During the early stages of civilization—when humans followed food resources, hunting and foraging in seasonal movements--the main concern about water was finding it. When people started building permanent settlements, it became necessary in many places to manage water resources.
Management systems can be divided into two basic types: systems built to ensure a sufficient and continuous water supply and systems built to protect against flooding--roughly, dams and dikes. Many cultures, including the Mayans and Southeast Asians, have impounded, diverted and transported water for agricultural and urban needs. Ancient ruins in the Middle East attest to early efforts in water resources management on a grand scale. For millenia, the Egyptians maintained extensive irrigation systems consisting of low earthen dikes along the Nile to control the water level for agricultural purposes. Large dam projects were also carried out in Egypt, though not always successfully. Some time during the Old Kingdom, a dam 361 feet (110 m) wide and 39 feet (12 m) high was built near present Helwan to impound the flash floods coming down the wadi in winter. Apparently, the barrage collapsed the first winter it was in use, but the abutments of the dam still remain. The Orontes dam, 500 feet (150 m) long and 20 feet (6 m) high, was built in 1300 B.C. with greater success. Both the dam and the 3-by-6-mile (5-by-10 km) lake it created still exist. Remains of similar age are also to be found in the Tigris river valley. Water diversion for agriculture and direct consumption also formed a major component of the infrastructure of the Roman Empire. A number of dams constructed in that era in Italy, Spain and North Africa are still in use. The next big project that has survived the test of time is the Tibi dam near Alicante in Spain, which was completed in 1594 and provides water for irrigation to this day.
A little earlier, in the second half of the fifteenth century, a major land reclamation project was completed in Holland. Although different societies, including the Dutch, had built dike systems in river areas to contain seasonal rises in water levels and had diked in tidal areas to keep out seawater at high tide, the Holland project gave a new dimension to land reclamation. For the first time, land below sea level was reclaimed, requiring pumping systems to be in constant operation after the so-called polders were created. For centuries, windmills maintained groundwater in these polders at levels suitable for agriculture, until they were replaced by steam engines.
With the advent of industrialization dam building once again became a major focus of interest in the Western world. The growth of urban centers overtaxed the naturally available supply of water in many places. Great Britain was the first country to start building great numbers of dams in the nineteenth century. Unfortunately, these efforts proceeded largely by trial and error, causing catastrophic flooding on a number of occasions. Most of these early nineteenth-century installations were earthen or rockfill dams that did not provide watertight barriers. The leakage would undermine the stability of the dam, and the weight of the water bearing down on it would eventually cause a collapse. It is estimated that about 150,000 dams across the world pose a potential hazard to lives or property.
There have been about 200 dam failures since 1900. One of the worst took place in Pennsylvania, near Johnstown, in 1889, when a dam built to impound water for the Pennsylvania Canal failed, sending a 70-foot (21 m) wall of water over Johnstown and seven other towns. About 2,200 people died in the disaster. With these failures in mind, the French, taking a more scientific approach, started developing masonry dams of different designs that proved much more reliable and laid the foundation of modern dam design.
The development of technology to generate hydroelectric power gave new impetus to dam projects, stimulating such gigantic multi-purpose achievements as the Hoover Dam in the Colorado river in Nevada. The Hoover Dam, which was finished in 1936, impounds water for irrigation purposes and supply and can generate 1.8 million horsepower per day. At 725 ft (221 m) it is the 17th highest dam in the world. Another modern giant is the High Dam at Aswan, Egypt, which was built with aid, not always very efficient, of the Soviet Union. The High Dam can impound 212 billion cubic yards (162 billion cubic meters) of water in a lake 310 miles (500 km) long and on average 6 miles (10 k) wide. With the exception of the Aswan High Dam, which is an earth and rockfill barrage, the modern dams are more like buildings, with galleries at different heights to enable continuous monitoring of the performance and stability of the structure. China has by far the largest number of dams, over 18,000 that are at least 50 ft (15 m) high. The United States has about 5,300.
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