Dams: What They Are and What They Do

Excerpted from Chapter 1 of Patrick McCully’s:
Silenced Rivers: The Ecology and Politics of Large Dams

"A reservoir is a man’s triumph over nature and the sight of a vast sheet of water brings an inner satisfaction to those who behold."
S.H.C. de Silva
Consultant to the Irrigation Department of Sri Lanka, 1991

Dams have two main functions. The first is to store water to compensate for fluctuations in river flow or in demand for water and energy. The second to raise the level of the water upstream to enable water to be diverted into a canal or to increase ’hydraulic head’ –– the difference in height between the surface of a reservoir and the river downstream. The creation of storage and head allow dams to generate electricity (hydropower provides nearly a fifth of the world’s electricity); to supply water for agriculture, industries and households; to control flooding; and to assist river navigation by providing regular flows and drowning rapids. Other reasons for building large dams include reservoir fisheries and leisure activities such as boating.

Hydropower generation capacity is a function of the amount of flow and hydraulic head. Although the head is usually related to the height of the dam, a low dam can have a high head if the powerhouse with its turbines and generators is located some distance downstream of the dam. Pipes known as ’penstocks’ direct water to the turbines. Once the water has spun a turbine it flows into the ’tailwater’ below the dam through a ’tailrace’ pipe.

One advantage of hydro over other forms of electricity generation is that reservoirs can store water during times of low demand and then quickly start generating during the peak hours of electricity use. Thermal power plants take much longer to start up from cold than hydro plants. Hydro’s suitability for generating valuable ’peaking’ power has in recent years encouraged a boom in what are known as pumped–storage plants. These involve two, normally relatively small, reservoirs, one above the other. During peak hours, the water from the upper reservoir falls through turbines into the lower one, generating electricity. The water is then pumped back uphill again using cheap off–peak electricity.

Weirs and barrages are different types of ’run–of–river’ dams, this means that while they raise the water level upstream they create only a small reservoir (’head pond’) and cannot effectively regulate downstream flows. A weir is normally a low wall of stone, concrete or wicker. A barrage can be a huge structure ten or twenty metres high extending for hundreds of metres across the bottom reaches of a wide river. The electricity generation of a ’run–of–river’ hydropower dam is proportional to the flow of the river at any one time.

While they tend to have less damaging consequences than storage dams, run–of–river dams are far from environmentally benign, and the distinction between a ’run–of–river’ and a ’storage’ dam is not always clear. Dam proponents have in some cases sought to downplay the impact of planned dams by claiming that they will be run–of–river. Thailand’s Pak Mun Dam, for example, is repeatedly described by officials as a run–of–river project yet for much of the time the dam’s gates remain closed and it operates as a storage dam. Despite years of protestations from its builders and funders that it would have minimal impacts on the river, Pak Mun managed within a couple of years to destroy one of the country’s richest freshwater fisheries.

Just as every river and watershed is unique, so is every dam site and every dam. There are, however, three main types of dam design –– embankment, gravity and arch –– selected mainly according to dam–site topography and geology. Earth and rock embankments, which are usually the cheapest to build, make up more than 80 per cent of all large dams. Embankments are generally built across broad valleys near sites where the large amounts of construction material they need can be quarried. Large embankment dams are the most massive structures humanity has ever erected. The most voluminous dam in the world, Tarbela in Pakistan, contains 106 million cubic metres of earth and rock, more than 40 times the volume of the Great Pyramid.

Gravity dams are basically thick, straight walls of concrete built across relatively narrow valleys with firm bedrock. Arch structures, also made from concrete, are limited to narrow canyons with strong rock walls and make up only around four per cent of large dams. An arch dam is in form like a normal architectural arch pushed onto its back, with its curved top facing upstream and its feet braced against the sides of its canyon. The inherent strength of the shape enables the thin wall of an arch dam to hold back a reservoir with only a fraction of the concrete needed for a gravity dam of similar height.

A dam contains a number of structural features other than the main wall itself. Spillways are used to discharge water when the reservoir threatens to become dangerously high. Dams built across broad plains may include long lengths of ancillary dams and dykes. The five reservoirs of Phase 1 of the La Grande hydropower scheme in northern Quebec, for example, are impounded by eleven dams and more than 200 accompanying dykes stretching for a total length of 124 kilometres.

A Short History of Damming

"Then nothing will remain of the iron age
And all these people but a thigh–bone or so, a poem
Stuck in the world’s thought, splinters of glass
In the rubbish dumps, a concrete dam far off in the
mountain . . ."
Robinson Jeffers
from Summer Holiday, 1925

Farmers in the foothills of the Zagros mountains on the eastern edge of Mesopotamia may have been the first dam builders. Eight thousand–year–old irrigation canals have been found in the area and it is not unlikely that small weirs of brushwood and earth were used to divert water from streams into the canals. By 6,500 years ago the Sumerians were criss–crossing the plains along the lower Tigris and Euphrates with networks of irrigation canals. Again no physical evidence of dams has been found from this period but it is likely that they were used to control flows of irrigation water.

