ANÁLISIS Y EVALUACIÓN DE RIESGOS PARA LOS EQUIPOS Y HERRAMIENTAS DE OBRA

The Thames Barrier, a series of giant gates positioned across the river downstream of don, was completed in 1982. It holds back estuarine floodwaters that would threaten Lon-don should high tides coincide with strong onshore winds (breezes blowing from sea to shore). The gates are raised to allow water and boat traffic through and lowered to hold them back. Because sea levels are probably rising due to global warming (see “Climate change,” pages 196–199), the barrier may need to be strengthened within the next 25 years.

Colorado River in the 1890s, but floods washed away most of their newly constructed dams and levees within a few years.

So silt-rich was the river in those days that it was commonly described as “too thick to drink, too thin to plow.”

The first large dam on the river, the Laguna Dam, was com-pleted in 1909, but by 1910 its reservoir had become congest-ed with silt. The Hoover Dam, a landmark achievement in civil engineering, was the first dam on the Colorado River to completely regulate the flow of the lower river. It was the world’s first truly multiuse dam, providing domestic and irri-gation water as well as hydroelectricity. Today at least 10 dams operate on the Colorado River system, with the Hoover and Glen Canyon Dams being by far the most important.

The effect of these two dams has been to turn the once cloudy, turbulent waters of the lower Colorado into a clear, regulated stream. In the lower Colorado, levels of suspended sediment have dropped by more than 98 percent since the construction of the Hoover Dam in 1935 and the Glen Canyon Dam in 1963. The Colorado River no longer feeds its delta with silt-rich water to replenish the material that waves and sea currents remove. As a result, the delta is gradually being washed away.

Water management on the Colorado River is governed by a complex legal framework, known as the “Law of the River,”

that has evolved piecemeal over more than 80 years. Today the Law of the River operates at three levels. At the top level, Mexico is guaranteed 0.44 cubic miles (1.85 cubic km) of water of sufficient quality per year to be usable for irrigation and domestic supplies. At the second level, water is divided for use between the upper and lower basins, and then to the various U.S. states within each basin. At the lowest level, water is allocated within each state.

The U.S. Bureau of Reclamation controls the allocation of water to the lower basin through the operation of the Hoover Dam. The Law of the River allocates a fixed quantity of water to Mexico but none specifically to maintain the integrity of the Colorado River delta. In 1962 Mexico complained about the quality and quantity of water the U.S. states were passing on. Since 1973 the United States has been responsible for ensuring the quality of the water flowing to Mexico. To meet

its obligations, the United States built and operates the Yuma Desalting Plant on the border between the United States and Mexico.

Until the early 1930s and the completion of the Hoover Dam, the Colorado River delta was a highly diverse and pro-ductive seasonal wetland, starved of water during the winter months and inundated in the late summer. It supported at least 200 species of vascular plants and a great diversity of waterfowl, fishes, and estuarine invertebrate species. When the Spanish arrived in the region in the 17th century C.E., they discovered some 20,000 Cocopah Native Americans inhabiting the delta region, coexisting with large mammals such as jaguars, beavers, deer, and coyotes. Until the mid-20th century some 3,000 Cocopah people continued a sus-tainable lifestyle by fishing in the freshwater lakes of the delta. Today fewer than 200 remain, and they fish in the sea because most of the lakes have disappeared.

In the mid- to late 1930s, when the Lake Mead reservoir behind the Hoover Dam was first filling with water, and then again between 1963 and 1981, when Lake Powell filled behind the Glen Canyon Dam, no freshwater reached the delta. In the years in between and since, much less than 1 percent of the river’s annual flow has reached the delta in most years, and the water that does reach it is murky, salty, and high in pesticide residues.

Without freshwater to flush salts and heavy metals from delta soils, these potentially harmful substances can become concentrated in food chains through the processes of bioac-cumulation and biomagnification to reach levels that impair animal reproduction (see sidebar “Bioaccumulation and bio-magnification,” page 207). In the late 1990s measured levels of the heavy metal selenium in delta water, sediment, and fish tissue were up to 14 times higher than the Environmen-tal Protection Agency’s (EPA’s) limits for wildlife protection.

Lack of flowing freshwater in the Colorado River also affects marine animals in the upper Gulf of California. Dur-ing the 1970s, when Lake Powell retained the freshwater that would have flowed into the delta, the upper gulf fishery for the totoaba fish (Cynoscion macdonaldii) collapsed, and the United States now classes the fish as endangered. Shrimp

catches in the late 1980s and early 1990s dropped to about 50 percent of the average over previous years. The Colorado delta clam (Mulinia coloradoensis) is found only in the Col-orado River delta and upper Gulf of California. Population densities have dropped by more than 90 percent over 40 years. The clam is now on the U.S. endangered species list.

