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002375 ...J W > W ...J <{ W (/) Z <{ W :a; w 6 lD <{ yj c:: UJ I- W ::E ~ Z o ~ > UJ ...J UJ 925 -85 m'/s 232 m'/s .~.- ~- 920 1299 m',s 2800 m'/s -'- _"'" -- ~ -_~~_~_:___~=~.~ <, : ~ 1273m'/s _ ......... ..~-.---............._......... -800m3/s P-32 - - - - - - - - - :.....'....~::..-.::.:::~~ ---- -- -- -- -:~.,-.,..:..,--:-_... . .... '- ~ '~~~~=~==~~---------- .~ 915 910 bed February 1996 range in bed elevation August 1992... August 1995 905 ~~ o 200 400 600 800 DISTANCE. IN METERS. FROM AN ARBITRARY DATUM, ALONG CHANNEL CENTERUNE Figure 27. Graph showing longitudinal profile of the water surface at Badger Creek Rapids at 3 discharges, surveyed in 1985 by J. C. Schmidt, and at 2 discharges, surveyed in 1996 by NAU. Elevations are those surveyed at the water's edge, and the flat or adverse slope downstream from the rapids is that of the upstream flowing eddies that occur along both banks. Discharge at time of measurement is based on unit values measured at Lees Ferry gage: 1299 m3ts determined from high water marks of the 1996 Controlled Flood; 1273 m3ts surveyed on May 19, 1985; -800 m3ts surveyed July 31, 1985; 232 m3ts surveyed March 22, 1996; -85 m3ts surveyed October 5, 1985. Water surface at 2800 m3ts based on field surveys of the estimated water's edge determined from a June 1952 photograph (Fig. 29A). Bed elevations measured by NAU on February 15, 1996, and at cross-section P32 by Graf et a!. (1995, 1997). "",~-.... widths in length and channel geometry effects produce substantial scatter in temporal patterns of specific sites. The average trend indicates that pools tend to scour during increasing discharge and to fill during decreasing dis- charge (Fig. 26). This behavior is similar to that observed in the ponded backwater imme- diately upstream from the Grand Canyon gage during the 1996 Controlled Flood (Topping et at, 2000b), and to that observed during pre- dam floods in the ponded backwater of the Lees Ferry gage (Colby 1964, Howard and Dolan 1981, Topping et at, 2000a). Resurvey of monurnented cross-sections between 1992 and 1999 also did not indicate accumulation of fine sediment on the bed (Flynn and Hornewer, 2003). Most of these cross-sections were located downstream from the Paria and Little Colorado Rivers; others were located elsewhere between Glen Canyon Dam and Lava Falls Rapid, which is down- stream from the study area. Most of these cross-sections describe the main channel in ponded backwaters, but some describe the main channel pool downstream from small rapids and a portion of the adjacent eddy. Flynn and Hornewer (2003) found that 61 of 83 cross-sections had a net loss of bed sedi- ment and only 19 had a net gain. Of 57 cross- sections located within 10 krn downstream from the Paria or Little Colorado Rivers, 55 had a net loss of bed sediment. The only time these cross-sections filled was immediately following tributary floods, and the duration of main channel aggraded conditions was short. 6.0 THE HISTORY OF FINE SEDIMENT STORAGE AT SPECIFIC SITES Temporally detailed records of sand bar volume were reconstructed for a few sites in the study area, based on integrating data from aerial and oblique photographs and field surveys. These data illustrate the processes that lead to aggradation and degradation of fme sediment. The description of Badger Creek Rapids emphasizes flow characteristics and the characteristics of sand deposits during the pre- dam and post-dam eras. The description of Eminence Break camp emphasizes patterns of long- and short-term bar change. 5.0 Changes in the Topography of the Main-Channel Bed 37