The earliest dams for which remains have been found were built around 3,000 BC as part of an elaborate water supply system for the town of Jawa in modern–day Jordan. The system included a 200 metre wide weir which diverted water via a canal into ten small reservoirs impounded by rock and earth dams. The largest of the dams was more than four metres high and 80 metres long. Some 400 years later, around the time of the first pyramids, Egyptian masons constructed the Sadd el–Kafra, or ’Dam of the Pagans’ across a seasonal stream near Cairo. This squat mass of sand, gravel and rock was 14 metres high and 113 metres long, and retained by some 17,000 cut stone blocks. After perhaps a decade of construction, but before it could be completed, the dam was partly washed away and was never repaired. The failed dam may been intended to supply water to local quarries. Because the Nile’s floods inundated their fields before the planting season each year, the farmers of Ancient Egypt did not need dams for irrigation.

By the late first millennium BC, stone and earth dams had been built around the Mediterranean, in the Middle East, China, and Central America. The ingenuity of Roman engineers is perhaps most visible in their dams and aqueducts. The most impressive surviving Roman dams are in Spain which continued to be preeminent in hydraulic engineering through the Moorish period and into modern times. A 46–metre–high stone dam near Alicante begun in 1580 and completed 14 years later was the highest in the world for the better part of three centuries.

South Asia, too, has a long history of dam building. Long earthen embankments were built to store water for Sri Lankan cities from the 4th century BC. One of these early embankments was raised in 460 AD to a height of 34 metres and was the world’s highest dam for more than a millennium later. King Parakrama Babu, a 12th century Sinhalese ruler notorious as a tyrant and megalomaniac, boasted to have built and restored more than 4,000 dams. One old embankment he enlarged to a height of 15 metres and an incredible length of nearly 14 kilometres. No dam equalled it in volume until the early 20th century. Parakrama Babu’s large dams, believes anthropologist Edward Leach, were of little use to most Sri Lankan villagers who relied for irrigation on small artificial ponds known as ’tanks’. The large dams, says Leach, ’are monuments, not utilitarian structures.’

Technologies to convert the energy of flowing water into mechanical energy have a history almost as long as that of irrigation. A type of waterwheel known as the Noria which has buckets around its rim to scoop up water from a river or canal was used in Ancient Egypt and Sumeria. By the first century BC, watermills were used to grind corn in Rome. The Domesday Book of 1086 records 5,624 watermills in England –– roughly one to every 250 people.

Watermills were not only built for raising water and grinding corn. During the later Middle Ages they performed numerous tasks in the great industrial centres of Germany and northern Italy including pulping rags for paper, hammering iron, beating hides in tanneries, spinning silk, crushing ores and pumping water from mines. Ores from the famous ’silver mountain’ at Potosí in Bolivia were ground in well over a hundred watermills. In the early 17th century, the dam holding back one of the largest of the 32 reservoirs supplying water to the mills collapsed, washing away 4,000 people along with almost all the mills. By the beginning of the industrial revolution some half a million watermills were powering Europe’s factories and mines.

Almost 200 dams higher than 15 metres were built in fast–industrializing 19th century Britain, mainly to store water for its expanding cities. In 1900, Britain had nearly as many large dams as the rest of the world put together. Nineteenth century dams were mainly earth embankments designed largely on the basis of trial and error –– until the 1930s there was little scientific understanding of how soil and rock behaved under pressure. Dam builders in the 19th century (and even today in some parts of the world) also had little streamflow or rainfall data, and few statistical tools to analyze what hydrological data had been gathered. As a consequence, their structures collapsed with alarming frequency. Two hundred and fifty people were killed when a water supply dam in Yorkshire burst in 1864. The US had a particularly bad safety record: nearly one in ten embankments built in the US before 1930 failed. More than 2,200 people were swept to their deaths when a dam above the town of Johnstown, Pennsylvania collapsed in 1889. The earthen embankment had held back the largest reservoir in the US.

French engineer Benoit Fourneyron perfected the first water turbine in 1832, hugely boosting the efficiency of watermills (a turbine, which converts the potential energy of falling water into mechanical energy, is far more efficient than a waterwheel which is powered by the kinetic energy of flowing water). The full significance of the turbine became clear in the latter part of the 19th century with advances in electrical engineering which led to the building of power stations and transmission lines. The world’s first hydro plant, a run–of–river dam in Appleton, Wisconsin, began producing power in 1882. The following year hydro dams were built in both Italy and Norway.

Over the next few decades small hydro dams proliferated on the swift–flowing rivers and streams of Europe, most notably in Scandinavia and the Alps. After the turn of the century, the size of the dams and power stations being built began rapidly to increase. Progress in turbine design increased the head at which turbines could operate from 30 metres in 1900 to more than 200 metres by the 1930s, and improvements in dam engineering allowed the high dams to be built to create this head.

