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<br />" ~ <br /> <br />-0.,' <br /> <br />J <br /> <br />N <br />-...l <br />t.Jl <br />~ <br /> <br />23. The volume of sediment deposited in John Martin Reservoir <br />since the 1942 survey is 70,654 acre-feet. The average annual sedi- <br />ment deposition for the 24.3-year period is 2,908 acre-feet. The <br />net increase in deposition for the 4.5-year period from March 1962 <br />to September 1966 is 11,269 acre-feet. <br /> <br />24. Spe~ia~ surveys at AT&SF Rai~way bridqe. A railroad bridge <br />crosses the Purgatoire River near the head of John Martin Reservoir <br />at Sediment Range 22. The bottom of the steel girder is at elevation <br />3,874.1, which is 4,1 feet above the maximum water surface, eleva- <br />tion 3,870. The lowest point in the channel under the bridge, as <br />shown on plate 16, was originally 10.5 feet below the maximum water <br />surfacee The opening between abutments, exclusive of the bridge <br />piers, was originally about 8,000 square feet. Although the pool <br />created by John Martin Reservoir has never been closer to the bridge <br />than about one mile downstream, resurveys at the bridge and at sec- <br />tions above and be low the bridge have been conducted frequent ly to <br />determine if the river channel in the vicinity of the bridge has <br />materially changed and if these changes could be attributed to the <br />operation of John Martin Reservoir, ^ summary of the results of these <br />resurveys is shown in t:ab Ie 5. The great:est change since the original <br />survey in 1942 was a reduct:Ion in the openIng of 1,053 square feet:, <br />as indicated by the 1957 resur~ey. The resurvey of 1951 showed an <br />increase in the opening of 630 square feet. The 1966 resurvey indi- <br />cates a reduction of 757 square feet, or 9.46 percent. All the <br />changes to date are attributed to natural causes of the streams and <br />not to the effects of John Martin Reservoir. <br /> <br />25. Suspended sediment sampling. The results of load computa- <br />tions from suspended sediment sampling at the inflow and outflow <br />stations at John Martin Reservoir are shown in table 6. Annual <br />suspended sediment loads for the inflow and outflow gaging stations. <br />are shown in table 7. The total suspended sediment inflow to John <br />Martin Reservoir is shown in table 8 and includes the estimated in- <br />tervening sediment inflow between the two gages at Las Animas, Colo- <br />rado, and the dam. The figures in this table were used to compute <br />the trap efficiency from 1942 to 1966, inclusive. Suspended sediment <br />sampling on a daily basis was discontinued at the inflow stations on <br />1 October 1962. The present program is one of intermittent samples <br />taken by the U.S. Geological Survey. Daily samples are taken at the <br />gage below the dam when releases are made from the reservoir. <br /> <br />26. Lake deposit samp~es. Eleven samples were taken from the <br />deposited material in John Martin Reservoir and were tested for <br />particle size, moisture content, and amount of soil solids. Results <br />of the tests for particle size distribution are shown in table 9. <br />The samples were taken under water and should be representative of I <br />sediment deposi ted in the period between the latest resurveys. S -"'-- <br /> <br />7 <br />