Chapter Nineteen 
Water Supply, Use and  
Management 

LEARNING OBJECTIVES        Although water is one of the most abundant resources on Earth, there are many important issues and problems involved in water management.  This section should foster your understanding of concepts presented in the text on this subject, including issues relating to water management and the impacts of water projects such as dams, reservoirs, canals, and channelization.  CASE STUDY  - The Colorado River.       From its headwaters in Wyoming to its discharge point into the Gulf of California, the Colorado River drains much of the southwestern United States.  The river basin is approximately 632,000 square kilometers (244,000 square miles).  Considering its size, the river has only a modest flow, but it is one of the most regulated and controversial bodies of water in the world.  The total flow of water in the river has been appropriated among the various users, including seven states and Mexico.  Today, Colorado River water only occasionally flows into the Gulf of California--it's all stored and used.  As a result, ecosystems of the lower river and delta, being deprived of water and nutrients, have been damaged.         The complex issues of water management for the Colorado River illustrate some of the major problems that are likely to be faced by other semiarid regions of the world in the coming years: How are we going to appropriate  water resources when they are scarce?  How can we best control water quality?  How can we protect river ecosystems?  There are no easy answers to these questions.        The water of the Colorado River basin is completely spoken for and distributed by canals and aqueducts to millions of urban people as well as to agricultural areas.  Management of the waters of the Colorado River has been frustrating in part because of inherent uncertainties concerning how much water can be expected in a given year.  Because water resources in the basin include snow melt, long-term winter precipitation, and short-term summer thunderstorms, the total water available on a year-to-year basis is tremendously variable.         Legal entitlements of the basin's water exceed the actual distribution.  The reason distribution can be maintained is that the Colorado River is one of the most managed rivers in the world.  The major dams and reservoirs in the system store approximately 70 million acre-feet of water.  The two largest reservoirs--Hoover and Glen Canyon dams--store about 80% of the total in the basin.  This storage provides a buffer of several years water supply, but if there are several drought years in a row, maintaining a sufficient water supply for all users may not be possible.         The Glen Canyon Dam, completed in 1963, is upstream from the Grand Canyon.  From a hydrologic viewpoint the Colorado River is changed forever by the dam.  The river is tamed; the higher flows have been reduced and the average flow has increased.  Changing the hydrology of the river also changed other aspects, including the rapids, distribution of sediments that form sandbars, and vegetation near the water's edge.  Because natural flooding of the river has been stopped, periodic release from the dam of flood waters has proven to be very beneficial because floods are a necessary part of the river ecosystem if it is to function on a sustainable basis.       Conflicts over Colorado River water use and water quality have spanned decades and extend far beyond the river basin to large urban centers and developing agricultural areas in California, Colorado, New Mexico, and Arizona.  The crucial need for water in these semiarid regions has resulted in overuse of the limited supplies of water as well as deterioration of water quality.  International agreements, interstate agreements, and court settlements have periodically intensified or eased tension among users of Colorado River water.  These court decisions and laws, along with changes in water use in urban and agricultural areas, continue to significantly influence the lives and livelihood of millions of people both in the United States and in Mexico.         FAQs . . Photo Credit: EPA; U.S. Geological Survey