Laserfiche WebLink
<br />0;)2;)33 <br /> <br />11 <br /> <br />.;:;.', <br /> <br />Sedimentary structures studied in flood deposits revealed that bed forms evolved <br />during the flood from climbing ripples to cross-stratified dunes. This shift correlated closely <br />with coarsened sediment textures seen in beach trenches. Usually, this bed-form evolution <br />would be associated with increased flood power associated with, but in this case the <br />phenomenon could only indicate sediment limitation, since flood magnitude was constant. <br /> <br />Debris Fans and Rapids: Twenty-five new debris flows occurred in Grand Canyon between <br />1987 and 1995, and these flows created 2 new rapids and narrowed 9 other rapids. Recently <br />aggraded debris fans and rapids were reworked by the 45,000 cfs flood to varying degrees, <br />depending on local site conditions and on the length of time since debris flows occurred. The <br />1996 Test Flow rearranged many of these fans and increased the width of many rapids. Most <br />of the reworking of the fans occurred during the rising limb of the 1996 Test Flow (Webb et <br />a1. 1997). Older deposits generally showed less response to flood reworking than did the. <br />most recent sites, such as Badger and Lava Falls Rapids. <br /> <br />Fan and rapid reworking was more dependent on peak flood magnitude than on flood <br />duration. A flood with a higher peak flow for a single day would likely have had a more <br />significant reworking impact than the 1996 Test Flow. If aggraded fans and rapids are a <br />management concern in the future. then larger flood peaks of shorter duration following <br />debris flows may be most effective in reversing aggradation off an-eddy complexes; however. <br />sllch flows are presently outside the flow criteria specified in the ROD. <br /> <br />None of the debris fans aggraded by debris flows since 1986 were completely restored <br />to pre-debris flow conditions. Increased fan elevations resulting from recent debris flows have <br />had an enhancing effect on low-velocity pools that form upstream of these natural check <br />dams. Enhancement of upper pools has been associated with increases in bed-storage capacity <br />for sand locally. Hence. long-term increases in channel-stored sand above aggraded fans will <br />likely occur through time as future debris flows occur; especially if such sites remain only <br />partially reworked by floods similar to the 1996 Test Flow. <br /> <br />Campsite Rejuvenation: The 1996 Test Flow greatly increased campsite availability, and <br />campsites remained enlarged for at least 6 months following the event. Fifty percent of all <br />campsites in all reaches increased in usable area (Kearsley and Quartaroli 1997). Eighty-four <br />percent of all beaches monitored by the Colorado River guides and located in critical reaches <br />increased in usable area (Thompson et al. 1997). However, the 1996 Test Flow did not <br />restore sandbars to their pre-dam condition. nor were they restored to their condition of the <br />mid- 1980s (Kearsley and Quartaroli 1997, Schmidt 1996). Prolonged high flows in 1996 and <br />1997 subsequently eroded many of those campsite beaches. <br /> <br />Sediment and Cultural Sites: It has been hypothesized that dam-related erosion of channel <br />margin sand deposits and reduced high flows, has resulted in headward erosion of arroyos and <br />the archeological sites they protect. Interuption of the process of spring-flood sediment <br />deposition by Glen Canyon Dam may have exacerbated erosion of these culturally significant <br />sites <br /> <br />"'''-T' <br /> <br />Final Draft - 12/12/97 - For AMWG Review <br />