Across the yawning gulf of 5,000 years, we see the sun-browned Sumerians beginning the endless task of breaking the rivers and the plain to the use of man. As century followed century, Mesopotamia came to be damascened by an azure web of canals, which tamed the mighty Euphrates, clothed the desert in rippling fields of golden grain, and moistened the roots of date palms planted along their banks in endless rows. (De Camp 1963: 52)
So L. Sprague de Camp depicts Mesopotamia in his book, The Ancient Engineers. Mesopotamia's land was blessed with life-giving water, and that is one of the leading reasons why civilization began there. Indeed, its very name means "the land between the rivers" (from the ancient Greek potamos, which meant "river," and the prefix meso, "between").
But it was not sufficient that the waters of the Tigris and Euphrates merely existed. The waters had to be bent to the "use of man" and delivered to his fields and pastures so that crops and herds could grow to sustain human life. As a result, the rivers of Mesopotamia inspired some of the earliest achievements in civil and mechanical engineering.
The Nature of the Rivers
When it came to water, the farmers of Egypt had an easier time than their ancient Iraqi brethren. The Nile flooded once a year with calendric precision, enabling farmers to prepare for its rise. When it did rise beginning in late July, its waters rose gradually, inundating the fields in August and September. And when they withdrew in early October, they left behind a fresh layer of rich silt that renewed the fertility of the land. It was then that the first seeds were sown and the agricultural cycle began. Indeed, there was time to plant both winter and summer crops before the Nile would rise again.
In Mesopotamia, however, the situation was very different. When the snows in the mountains to the north melted, the waters of the Tigris and Euphrates rose, but their annual flooding was unpredictable, occurring anytime between April and early June, too late to help any winter crops, and by then the seeds for summer crops had already been sown. And when the deluge came, it could arrive suddenly with an almost capricious fury, destroying the young plants as well as everything in its path. No wonder, then, that Mesopotamia was the home of the world's oldest story of a cataclysmic flood.
Of the two rivers, the Tigris was the more violent, flowing faster and flooding sooner. The Euphrates, on the other hand, with a shallower channel flowed more slowly and was less violent. Because of its more gentle nature, the Euphrates was more readily turned to by farmers for aid. Though the Tigris was used to water the lands that lay to its east, the waters of the more friendly Euphrates nourished most of Mesopotamia's cultivated soil.
Wherever feasible, levees were built to raise the height of the rivers' embankments, which were at times reinforced with reed mats or, for greater stability, with walls of baked brick set in bitumen. Dams were usually ineffectual because the earth out of which they were made eventually washed away.
Projects and Management
In addition to defensive earthworks to hold back floodwaters, networks of canals were constructed to distribute river water safely and efficiently. These public works projects could not have been accomplished without the efforts of an organized work force operating under centralized authority. To be sure, the populace knew their lives and welfare depended upon their collaborative efforts. But their kings also regarded flood control and irrigation as their highest responsibility and took the greatest pride in their hydraulic achievements. Hammurabi of Babylon, for example, devoted most of the last nine years of his reign to such projects and even ceremonially honored one of the years of his reign by naming it for a great canal he had built. In fact, three of the statutes in his famous code dealt specifically with the control of water, including punishments meted out to those who out of laziness failed to maintain the levees near their property.
Methods of Irrigation
Not only did canals need to be laboriously dug by hand; they also needed to be laboriously maintained. The fact is echoed in a favorite Babylonian curse: "May your canal become clogged with sand!"
The slope of a canal was critical to its operation because the flow of water depended upon gravity. If the slope was too steep, erosion from fast-flowing water would eat away the bed of the channel and make the level of the water too low for it to spill into the fields; if the slope was too gradual, silt would build up or reeds grow, clogging the flow. Thus, surveying played an important role in the construction of canals, just as regular dredging and reed-pulling did once they were dug. In addition, the embankments of canals had to be preserved to insure they would not collapse. When canals formed networks, the problems and challenges only multiplied.
At inlet points along the riverside, sluice gates controlled the entry of water into the canal system. But when the level of the water in the river dropped below the inlet point, the water had to be raised. This was accomplished by an ingenious device known in Arabic as a shaduf, a device still used in the Near East today.
