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<br />I <br />t <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I, <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />OD254r~ <br /> <br />The basin is presently about 50 percent full, and 1 million cubic yards of material will be <br />excavated and disposed of in the adjacent established disposal sites within the next 3-5 years. <br />The work in the Laguna Basin is required periodically as a routine maintenance function. <br /> <br />The time span between dredging depends on the sediment load. Under the present forecasts <br />it appears the work may be scheduled at 5 year intervals. The settling basin is approximately <br />4,000 feet long by 500 feet wide and is normally excavated to a depth of about 25 feet. The <br />total capacity is approximately 2 million cubic yards. Working a two-shift schedule it takes <br />approximately 12 months to excavate the basin when it is half full. It is necessary to keep <br />the basin at about half capacity or less, since the trap efficiency (ability to capture sediment) <br />drops off dramatically as it passes the half-way point. <br /> <br />Included within this division is Mittry Lake, a shallow lake east of the Colorado River <br />channel and north of Laguna Dam. The lake has a surface area of approximately 750 acres <br />and is fed by an inlet structure originating at the head works of the Gila Gravity Main Canal. <br /> <br />Yuma Division <br /> <br />The Yuma Division is the reach of the river located between Laguna Dam and Morelos Dam. <br />The river channel extending from Laguna Dam to the upper end of the diversion pool above <br />Morelos Dam was formed by the undiminished natural flow of the river before the dams <br />were constructed. This dominant flow, the flood flows most affecting the channel shape, <br />averaged about 20,000 cfs with maximum flows in the early 1900s exceeding 200,000 cfs, <br />depending upon the time of year and location within the division. While the historic riverbed <br />averages 600 feet in width, only about 120 feet is presently occupied by river flows. The <br />remaining portions of the riverbed, at or near the elevation of ground water, support various <br />growths of vegetation: cattails, cane, arrowweed, saltcedar, mesquite, cottonwood, etc. <br />Above Yuma, vegetation has been partially controlled by intermittent programs of vegetative <br />control - mowing or cultivating. <br /> <br />A 1969 plan for this division called for renovation of the low-flow channel by dredging, <br />reshaping, and lowering the water table under the remainder of the riverbed, and instituting a <br />program of vegetative control. After completing most of the work in the upper 6 of the <br />20 miles of river channel in the division, the work was suspended pending resolution of <br />environmental concerns. These concerns were met by dredging the area which is currently <br />the open water in Mittry Lake. Prior to that, little open water existed. The lake is now <br />heavily used for fishing. <br /> <br />During the high flows of 1983-1984 the channelization work was destroyed, and the river <br />attacked the levees in several places, which resulted in emergency maintenance. The whole <br />floodplain was essentially inundated, and farm drainage was severely affected. <br /> <br />The 1993 Gila River flood deposited 10 million cubic yards of sediment in the Colorado <br />River channel from the confluence of the Gila River to Morelos Dam and raised the river <br /> <br />Appendix C - 9 <br />