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2. STUDY AREA The Colorado River was divided into an upper and lower basin by the Colorado River Compact of 1922 with Lee's Ferry as the boundary division between basins (Figure 2.1). The study area includes 26 km of renowned trout fishery in the upper basin above Lee's Ferry to Glen Canyon Dam and 384 km in the lower basin below Lee's Ferry to Diamond Creek. The study area was divided into five reaches, numbered 10, 20, 30, 40, and 50, respectively (Table 2.1). These correspond to reaches established by the United States Geological Survey (USGS) in their study of sediment transport. The 26 km of river from Glen Canyon Dam to the Paria River constitutes Reach 10 and includes the first tributary below Glen Canyon Dam. Reach 20 extends from below the Paria River to just below the confluence of the Colorado and Little Colorado rivers. Reach 20 includes Nankoweap Creek and the Little Colorado River. The third reach, Reach 30, extends from that point to Bright Angel Creek near Phantom Ranch and the heart of the Grand Canyon. Tributaries in Reach 30 include Clear and Bright Angel creeks. Reach 40 begins below Bright Angel Creek and ends at National Canyon, River Mile (RM) 166.5. Reach 40 contains the most perennial tributaries, including Crystal, Shinumo, Tapeats, Deer, Kanab, and Havasu creeks. Reach 50 begins at National Canyon and extends 109 km to above Diamond Creek. Reach 50 is the only one that does not contain a perennial tributary. The river between Glen Canyon Dam and Diamond Creek has been classified into four categories (Howard and Dolan 1981). The first is wide valleys with freely meandering channel. This category is characterized by wide, shallow areas where cobble bars average 215 m in width compared to an average width of 129 m at average flood state (3,500 m2/sec). Such areas include Reach 10, and the upper portion of reaches 30 and 50. The second classification is valleys of intermediate width. These are valleys constricted somewhat by resistant sandstone or limestone. Cobble bars in these areas are less abundant and limited to the wider portions of the river. The third type is narrow valleys in fractured igneous and metamorphic rocks. These are characterized by a narrow, steep-gradient channel and deep pools. The shoreline is often cliff, and cobble bars are rare. This area is often called the "granite narrows" and extends from RM 77 to RM 112. The last category is narrow valley in massive -14-