The delta still sustains the largest remaining populations of an endangered bird, the Yuma clapper rail (Rallus longirostris yumanensis), and an endangered fish, the desert pupfish (Cyprinodon macularius). In most years the delta supports more than 60,000 resident birds and at least 40,000 migrato-ry birds.

However, in years when freshwater flows into the delta have been well above average, wetlands containing cattails and other emergent plant species and bordering land con-taining cottonwood trees and willows have shown a remark-able capacity for recovery. And in 1993 local freshwater floods in the delta region caused a resurgence in the upper gulf’s schools of corvina fish (Cynoscion xanthalus) after an absence of more than 30 years.

Until recently, the conservation of the Colorado delta and the lifestyles of its indigenous population had not been high on the agenda of the U.S. or Mexican governments. However, environmental groups such as the Living Rivers/Colorado Riverkeeper organization have raised international awareness of the plight of the delta’s indigenous people and its endan-gered animal species. International environmental agencies estimate that only 1 percent of the lower river’s average flow, allocated to the delta region, would be enough to help the delta flourish once again. Comparatively small changes to the allocation of water among U.S. states bordering the lower Colorado could have a big impact in restoring the delta ecosystem.

The lower basin of the Colorado River provides water for about 17 million people and irrigates more than 1 million acres (405,000 ha) in Arizona, California, and Nevada. Since 1929, the Colorado River Water Pact has sought to limit Cali-fornia’s annual share to 1.3 cubic miles (5.4 cubic km). Since the construction of the Colorado River Aqueduct, however, the state has regularly overdrawn its quota. In October 2003

the U.S. Department of the Interior and four California water agencies agreed to a 14-year plan for California to reduce the amount of water it removed from the Colorado River, by more efficiently utilizing water for irrigation and reducing losses through seepage. This agreement, the Colorado River Quantification Settlement Agreement (QSA), honors the pact California signed in 1929 but has since failed to keep.

Scientific cooperation between the United States and Mex-ico governments is underway, alongside negotiations on institutional arrangements for allocating water. Conse-quently, international agencies and environmental groups now project with a degree of confidence that further degra-dation of the Colorado River delta could be halted within the next decade.

As the descriptions of the Colorado, Thames, and Nile Rivers illustrate, any river system serves a multiplicity of uses that impact on its biological communities. Moreover, the size and nature of these operations changes over time scales of only a few decades. As a result, any consideration of how freshwater ecosystems should be managed must take into account rapidly changing human priorities.

Water is an invaluable resource. Adult humans need to con-sume at least 2.1 U.S. pints (1 L) of water a day to maintain the balance of water and salts in the body and prevent dehy-dration. In a hot, dry environment where water loss from sweat and breathed-out air is greater, individuals need a larg-er daily quota. The watlarg-er—if drunk as freshwatlarg-er—needs to be clean and safe, which in practice means that it does not harbor harmful organisms, high levels of injurious chemi-cals, or high concentrations of dissolved salts. In reality, many people across the world consume their daily intake of water in other beverages and in food, rather than by drinking clean, clear water.

People also need freshwater supplies for washing them-selves, for cleaning clothes and other household items, and for disposing of human wastes. This is domestic water usage (see table on page 172). This usage does not take into account the large amounts of water used to produce the food and drink that people consume or to manufacture and operate items that people use daily, such as vehicles, clothes, electri-cal items, and so on. When these uses are also computed, people in highly developed countries such as Canada and the United States use in excess of 1,850 U.S. gallons (7,000 L) per day, while in stark contrast, those in developing countries such as Angola, Ethiopia, and Laos use less than 26 gallons (100 L) daily. In developed countries, access to clean, clear water is taken for granted by most people. In many commu-nities of the poorest developing countries of Africa and Asia, clean clear water is considered a luxury, as yet unattainable.

Freshwater is used in such a variety of ways that one aquat-ic science expert aptly dubbed it a “pillar of our civilization.”

Freshwater bodies serve as mediums for transport and as political boundaries. People use freshwater to irrigate crops,

USES OF

LAKES AND RIVERS

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prepare food, and wash themselves and their possessions clean. Freshwater environments store water, helping to pre-vent damaging floods, and disperse wastes. Rivers and lakes

Local girls obtaining water from a well beneath a dry riverbed on the African offshore island of Madagascar.

Lack of clean freshwater threatens the lives of millions of people each year in developing countries. (Frans

Lanting/Minden Pictures)

provide edible items such as fish and aquatic plants and can be a source of hydroelectricity. They also provide some of the world’s most scenic destinations and exciting boating experi-ences. The multiplicity of uses of lakes and rivers, not all of which can be easily costed, nevertheless reveals their very high value as resources.