A shaduf is a seesaw-like contraption consisting of a long pole with an empty bucket at one end and a counterweight at the other. The counterweight can be a bucket or sack filled with clay or rubble. The pole, for its part, rests horizontally atop a simple wooden framework that holds it loosely and allows it to swivel or bend as though on a fulcrum. Using his own body weight, the worker pulls the bucket end of the pole downward, swings it out over the water, and dips it into the river until the bucket is full. Loosening his grip on the pole, he then lets the counterweight raise the heavy bucket, and swings it out over the canal, into which he empties the water. Because the counterweight does the work of lifting the full bucket, the job is made easier. In fact, by using more than one shaduf, water can be raised from one level to another. Besides moving water from river to canal, the shaduf could help a farmer transfer water from a major canal and pour it into a minor one to irrigate his fields.
In addition to canals, wells have been found at a number of Mesopotamian sites. Here, a bucket would simply be lowered and then pulled up. This job too was made easier by an inventionthe pulleysometime before 1500 BCE. The pulley itself represented an application of an even earlier invention, the Sumerian wheel. For a more rapid water supply, King Nebuchadnezzar of Babylon installed a chain pump in the basement of his palace: a series of buckets attached to a continuous metal chain. As the buckets at the bottom filled up with water, the ones at the top emptied out.
According to the Greek historian Herodotus, an Arabian king of the sixth century BCE once transported water across the desert in a pipe sewn together from animal skins. The pipe was a long one: it would have taken 12 days to cover the same distance by camel caravan, a distance of more than 300 miles.
Whether the story is true or not, we'll never know. What we do know is that water was regularly transported for long distances across northern Mesopotamia by underground conduit, especially when a growing population center needed water and there was no handy river nearby. Once again an engineering solution solved a problem, in this case a solution even more ingenious than the shaduf.
On the one hand, transporting water over long distances in a closed conduit (to avoid evaporation) is not hard, for gravity will do the job. As long as the source of watersay, a mountain springis always higher than the point of deliverysay, a city in a valley, the water will reach its destination by simply flowing downhill. The problem, however, is to maintain the proper degree of slope. If the slope is not constant, for example, if the conduit goes up instead of down, flow will be impeded. If the slope is too gradual, flow will be too slow and sedimentation may clog the channel. If the slope is too steep, the rapid flow of the water may erode the channel and cause it to collapse. How, then, does one insure that the slope will be appropriate? Only by putting it underground, where its course is determined by excavation rather than by the landscape above. But how does one insure the right slope when the conduit is being cut many feet, perhaps even hundreds of feet, below ground? And, even if that can be accomplished, how can we be sure the workmen digging the conduit underground will point it in the right direction and keep it going straight? Besides, if we're talking about a conduit stretching for miles, how long will it take a few workmen in a crowded pit to cut through miles of solid bedrock? All of these frustrating questions show us why the underground water conduit, or subterranean aqueduct, was a long time in coming.
The solution was discovered by an eighth century BCE Assyrian king named Sargon II during his conquest of Urartu, the ancient name for Armenia. Urartu was mining country, and the miners of Urartu had found the answers to all our questions. Once Sargon II learned them, he punished the Urartians by destroying all their aqueducts and then, when he returned triumphantly to Assyria, built underground aqueducts of his own. Later, the Persians would learn the secret, and in successive centuries it became common knowledge throughout the Mideast, where it is still in use today.
The device in question is called in Arabic a qanat, and in Persian a kariz. Basically, it is an underground water conduit with a constant slope. Not only that, but it has a regular series of access holes for maintenance (in case of blockages), holes that also release the air pressure that can build up and impede flow when the rush of water through the pipe becomes too fast.
But back to our problem: how, then, does one make a qanat?
The answer is to dig vertical channels into the rock at regular intervals. A surveyor on the ground marks out a straight line in the direction the water is meant to travel in. Wherever he then plants a stake, a hole is dug and a channel cut down through the rock. The straight up-and-downness of each channel can be easily monitored with a plumb line dropped down and dangled from the hole at the top. The proper depth of each channel can also be accurately predetermined by measuring from the horizontal plane above the series of holes. When different teams of miners have dug their vertical channels to the right depth, short horizontal channels can then be cut to connect them at the bottom. These short channels can be subsequently smoothed out to achieve a graduated slope. By employing multiple teams of diggers, the complete project can be executed more quickly and accurately than if a single set of miners burrowed slowly ahead in mole-like fashion.
In Afghanistan, such tunnels, dried out and abandoned, were exploited and expanded as hiding places and storage facilities by the militants of modern times.
