Created in the 1963 with the Completion of the Glen Canyon Dam Construction, Lake Powell Became the Second Largest Reservoir I
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Geo/SAT 2 DROUGHT IN THE COLORADO RIVER BASIN: SHRINKAGE OF LAKE POWELL Professor Paul R. Baumann Department of Geography State University of New York College at Oneonta Oneonta, New York 13820 USA COPYRIGHT © 2008 Paul R. Baumann INTRODUCTION: Created in 1963 with the completion of the Glen Canyon Dam, Lake Powell became the second largest reservoir in the United States following Lake Mead. Construction on Glen Canyon Dam was started in 1956 and completed seven years later in 1963, after which the water from the Colorado River proceeded to backup behind the dam to form the lake. In June 1980, after seventeen years, Lake Powell reached full pool size with a volume of 27 million acre-feet (MAF)* and a surface area of 266 square miles (689 sq. km.). At full size the reservoir is nearly 186 miles (299 km.) in length with a water depth of 560 feet (170.7 m) at the dam. FIGURE 1: Colorado River Basin With Lake Powell being located in an arid and semiarid region its water level varies considerably and provides a good barometer of water conditions within the Colorado River’s 246,000 square mile (637,137 sq. km.) basin (Figure 1). From 1995 through 1999 * An acre-foot is roughly 326,000 gallons of water, enough to supply an average family of four for a year. its water level was above average and as late as September 1999, the reservoir was still 95 percent full. However, precipitation levels in the upper Colorado River basin from October through December 1999 fell to 70 percent below average, signaling a low runoff for 2000 and the beginning of an extreme drought. FIGURE 2: Lake Powell – Study Area Outlined in Red The goal of this instructional module is to measure the impact of the drought on the lower portion of Lake Powell between 1999 and 2002. The objective is to determine the surface area of the lower portion of the reservoir when the drought started and the amount of change in the surface conditions of the reservoir four years into the drought. Two Landsat Enhanced Thematic Mapper (ETM+) data sets, one taken at the beginning of the drought (October 10, 1999) and the other four years into the drought (June 13, 2002), are used in this exercise. Subsets from these ETM+ data sets were formed to create a study area, the lower portion of Lake Powell. The study area is 676.8 square miles (1089.41 sq. km) in size (Figure 2). BACKGROUND: Colorado River Compact Lake Powell came into existence as part of a larger project to control flooding on the Colorado River and provide water and electrical power throughout the southwest United States. Between 1905 and 1907, several large floods on the Colorado River destroyed crops and fields in southern California, mainly in the Imperial Valley. Floodwaters from the river broke through the irrigation floodgates and flowed into the valley forming the Salton Sea, a 450 square mile (1165.5 sq. km.) lake. From these disasters the idea of building dams to control the river and use its water to meet the growing needs of the dry West was formed. By obtaining money from western land sales and irrigation water the Newlands Reclamation Act of 1902 provided the financial means to build these dams. In 1922, the Colorado River Compact was established to control the river, and in the process, divided the river into the Lower Basin (Arizona, Nevada, and California) and the Upper Basin (Utah, Colorado, Wyoming, and New Mexico). See Figure 1. Shortly after the compact was formulized, dam construction in the Lower Basin started. Completed in 1936, Hoover Dam was built to regulate flooding and erosion and provide a dependable water supply and hydroelectric power. Downriver from Hoover Dam, the Davis, Parker, and Imperial dams were built to assist in controlling floods. As part of the compact agreement the Upper Basin had to provide the Lower Basin each year with 7.5 MAF of water. In addition, the 1944 Mexican Water Treaty required the United States to release to Mexico annually .73 MAF of Colorado River water, later increased to 1.5 MAF. This water also had to come from the Upper Basin. Because moisture conditions within the Upper Basin varied greatly from one year to another, the Upper Basin states frequently found it difficult to supply the annual 9.0 MAF of water. To alleviate this situation the U.S. Congress passed in 1956 a bill to build several dams in the Upper Basin. The largest of these dams was the Glen Canyon Dam. Two more large dams, built farther upriver, were the Flaming Gorge Dam on the Green River and the Navajo Dam on the San Juan River. Both of these dams are in headwater sections of the Colorado River