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<br />![ - - ---'=c-::_-" <br /> <br /> <br />;1 <br />.-:1 <br />.' ,,;1 <br />!'-' <br />, <br />I <br /> <br />" <br />I <br />I <br />'I <br />:1 <br />I <br /> <br />:1 <br /> <br />. I <br />, " <br />'f I <br />. , <br /> ! <br /> I <br />'. , <br /> , <br /> :1 <br /> " <br /> :1 <br /> :\ <br /> , <br /> I <br /> ,I <br /> ;, <br /> " <br /> I <br /> I <br /> " <br /> ,; <br /> I <br />~, :1 <br />'. <br />. <br />, ': <br /> " <br /> :1 <br /> " <br /> [ <br /> <br />oonnH <br /> <br />- <br />.. -, <br />, <br /> <br />-11- <br /> <br />fluctuation be~ween drouth years and normal years is very apparent. <br /> <br />One of the most significant conditions affecting the area is <br /> <br />the increasing development of irrigation water from wells rather <br /> <br />than from surface diversions. Since the construction of any large' <br /> <br />reservoir in the area is going to change ground water conditions <br /> <br />significantly, the effect must be considered as a part of the <br /> <br />dam site investigations. <br /> <br />SITE GEOLOGY <br /> <br />Narrows &xis (see Figure 2) <br /> <br />The Narrows site is located on a portion of the South Platte <br /> <br />River where the present river channel is several miles to the <br /> <br />north of the old deep channel. The present river flows in a broad <br /> <br />bnt shallow channel containing alluvium averaging 10 to 12 feet <br /> <br />in thicl<ness above the bedrocl<. Drilling at the Narrows site <br /> <br />shows the bedrock to be shOlly siltstone and claystone of the <br /> <br />Pierre formation. On the left abutment bedrock is exposed in <br /> <br />some places and is overlain in other places by dune sand and loess <br /> <br />overburden with thicknesses ranging up to 20 feet. The bedrock <br /> <br />shale at the right side of the present channel rises steeply 25 <br /> <br />to 30 feet, then inclines downward to the south toward the above- <br /> <br />mentioned deep point in the old river channel, which is located <br /> <br />approximately four and one-half to five miles to the south. Along <br /> <br />the proposed axis this deep channel is located close to the pre- <br /> <br />sent surface course of Bijou Creek. The elevation of the bottom <br /> <br />of the deep channel is believed to be around elevation 4215 as <br /> <br />compared to an elevation of about 4300 for bedrock under the pre- <br /> <br />sent river channel, which would be under the main part of the pro- <br /> <br />posed dam. As the shale surface slopes down to the south under <br /> <br />"', '" "'l ~..' <br />