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<br />002371 <br /> <br />Figure 22. (Below) Graph showing longitudinal profile of the thalweg and contact of sand and gravel that existed in <br />1956. Water surface profiles are those surveyed in 1956 at 79 m3/s and 1966 at 238 m3/s (Grams et aI., 2004, fig. 7). <br />960 <br /> <br /> <br />. 955 <br /> <br />. <br />... : y y <br />. <br /> <br />. <br />. <br /> <br />. <br />. <br /> <br />Ul <br />'" <br />W <br />I- <br />W <br />::;; 950 <br />~ <br />Z <br />o <br />~ <br />a; 945 <br />...J <br />w <br /> <br />-Ir-- Water surface-1956 <br />. Water surface-l966 <br /> <br />. Bed-1956 <br />.-- Gravel interface-1956 <br /> <br />. Bed-I965 <br /> <br />- Bed-200D <br /> <br />. <br /> <br />940 <br /> <br />935 <br /> <br />10 <br /> <br />. <br /> <br />.. . <br />. <br /> <br />15 <br /> <br />25 <br /> <br />DISTANCE DOWNSTREAM FROM GLEN CANYON DAM. IN KILOMETERS <br /> <br />20 <br /> <br />Table 7. Summary of channel cross-section data at Marble Canyon Dam sites <br /> <br />Location <br /> <br />Average change in bed elevation <br />between 1998 and 2000, in meters <br /> <br />....,..,.... <br /> <br />RM 32.8A <br />RM 32.8B <br />RM 39.5A <br />RM 39.5B <br />RM 39.5C <br />RM 39.50 <br /> <br />Average change in bed elevation <br />between 1950 and 2000, in meters <br />o <br />+0.1 <br />- 0.9 <br />- 1.0 <br />- 1.0 <br />-0.4 <br /> <br />5.3.1 Bed Changes in Marble and Grand <br />Canyons That Are Probably Post-dam <br /> <br />o <br />o <br />+ 0.1 <br /> <br /> <br />Comparison of the water surface elevation <br />of the Colorado River between Lees Ferry and <br />Lake Mead reservoir in 1923 and 2000 indi- <br />cates that there was net aggradation of rapids, <br />and that the rate of aggradation probably <br />increased after completion of the dam. Magid <br />et al. (unpubl. manuscript) compared Light <br />Detection and Ranging (LiDaR) data of the <br />elevation of the Colorado River with that <br />surveyed by the USGS in 1923 and compared <br />the elevations of 145 rapids. They found that <br />39 rapids aggraded, 16 degraded, and that the <br />average for all measured rapids was an in- <br />crease in water surface elevation of +0.18 m <br />T 0.55. Magid et al. (unpubl. manuscript) <br />argued that aggradation in rapids increased <br />after completion of the dam, based on statisti- Figure 23. Photograph of one of the alternative Marble <br />cal comparison of the rapids where debris Canyon dam sites, near River Mile 39.5. <br /> <br />5.0 Changes in the Topography of the Main-Channel Bed 33 <br />