Basin. Today, with a dam almost every hundred miles, the Colorado River is the most dammed river in the United States, which results in it no longer providing water to the Gulf of California. Except in very wet years, the river’s delta is a desert, and what water does reach the area simply disappears into the ground in northern Mexico. In addition to controlling the river the compact was also established to make sure that each state within the Colorado River Basin received a fair share of the river’s water. In the early 1920s the states within the basin were concerned about California’s growth, and thereby, its increasing consumption and appropriation of the water within the river. This concern was further exacerbated by the fact that California contributed little water to the river. This concern still exists as California continues to grow and take unused water from the river, beyond its allotment. Arizona was especially disturbed about California’s growing water demands and did not ratify the compact until 1944, 22 years after it was initially negotiated. Arizona’s ratification of the compact was linked to the development of the Central Arizona Project, a 336-mile long system of aqueducts designed to deliver 1.8 MAF of water per year to the state’s southern growth area. However, before this project commenced, California and Arizona had to resolve their differences as to how much water each state would receive from the river. These differences resulted in an 11-year, complicated court case that eventually went to the U.S. Supreme Court. Finally, the case was resolved with California receiving 4.4 MAF, Arizona 2.8 MAF and Nevada .3 MAF. See Table 1. In addition, each state was allowed to use all the water in the tributaries located within the state’s boundaries. Relative to its population size, Arizona was the big winner in this case. In the early 1950s when the case was being litigated, Nevada did not visualize the recent rapid growth of Las Vegas and environs. Today, Nevada might argue for a larger allocation. The Upper Basin states worked together in a more cooperative manner than the Lower Basin states and quickly formulated a contract that allotted 51.75% of the Upper Basin water to Colorado, 23% to Utah, 14% to Wyoming, and 11.25% to New Mexico. Percentages were used rather than actual amounts since the states did not know how much water would be available to the Upper Basin each year due to the combination of precipitation variability and the requirement of providing 9.0 MAF to the Lower Basin and Mexico. Table 1: Colorado River Allocations Political Entity Annual allocation (in acre-feet) Upper Basin States 7,500,000* Colorado 3,900,000* New Mexico 800,000* Utah 1,700,000* Wyoming 1,000,000* Lower Basin States 7,500,000 California 4,400,000 Arizona 2,800,000 Nevada 300,000 Mexico 1,500,000 Total 16,500,000 Source: U.S. Bureau of Reclamation. *Upper Basin states’ allocations based on using percentage values with 7.5 million acre-feet. Precipitation Patterns The arid and semiarid American Southwest constantly faces precipitation variability. What moisture the region receives to feed the Colorado River and its major tributaries, the Green River and San Juan River, is the result of various climatic conditions. A change in any one of these conditions could bring on a flood or drought. The Upper Basin falls mainly on the Colorado Plateau, which experiences both a winter and summer precipitation regime. In the basin’s higher elevations that form its headwaters precipitation falls rather evenly throughout the year, building large snowpacks during the cold months. Cold frontal systems developing over the North Pacific Ocean bring large amounts of precipitation during the winter and spring months. These systems acting like large rivers flowing eastward across western United States carry moisture at high levels in the atmosphere. As these atmospheric rivers encounter the high elevations of the Colorado Plateau, orographic conditions occur, resulting in increasing amounts of precipitation with the increase in elevation. In the San Juan, Uinta, and Wind River mountains these systems create large snowpacks that normally meltdown at a gradual rate during the late spring and early summer to provide water for the Colorado River throughout the summer and into the fall. If these winter frontal systems originate over warmer waters in the Pacific Ocean, precipitation in the form of rain might fall on the mountain snowpacks producing fast, high runoff and floods on the rivers. During the summer regime rain over the Colorado River Basin comes from convectional systems. Low-level moisture arriving from the Gulf of Mexico, the Gulf of California, and the eastern Pacific Ocean generate thunderstorms in July and August.