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" O I" ,;,) '7 0) U.L {... c' " often coated with a R~iceable white coating. Flat l~g surfaces of the more recently exposed rocks have a soft, powdery coating of mossy plant growth dis- colored somewhat by carbonate evapori tes, all of which can be uiped away by a bare hand. Near the high water line, carbonate evaporites from wi eking or wave , splash from lake water leaves a thin whitish mark. Some rock surfaces seem to be too non-porous, hard, smooth or o,therwise incompatible to the formation of a high water mark and development of the plant coating. The Kayenta sandstone in the walls along the streambed under Rainbow Bridge is a good example of an ,unaffected rock type. Dislocation of gravel, cobbles and small boulders by visitors and the constant ~tirring of the remaining sand by their feet is having a noticeable cumulative effect near the base of the east leg of Rainbow Bridge. A measure of change since man has made heavy use of, the site is the position of a very old fire pit a few feet from the leg. The fire pit (Indian or early whites?) is under- cut on the trail side (downhill side) about 4 to 5 feet. Most of the material here is sand and is lost by wind erosion. It is noted that the thin and scattered cobble surface pavement helps hold the sand. When the cobbles are carried away and the sand blows away, roots of the few desert plants are exposed to where the plant is lost, whereby erosion further increases. Some plants are tenaciously clinging to life on pedestal root columns. This process is causing a very slight loss of alluvial armoringat the underside toe of the Bridge; however, the only noticeable erosion to the sandstone here is by name carving by visitors. Name carving of significance continues on top of the Bridge where the sandstone surface is softer than usual due to weathering. There seems to be some notoriety to leaving one's initials on top of the Bridge similarly to the cairns on the'mountain peaks. The name carving is illegal but unpreventable at present and the loss of rock material in a few spots is quite noticeable from year to year. Sedimentation Surveys A flash flood occurred along Bridge Creek on July 31, 1976. Lake Powell was at about its maximum level of 3,670 feet and shallow backwater had progressed barely under Rainbow Bridge. As the flood reached the slack, lake water, dropped its suspended solids which formed a deltaic deposit a few feet thick and several hundred feet along the streambed was formed from sediments caused by the flood water. As the lake receded, mud became visible and the creek is presently , slowly washing it dOlnl its natural course. ' Meteorological Data The meteorological raft with appropriate instruments stationed at the Courtesy Boat Dock in Bridge Creek Canyon is operating but has a few problems. The equipment is continuously recording air and water temperatures satisfactorily. The instruments for wind and relative humidity have not functioned well and are giving incomplete data at best, partly due to known vandalism. Repair and replacement of the equipment is on a continual basis. Summaries for the four report periods #6, #7, #8 and #9 are as follows: 4 ~;t , ~~!~ i~~ t1P' ~~';$!t r~ )!Jf'~ I , !~~ :>~'8-~ kl~~:~ :<'.>.~ f:.ti~? ~:.~~~,~" > - - -.~-." rA~':':i ;:_,/,:~<;: ~;>.(~. ..... ..~:: -~._>.;; .' :.~.::.(~~ '-<";.,:~;2:, ~'~{~.f 1;;r1;i~r{:~~~Z;~it~~1i~~~i~\$~ift:;;~lt\t'i;fli;r0~8~r1~}~:1.~;~~~j~i~~~0~~!~f:~I?fi~1t1f~I~~t;~tfif~~f~~I~ffa;,~~tt~Jit}Z~iI~~!l,~