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"i The Colorado River Basin can be naturally divided into two major basins; an upper basin and a lower basin. It is often described as an hourglass with the neck being the river section before it enters the Grand Canyon near the historic site of Lee's Ferry. The upper river drains the high mountains that rim the upper basin and the high mesas and plateaus in between. The lower river begins as the river enters the Grand Canyon and runs to its mouth in the Gulf of California. The upper and lower river basins can be further divided by their major tributaries. Lower Basin Tributaries The Gila River is the major tributary on the lower river. It drains over 53,000 square miles of central and southern Arizona and western New Mexico.22 While it is a very large drainage area, the Gila River System watershed is primarily desert. Almost all of the runoff originates on a very small portion of the watershed along the Mogollon Rim and the White Mountains. The Mogollon Rim runs from central Arizona south and eastward into New Mexico. The elevation ranges from 6,000' to 8,000'. The White Mountains in eastern Arizona and western New Mexico have a handful of peaks that exceed 10,000'. The Gila River is a "wasting" river. In 1947, Reclamation estimated that the natural flow of the Gila River at its confluence with the Colorado River near Yuma was 1,272,000 afper year.23 Because the study period was 1897-1943, this is probably a high estimate. Other studies have suggested a natural flow more in the range of 900,000 afper year.24 However, the estimated natural flow of the Gila River as it enters the Phoenix, Arizona area for that same 1897 to 1943 period is 2,280,000 af per year, over a million acre feet more that its flow at the mouth.25 Thus, as it flows from Phoenix to its mouth, it naturally loses over a million acre feet of water. Although it may contribute a relatively small portion of the flow of the Colorado River at its mouth, the Gila River is the most politically sensitive and difficult tributary of the Colorado River. In all but the very wettest months of rare years the Gila River is completely used and no water makes it past the Phoenix area, let alone all the way to Yuma.26 The Gila River system is almost exclusively used in Arizona with a little bit of use in New Mexico. While the issue of how much water Arizona uses upstream versus the amount that would "waste away" if it were not being used may seem like trivia, it is at the very heart of the unresolved legal issues still simmering within the Basin. The Gila River was Arizona's primary motivation for 22House Document 419, page 172. 23House Document 419, TABLE CXL VI, page 284. 24House Document 419 estimates the virgin flow of the Colorado River at 16,270,000 afat Lee Ferry for the same 1897-1943 period. (See TABLE CXL, page 281). 25See footnotes #7 and #8. Again, this estimate is probably high based on the 1897 to 1943 period of record. 26The USGS website, http:/waterdatausgs.gov. Look at the historic flows for the Gila River below Painted Rock and the Gila River near Dateland, Arizona Page -11-