to high rates of municipal or industrial wastewater discharge, or to agricultural runoff, this can lead to significant impairment of water quality in the reservoir itself, and in upstream tributaries (for example, see Three Gorges: A “Mega-Dam” and its Impacts). High nutrient fluxes can also lead to eutrophication of the reservoir. Additionally, the increased surface area of reservoirs leads to large evaporative losses and subsequent increases in water salinity, especially in arid and semi-arid climates.
As noted previously, dams have wide-ranging effects on downstream habitat through changes they cause to water turbidity and sedimentation and erosion patterns. These changes threaten certain species that have evolved to thrive in the natural system – like the humpback chub – through a combination of decreased or degraded breeding habitat and increased predation by non-native species. Additionally, because water released from dams through intakes flows from the deep part of the reservoir, it is commonly colder than the natural river flow – and its temperature is less variable than in the river’s natural state. For example, prior to construction of the Glen Canyon Dam, water temperature in the Colorado River varied from ~0 to 27° C over the course of the year; water discharged from the dam now averages ~8°C and varies little seasonally. The changes in water temperature and its variability impact some fish species, which rely on temperature cues to trigger key lifecycle events. The lower oxygen levels in waters released from storage behind dams also impact fish downstream.
Finally, dams present physical barriers to catadromous and anadromous fish species (those that spawn in saltwater and live in freshwater, and vice-versa, respectively). These fish migrate either upriver from the ocean (anadromous), or downriver to the ocean (catadromous) to spawn. After hatching, the young fish migrate in the opposite direction. Of these, perhaps the best known is the salmon, which migrates up-river to spawn, commonly over hundreds or thousands of km – for example, although greatly reduced due to major dams along the Columbia and Snake Rivers, Chinook salmon runs commonly extend from the Pacific Ocean all the way to Idaho! Structures designed to allow fish to navigate dams, such as fish ladders, are one solution, but they still present a barrier that reduces the likelihood of safe passage, and thus fish numbers.
The large reservoirs impounded by dams provide breeding grounds for some water-borne diseases and parasites, especially in tropical climates. Among the most prevalent of these is schistosomiasis, a disease caused by parasitic worms. The parasite is spread by freshwater snails, and has come to be known as the “disease of hydroelectric dams”. It infects an estimated 200 million people per year (with 200,000 fatalities), primarily in Asia, Africa, and South America. Through the expansion of habitat for the disease vector by large slackwater reservoirs, the incidence of this and other diseases is greatly increased. For example, in the Yangtze River Basin, the incidence of schistosomiasis is near 5%, versus less than 1% in less or undeveloped areas.
Dams and major irrigation projects also provide expanded breeding habitat for insects (mosquitos) that serve as vectors of Dengue fever, malaria, and West Nile virus, among others. The World Health Organization (WHO) estimates that malaria cases in villages near the Bargi reservoir in India increased more than twofold following the dam’s construction, and up to four-fold in villages closest to the dam itself. Likewise, malaria incidence increased by seven times in proximal Ethiopian villages following the construction of small dams on the Tigray River. A similar increased incidence of West Nile virus has been documented as a result of increased mosquito breeding area in many parts of the world, including the Midwestern U.S., California, and Oregon.
Impoundment of water behind major dams changes the distribution of stress in the Earth’s crust, and in combination with downward percolation of impounded water, can trigger seismicity. For the most part, this phenomenon is restricted to increased numbers of small (magnitude <3.5) earthquakes triggered by the increased load of millions of m3 of water, associated warping – or flexure – of the crust, and diffusion of water pressure from the reservoir along fractures and fault lines (Figure 10). Although hotly debated, reservoir-induced seismicity has even been invoked as a possible mechanism for the devastating 2008 magnitude 7.9 Wenchuan earthquake that killed an estimated 80,000 people (see one news article discussing this issue here [1]). The reservoir impounded behind the 156 m-tall Zipingpu Dam lies above the Beichuan- Yinxiu fault, which extends to the Northeast to the earthquake hypocenter, located ~50 km away.
Although remote, there is also a potential risk of dam failure to life and property downstream. In some cases, the causes of such failures are not known with much certainty. For example, the collapse of the St. Francis Dam Northeast of Los Angeles in 1928 resulted in the catastrophic release of over 12 billion gallons of water. The flood wave, which was over 140 feet high, killed an estimated 600 people and scoured the valley below, transporting fragments of the dam as large as 10,000 tons for almost a mile downstream. In other cases, upstream flooding or inadequate ability to release water and relieve pressure on the dam are the culprit, as in the famous collapse of the South Fork dam and resulting 1889 Johnstown PA flood that killed over 2000 people and triggered changes to liability laws in the U.S. In 1986, a similar disaster at the Glen Canyon Dam was narrowly avoided.
Rivers are not restricted by state and national borders, whereas dams are rarely constructed or managed by collaboration between governments. As a result, alteration, interruption, and control of river discharge by dams naturally leads to political and legal conflict. In the case of the Colorado River, which we will cover in more detail in Module 8 (Cities in Peril), the allocation of water between states within the drainage basin is governed by the 1922 Colorado River Compact. Allocation of water between the U.S. and Mexico is governed by an international treaty established in 1944 and recently revised in 2012. Even though well established, the water allocation of the Colorado, and its fairness are widely debated. The compact is also the focus of lawsuits over water rights for Native American reservations, which were not explicitly included in the original agreement. Court battles have also arisen over other river flows in recent years (for example in Florida and Georgia, and along the San Joaquin and Sacramento Rivers) pitting access for communities or farmers against minimum limits on flow required to support endangered species.
Globally, other rivers and dams are the source of equal – or more – controversy. As one example, at the 1992 opening ceremonies for the Atatürk Dam on the Euphrates River in Turkey, the president of Turkey is reported to have said, "Neither Syria nor Iraq can lay claim to Turkey’s rivers any more than Ankara could claim their oil. This is a matter of sovereignty. We have a right to do anything we like. The water resources are Turkey’s, the oil resources are theirs. We don’t say we share their oil resources, and they can’t say they share our water resources." The conflict over waters of the Tigris-Euphrates continues (you can listen to a story about this dispute here [2]). Dams and control of river flows in the headwaters of the river system, and subsequent impacts on water access to supply populations with drinking water, to grow food, and support industry in the downstream nations of Iraq and Syria, are at the heart of the dispute. Similar tensions are now arising along the Mekong River between China (upstream) and downstream neighboring countries of Myanmar, Thailand, Laos, Cambodia, and Vietnam that rely on the river.