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00235~ Figure 11. Diagram showing the method of calculation of EDZs. EDZ is shown at bottom of figure. Ferry and Badger Creek Rapids (Sondossi, 2001), in the Redwall Gorge Reach between Twentynine Mile Rapids and Nautiloid Canyon (Sondossi et aI., 2002), in the Point Hansborough Reach near Point Hansborough (Schmidt and Leschin, 1995; Schmidt et aI., 1999b), in the Tapeats Gorge Reach between Sixtymile Rapids and Lava Chuar Rapids (Schmidt and Leschin, 1995; Schmidt et aI., 1999b), and the Big Bend Reach between Lava Chuar Rapids and Unkar Rapids (Schmidt and Leschin, 1995; Schmidt et aI., 1999b) (Table 3). These maps were overlain in a GIS, and the polygon enclosing the area where eddy bars occurred in any of the years of available photography was determined (Fig. 11). This polygon, referred to as the eddy deposition zone (EDZ), is a surrogate for the area of recirculating flow. Although this polygon is smaller than the total area of recirculating flow, it has the advantage of being defmed without the necessity of field observations at many discharges, and EDZ areas can be com- pared among different parts of the study area. There are 287 EDZs in the mapped reaches in Marble and upper Grand Canyons, of which 183 are larger than 1000 m2 and for which we measured the area of sand in each aerial photograph series. The average number ofEDZs larger than 1000 m2fkm in Marble Canyon is approximately 3, which leads to an estimate that there are approximately 300 EDZs larger than 1000 m2 in Marble Canyon. The total area ofEDZs varies between 16,700 and 50,900 m2fkm. The total area ofEDZs in Marble Canyon is about 26,000 m2fkm, which leads to an estimate that there is approximately 2.6 x 106 m2 ofEDZ in Marble Canyon. EDZs comprise approximately 20% of the water surface area at the approximate discharge of the pre-dam average flood, because the aver- age width ofthe channel at 2700 m3/s in Marble Canyon is about 130 m. There is more uncertainty in the estimate of these values in upper Grand Canyon, be- cause the distribution of EDZs is unknown in Upper Granite Gorge. The total area of EDZs is approximately 1.3 x 106 m2, based on the actual data for EDZs in the Tapeats Gorge Reach and Big Bend Reach, application ofthe Big Bend Reach average to the remainder of the Big Bend, and application of the Redwall Gorge Reach average to the Upper Granite Gorge. 16 System-wide Changes in the Distribution of Fine Sediment in the Colorado River Corridor ...