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<br />watershed have caused frequent flood problems along areas now protected <br />by John Martin Dam. Completion of the dam, which began storing water <br />in December 1948, greatly reduced the severity and frequency of <br />flooding in the lower Arkansas valley. Flooding from major tribu- <br />taries that enter the Arkansas' main stem below the dam has continued <br />to plague the lower communities, as the unusual June 196~ flood demon- <br />strated. The high peaks resulting from this flood led many local <br />people to believe that "the dam had broken." Actually, the vast <br />reservoir storage area behind the dam had contained the destructive <br />flood originating in the upper watershed and prevented a disaster of <br />major proportions in the lower valley. <br />7. The major uncontrolled areas that contributed most towards <br />flood peaks in 1965 lie south of the Arkansas' main stem. Although <br />Big Sandy Creek (a north-bank tributary and the largest single drain- <br />age area consisting of 3,426 square miles) attained peaks of 60,700 <br />cubic feet per second near Calhan, Colorado, only 3,600 cubic feet per <br />second were recorded at the creek's mouth. Valley storage and high <br />infiltration, along with numerous undefined noncontributing areas <br />and several small Soil Conservation Service flood-retarding structures, <br />act to substantially reduce flood peaks throughout this major drainage <br />basin. The total uncontrolled contributing drainage area of about <br />5,570 square miles (including Big Sandy Creek) exists within the first <br />60 river miles from John Martin Dam to about the Colorado-Kansas State <br />line. In contrast, only about 1,700 square miles of contributing <br />drainage area enter the reach between the State line and Kinsley, <br />Kansas, a distance of about 180 river miles. This reduction results <br />from the White Woman Creek closed basin, which contains 2,250 square <br />miles of noncontributing drainage area. Surface runoff from intense <br />storms occurring over the central portion of the watershed is greatly <br />reduced by infiltration and surface storage within this closed basin. <br />8. Broad flat valleys and mild channel gradients combine effec- <br />tively to create temporary storage areas in the Arkansas River flood <br />plains. The detention of significant floodwater volumes extends <br /> <br />" <br /> <br />; <br /> <br />, <br /> <br />A-4 <br />