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<br />. <br /> <br />23'2 (~ <br /> <br />t" <br />, ' <br />" ,;,., <br /> <br />C'~struction Methods. <br /> <br />Borings made in 1903 and 1904 on the lower river showed <br /> <br />that at Bulls Head, Williams Fork, Picacho, and other points tested, <br /> <br />the subterranean channel of the river had been eroded to a great <br /> <br />depth, so that the foundation of dams at any of these points on <br /> <br />bed rock was considered infeasible. In view of thi s fact and <br /> <br />of the silt problem, some engineers were led to conclude that <br /> <br />storage a~ aqy point in the lower basin of the Colorado River <br /> <br />was not feasible. Further consideration, however, led to the <br /> <br />evolution of a plan for building a high dam wi thout excavating <br /> <br />the river to bed rock, which, owing to the peculiar topographw <br /> <br />of Boulder Canyon seemed to be at least worthy of consideration. <br /> <br />. <br /> <br />At this point the canyon is about 300 fest \nde at the river <br /> <br />level, and cliffs of massive granite reach upward nearly vertical <br /> <br />to a height of over 2,000 feet above the river. Plans were <br /> <br />evolved for a loose rock dam at this point constructed by the <br /> <br />following method: <br /> <br />It is proposed to pierce the cliffs with large tunnels <br /> <br />on each side of the dam site a short distance above the low <br /> <br />level of the river at medium stages in order that the river <br /> <br />might be diverted through these tunnels at moderate stages <br /> <br />if desired. These tunnels were to be equipped with control_ <br /> <br />ling works. For a dam say 600 feet above the r'ver Jevel <br /> <br />it was proposed to provide slopes for a rock fill dam of <br /> <br />three to one on each side when counted from becl tock to <br /> <br />. <br />