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<br />II. DRAINAGE BASINS <br /> <br />". <br /> <br />A. Major Basin Characteristics <br />The major drainageway that traverses through the Yuma area heads <br />approximately nine miles west-southwest of the Town. The apparent <br />total basin boundary down to State Highway 59 (northeast edge of the Town <br />of Yuma) encompasses approximately 71 square miles, however a significant <br />portion of this basin drains into closed depressions and does not contri- <br />bute surface flow, These areas and ponding depressions have been identified <br />by review of the Soil Conservation Service soil survey work for Washington <br />and Yuma Counties plus Hydro-Triad, Ltd. field reconnaissance work. The <br />actual surface flow contributing basin totals only 16,7 square miles. This <br />area breakdown does not include the sub-basins immediately south of the <br /> <br />Town of Yuma, and those within the corporate limits that drain either to <br />Lake Yuma or eastward across State Highway 59. The drainage area of these <br />sub-basins totals 1,6 square miles. <br />The basal sedimentary rock throughout this area, the Ogallala <br />formation, is of late Tertiary age. The bedrock is mantled to varying <br />depths by some residual soils derived from the basal formation but primarily <br />by 10essia1 deposits and sand that has been wind transported and assorted <br />into low hummocks and dune1ike sequences. Some of the depressions formed <br />by the wind movement are as much as thirty feet deep although the average <br /> <br />is more on the order of six to eight feet. <br /> <br />-2- <br />