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<br />4223 <br /> <br />. <br /> <br />The flood of June 17, 1965, most of which entered the main stem <br />of the Arkansas River below Pueblo damsite, was the result of a <br />much more intense storm cell centered on the divide between the <br />Arkansas and South Platte drainage basins near Colorado Springs, <br />plus an additional cell in the Springfield area. The June <br />storm caused extensive flooding in Chico Creek, a tributary of <br />the Arkansas River, and in Jimmy Camp Creek, a tributary of <br />Fountain Creek. It had a 24-hour intensity of up to 14 inches <br />at some locations in El Paso County. Part of the runoff of <br />this storm flowed into the South Platte River Basin. Had this <br />intense storm been centered slightly more to the south, the <br />Arkansas River Basin would have received the entire brunt of <br />it. <br /> <br />b. Ground water <br /> <br />Ground water is an important source of irrigation supply in the <br />Arkansas Valley. Water pumped from the valley-fill aquifer <br />between Pueblo, Colorado, and the Colorado-Kansas state line is <br />estimated to have increased from 2,300 acre-feet in 1940 to as <br />high as 181,000 acre-feet in 1964. During the 1959-1968 period, <br />an average near 140,000 acre-feet annually was pumped from the <br />aquifer in this reach of the valley. Return flows from irriga- <br />tion with ground water contribute to the Arkansas River stream; <br />flow. <br /> <br />. <br /> <br />The principal ground-water occurrence in the Arkansas River <br />Basin is in the valley-fill aquifer. The reach of the Arkansas <br />Valley from Pueblo to the Kansas state line is underlain by <br />unconsolidar:ed clay, silt, sand, Cind gravel ue.!-,us.ils. Tl11.s <br />alluvium occupies a trough eroded in the shalet limestone, and <br />sandstone bedrock. The bedrock is relatively impermeable and <br />acts as a barrier to downward ground-water movement. That part <br />of these unconsolidated deposits that is saturated forms an <br />unconfined aquifer. The valley-fill aquifer is from 0 to more <br />than 250 feet thick and ranges from 1 to 8 miles wide. It <br />contains about two million acre-feet of water in storage. The <br />aquifer is hydrologically connected to the Arkansas River. <br /> <br />Ground water also occurs in the Ogallala formation on the <br />uplands of the Arkansas River drainage basin in eastern <br />Colorado. The Dakota and Cheyenne sandstones are also aquifers <br />in this part of the State. <br /> <br />lII-35 <br /> <br />. <br />