Of course, the qanat could not be used in southern Mesopotamia's alluvial plain where the earth was soft and tunneling would have been risky. There, aboveground canals were the method of choice for moving water. But in the north where there was substantial bedrock, the qanat was the answer.
In later centuries, the ancient Romans excavated underground conduits to transport water to their cities, but they also carried the water through conduits supported far above ground by tall arches. The most remarkable of these aqueducts can still be seen in Spain at Segovia and in France at Nîmes. Nîmes's celebrated Pont du Gard was, in fact, originally designed to carry not traffic but water. In their heyday, the aqueducts of Romeboth underground and elevatedbrought 250 million gallons of freshwater each day to the capital's urban masses. Amazingly, four are still in operation, including one that feeds the romantic Trevi Fountain. The Romans were able to erect aboveground aqueducts because they had access to large local supplies of quarriable stone, especially limestone, a luxury that the Mesopotamian engineer did not generally enjoy.
One Mesopotamian exception can be viewed near the modern city of Jerwan, located north of ancient Nineveh. A 30-mile-long underground conduit was built by the Assyrian king Sennacherib to supply Nineveh with water, but its slope required that it cross a small river valley. Sennacherib's solution was to build a 90-foot-long bridge to carry the conduit 30 feet above the stream. Ruins can still be seen of the five corbelled arches that supported the bridge, each constructed of cubic blocks of stone measuring 20 inches on each side. The whole aqueduct took a year and a quarter to complete, and Sennacherib planned a special ceremony to mark its opening. The monarch's thunder, however, was apparently stolen by a defective sluice-gate that allowed the water to flow before the ceremony could begin. Ever the opportunist, Sennacherib took this to be a sign from the gods validating his project and so he did not punish the seemingly negligent workers. Instead, as he proudly tells us in his annals, he rewarded them with fine clothes and golden rings and daggars.
Then and Now
Thanks to a complex and extensive system of irrigation that maximized the fertile potential of its soil, Mesopotamia enjoyed an abundance of agricultural produce. The land cultivated in southern Mesopotamia alone may have exceeded 12,000 square miles and the population density of the country as a whole may have even surpassed that of present-day Iraq.
This system was surely one of the glorious triumphs of ancient engineering. But what, then, became of it? And why must Iraq today import food to feed its people?
In part, the answer can be traced to the malevolence of man and nature.
Sennacherib, the same Assyrian king who built the aqueduct at Jerwan, was also capable of hydraulic destruction. Angry at Babylon for revolting against his rule, he massacred its people, dammed up the Euphrates, and then diverted its waters, sending them hurtling through the city. Later, he repented and rebuilt what he had destroyed, as did his son who succeeded him.
Less forgiving, however, were the Mongol hordes led by Hulagu Khan. After capturing Baghdad in 1258 CE and conquering Mesopotamia, they ravaged the country's canal system, leaving the people to starve. Politically and militarily vulnerable, Mesopotamia was plundered in successive centuries by foreign people as its population declined and social order broke down.
No enemy, however, foreign or domestic proved as unforgiving as nature itself.
Over time, the rivers meandered and their courses altered. Old canals became useless and were abandoned. With persistent neglect, even canals that still functioned slowly filled up with silt and reeds.
In prehistoric times, much of what is southern Iraq and Kuwait today lay submerged beneath the waters of the Persian Gulf. As a result, there are thick beds of sea salt beneath the soil. In ancient Mesopotamia, the intensive irrigation of the soil dissolved the salt and brought it to the surface. When the water evaporated, traces of salt were left behind, traces that accumulated over many centuries and, over time, chemically inhibited seeds from germinating. The problem was compounded by the Tigris and Euphrates themselves, which carried dissolved salts they had picked up from the mountains to the north that were their source. As their waters flowed through Mesopotamia's canals and evaporated from its soil, the salts were left behind. Thus, what the Mongols and others had been incapable of achieving, salinization accomplished.
Today, parallel ridges can be seen traversing the landscape of Iraq, tracing in dry wasteland the lines of canals that once flowed with life-giving water. From outer space, satellite imagery can detect the remains of now-desiccated watercourses invisible at ground level to the naked eye. Nearby, a weary peasant bends over, lowering the pole of a shaduf just as his ancestors did 5,000 years ago. The Euphrates rolls silently by him, mindless of human struggle and folly.