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<br />e' recharge from the Colorado niver. It has been estimated that <br />_ the groundwater flow westward from the lit:!itrophe section of <br />the Colorado River was about 100,000 AF/yr during the 1939 <br />conditions. <br /> <br />Figure A-9 Sh01>'S significant changes in the groundwater as <br />compared to those which occurred in 1960. Leakage from the <br />All-American Canal was the main cause for changes in flow <br />direction. The water level had risen an average of 30 feet <br />along a 25 mile stretch west of Pilot Knob. This increase of <br />'vater in the subsurface caused the groundwater to flm,' southerly <br />from California' to Baja California in ~lexico. Another major <br />change ,vas the development of a groundl,ater mound beneath Yuma <br />Mesa which rose to a height of 160 feet (Illsl) due to the reellarge <br />from irrigation ,wter which was diverted onto the Yuma Mesa <br />lands. This development resulted in the establishment of steep <br />gradients radiating outward in all directions from the mound. <br />Thus, large amounts of water were added to the groundwater <br />storage reservoir by the diversion of Colorado River water for <br />irrigation purposes. <br /> <br />Figure 10 is a composite groundl,'a ter contour map of the <br />Colorado delta region 1'Ihich depicts conditions as they existed <br />in the early 1970 's. The vintage of the ,..-ater level data ranges <br />f-rom-the-l'a-t-e-l:96e"-s-t-o-abO'U-t-i-972-tSonora-Mesa-wer-l-s-)--;-burit <br />provides a reasonable manifestation of thc knmm groundwater <br />conditions for the purposes of this report. Examination of <br />water levels reveals that the groundwater mounds along the All- <br />AJ:lerican Canal system and beneath Yuma ~lesa have reaehed near <br />"steady-state eondi tions" and that the drainage ,>,ells along the <br />east side of Yuma Valley and in South Gila Valley l>'ere pumping <br />nearly unifol'L1 amounts in the late 1960 I S al:d early 1970' s. <br />These eonditions provide n basis to examinc the quantitativc <br />aspects of the groundwater system in the upper part of the <br />Colorado dclta region in United States. <br />