Engineering Wonders of the World
Volume I

IRRIGATION WORK IN THE UNITED STATES. 89 clear cut, but shifts from season to season backward and forward over a vast range of country, extending practically from the 97th to the 104th degree of west longitude. This broad, indeterminate belt embraces nearly one-tenth of the whole United States, ex- clusive of Alaska, and is usually known as the semi-arid or sub-humid region. Its elevation varies from 1,500 feet along the eastern border to from 4,000 to 6,000 feet near the base of the mountains. It is desti- tute of forest cover, and for many years has furnished free grazing for numerous nomadic flocks and herds. Under improved methods of cultivation, known as dry farming, settlements have gradually moved westward, and large areas, which a few years ago were regarded as value- less except for grazing, are now producing average yields. Irrigation in the Great Plains began near the foothills, where the streams had the greatest fall, and where the water could be easily diverted. With the increase ^PI ^reat of population, the small ditches were linked up and enlarged. New and larger systems, including reservoirs and important masonry structures, were con- structed, until practically the entire available water supply of the Plains streams has been appropriated for irrigation. The principal rivers of the Plains region are the Missouri River and its tributaries the Platte and Yellowstone Rivers, and the Ar- kansas River and its branches. The Platte Qf tfæ tributaries of the Mis- River. . . souri the Platte River is the most important. This stream has its source in Northern Colorado, where the Rockies attain their highest general elevation. Among these lofty snow-clad peaks are its two heads, known as the north and south forks. The Platte River is born in a region of grandest scenery, of profound canyons, and of wonderful level- floored parks that were once occupied by vast lakes. Fed by melting snows, it gathers volume as it passes through gorge and canyon and over level beds of uplifted seas, until, on reach- ing the foothills, where the gradient diminishes, the rushing mountain torrent becomes a stately river. Leaving the mountains far behind, it emerges on to a vast plain whose horizons are 1,000 miles apart, and upon whose bosom are hundreds of cities and towns separated by broad areas of agricultural lands. It drains 90,000 square miles in three States, and at the present time furnishes water to approximately 2,000,000 acres of land wholly dependent upon it for crops. It protects more than 20,000 farms against drought, furnishing life-giving water through 15,000 miles of main canals and ditches. A conservative estimate of the total value of farms in the drainage basin of this river is $100,000,000. With the com- pletion of the irrigation works now under way in this drainage basin, the entire flood flow of the stream in the semi-arid region will be controlled. The Yellowstone River is the principal northern tributary of the Missouri, and, with its numerous branches in Wyoming and Mon- tana, is an important factor in the agricultural development ^he Y®Jlow" of both States. Three-fourths of the volume of this stream come from the Yellowstone National Park, one of the prin- cipal catchment basins in the arid region. The park has an area of 2,000 square miles, encircled completely by lofty mountain ranges with peaks towering from 10,000 to 12,000 feet above sea-level. The heavy rainfall on these mountains contributes, by two great rivers, the Missouri and the Snake, to both the Atlantic and Pacific Oceans. The agricul- tural feature of a large part of five States depends upon the quantity, control, and dis- posal of these waters. Two large irrigation projects of the Federal Government have been completed on the Yellowstone, and another is under way on the Shoshone River, a tributary.