Throwing Water Across the Land: Big Dams in the US

"Now what we need is a great big dam,
To throw a lot o’ water out across that land,
People could work and stuff would grow,
And you could wave goodbye to the old skid row."
Woody Guthrie
Washington Talkin’ Blues, 1941

The conquest and settlement of the dry US West in the late 19th century owes more to dams than cowboys. Early settlers saw the damming and redirecting of desert streams onto their fields as both an economic necessity and a spiritual duty, furthering God’s work by turning the wilderness into a garden. By the end of the 19th century, most of the best sites for the small dam and irrigation schemes which groups of farmers or private companies could afford had been exploited and many irrigation companies were going to the wall.

In 1902, Congress passed the National Reclamation or ’Newlands’ Act, described by environmental historian Donald Worster as ’the most important single piece of legislation in the history of the [US] West’. The Act set up the Reclamation Service –– later to become the Department of Interior Bureau of Reclamation or BuRec –– to build irrigation projects to be financed by selling government land and later by selling water and electricity (’reclamation’ is a semantically curious term which in the US usually means bringing irrigation to arid land).

The Newlands Act passed amidst a spate of rhetoric on how irrigation in the West would prove a magnet for the homeless and landless in the East, serving as a safety valve for the discontented and a bolster to democracy. Irrigation would also allow the US to settle the sparsely populated Western half of their country. Within a few years of the Act passing, however, it became clear that there were no legions of impoverished Easterners eager to become farmers in the desert and that federal irrigation was no more economic than its private counterpart. In the words of Donald Worster, the federal reclamation programme was ’hopelessly unrealistic, expensive, unworkable, and naive’. By 1930, says Worster, ’it was so manifest a failure that, had there not been powerful groups and strong cultural imperatives supporting it, federal reclamation would have died an ignominious death.’

To limit the concentration of ownership of federally irrigated land, no farmer was supposed to be allowed to own more than 160 acres in a reclamation project. This requirement, however, was studiously ignored or reinterpreted so that speculators and large landowners –– together with construction companies –– were the greatest beneficiaries of Western water development. The greatest losers were the federal taxpayers who had to subsidize the schemes and the Native Americans who lost untold numbers of sacred sites under the reservoirs, reservation land, water to which they had treaty rights, and most of the prodigious salmon fishery of the Pacific Northwest.

The glory years of the Bureau of Reclamation began with the first blasting at the site of Hoover Dam in 1931. The Bureau had already engineered 50 concrete dams, but Hoover was something else –– the 60 million tons of concrete it contained outweighed all their previous dams put together. Hoover stood an incredible 85 metres higher than any other dam in the world. Yet before Hoover was even finished the Bureau was overseeing the construction of Shasta Dam on California’s Sacramento River with a volume of concrete twice that of Hoover and the even more massive Grand Coulee Dam in Washington state, a 1500–metre–long, 168–metre–high monster described by one hyperbolic Western Senator as ’the biggest thing on earth’.

Electricity from the big Western dams helped to win the Second World War. By June 1942, almost all the power from Grand Coulee and Bonneville Dam, built by the Army Corps of Engineers on the Lower Columbia, was going to war production, most of it to producing aluminium for aeroplanes. Later the hydropower of the Northwest was turned to another use: the highly energy–intensive production of plutonium for nuclear bombs. In 1945 the first and second biggest sources of electricity on the planet were Grand Coulee and Hoover with respective generating capacities of 2,138 and 1,250 megawatts.

While the activities of BuRec are confined to the Western US, the hundreds of dams of the US Army Corps of Engineers have been built all over the country. In the 19th century, the Corps’ mission was to engineer rivers to accommodate river traffic and control floods. Like BuRec, however, it expanded its role, taking on hydropower production, ’reservoir recreation’ and irrigation. The four huge dams the Corps constructed on the Missouri –– Garrison, Oahe, Fort Peck and Fort Randall –– are respectively the third, fourth, fifth and seventh largest capacity reservoirs in the US (the other top seven places are occupied by BuRec’s Hoover, Glen Canyon and Grand Coulee dams).

Although it has built dams in only one river basin, the Tennessee Valley Authority may have had the most influence worldwide of any of the US dam–building bureaucracies. Established by the federal government in 1933 as a largely autonomous agency with wide powers over the lives of the valley residents, including the right to expropriate land, the TVA has inspired numerous river basin development authorities around the world. While the TVA is still regarded as synonymous with dam building, the authority built most of its 38 large dams before 1945, after which it turned to coal and nuclear plants. Despite the tens of billions of dollars spent by the TVA, the population of the Tennessee Basin is in many ways poorer than those living in nearby areas who did not ‘benefit’ from TVA development.