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002355 Table 4. Estimates of the potential fine-sedimentstorage volume in 9 EDZs. EDZ name EDZ Area Area of Void volume Percent Thickness (NAU designation) area, in surveyed comparison, between the stage overlap of void square by NAU, in square of 100 m3/s and the between volume, in meters in square meters minimum elevations EDZ and meters meters surveyed by NAU, area in cubic meters surveyed by NAU Cathedral 11,658 8392 7124 25,122 72 3.53 (RM 3) Fence Fault 11,479 9448 4954 8949 82 1.81 (RM 30) South Canyon 10,837 9536 4316 11,877 88 2.75 (RM 32) Anasazi Bridge 25,348 11 ,318 4545 12,412 45 2.73 (RM43) Eminence Break 80,259 30,377 12,884 34,776 38 2.70 (RM 45) Saddle Canyon 44,977 29,935 21,831 92,797 67 4.25 (RM 47) Crash Canyon 20,103 17,816 14,878 92,787 89 6.24 (RM 62) Carbon 20,253 18,123 10,971 24,451 89 2.23 (RM 65) Tanner 11,476 9422 4269 11,822 82 2.77 (RM 68) p..,.' Eddies have an enormous capacity to store fine sediment. They have the potential to store as much fine sediment as Topping et a1. (2000b) estimated accumulated seasonally prior to completion of Glen Canyon Dam. We determined the composite lowest elevation surface surveyed by NAU in the 1990s in 9 EDZs by integrating all surveys and .dete~in- ing the lowest measured value at each grid node. We subtracted this topographic surface from a flat surface whose elevation is the stage at 100 m3/s. This flat surface conforms to the lowest stage used in the calculation of EDZs. The difference between these two surfaces is a conservative estimate of the fine sediment storage potential in the eddy. The volume between these two surfaces, divided by the area of comparison, is between 1.8 and 6.2 m (Table 4). The mean thickness is 3.2 m. Application of this thickness to the total estimated area ofEDZs yields estimates of 8.32 x 106 and 4.2 x 106 m3 for the potential storage volume of eddies in Marble and upper Grand Canyons, respectively. The potential storage volume is 13.1 x 106 and 6.5 x 106 metric tons for Marble and upper Grand Canyons, respectively. These values approxi- mate the upper range of seasonal fme-sediment accumulation estimated by Topping et a1. (2000b ). There is substantial longitudinal variation in the number and size of EDZs. The fre- quency of EDZs largeF than 1000 m2 varies twofold among the reaches (Table 3). There are 2.2 EDZs larger than 1000 m2Jkm in the Lees Ferry Reach, and there are 5.6 EDZs per kilometer in the Tapeats Gorge Reach. The largest EDZ in these reaches is 67,000 m2 and occurs immediately downstream from Paria Riffle, at RM I.2R (Fig. 7). The second largest EDZ is located between Comanche and Tanner Creeks in the Big Bend Reach at RM68.2L~ this EDZ is 52,800 m2 in area. Other large EDZs are Saddle Canyon (RM47.6R; 45,000 m2) and Triple Alcoves (RM47.1R; 43,300 m2). Although there are some very large EDZs in the study area, these are so few that they do not account for a large proportion of the total area 3.0 The Valley of the Colorado River 17