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5 Nl. ~", ~ ~ Chapter 2 DESCRIPTION OF BASIN The construction and filling of the mainstem reservoirs of the Colorado River Basin (figure 1) have brought about significant changes in the flow patterns of the river. The Colorado River Basin encompasses portions of seven Basin States: Wyoming, Utah, Colorado, Nevada, New Mexico, Arizona, and California. The river flows over 1,400 miles from its headwaters in Wyoming and Colorado. It joins with tributaries from Utah and New Mexico; flows through the Grand Canyon; provides State boundaries for Nevada, Arizona, and California; flows through the Republic of Mexico; and terminates in the Gulf of California. The mainstem of the Colorado River provides municipal and industrial water for more than 18 million people and irrigation water to 1.7 million acres in the United States. CLIMATE Extremes of temperature in the Colorado River Basin (Basin) range from -50 to 130 degrees Fahrenheit. The northern portion of the Basin is characterized by short, warm summers and long, cold winters; and many mountain areas are blanketed by deep snow all winter. Much of the area consists of high basins or valleys with cold winters and hot, dry summers. The southern desert portion of the Basin has long, hot summers, practically continuous sunshine, and almost complete absence of freezing temperatures. Rainfall averages 2.5 inches per year in the southern end of the Basin, while total precipitation in the mountains reaches 40 to 60 inches annually. HYDROLOGY The Colorado River begins where peaks rise more than 14,000 feet in the northwest portion of Rocky Mountain National Park in Colorado, 70 miles northwest of Denver. It meanders southwest for 640 miles through the Upper Basin to Lee Ferry, the dividing point for the upper and lower portions of the Colorado River Basin. The Green River, the major tributary to the Colorado River, rises in western Wyoming and discharges into the river in southeastern Utah-730 river miles south of its origin and 220 miles above Lee Ferry. The Green River drains 70 percent more area than the Colorado River above their junction but produces only about three-fourths as much water. The Gunnison and San Juan Rivers are the other principal tributaries of the Colorado River in the Upper Basin. The Colorado River Basin has a total area of approximately 244,000 square miles, carrying an average annual natural flow of about 15 million acre-feet at Lee Ferry. Of this flow, more than 5 million acre-feet per year are exported to the Arkansas and Missouri River Basins, the Great Basin, southern California, and the Rio Grande Basin.