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<br />"' <br /> <br />Yu[!]a Nesa ,,'ater Hound Development <br /> <br />A groundwater mound has been created beneath the Yuma Hesa <br />by return flows from irrigation waters on Yuma Mesa. The re- <br />sulting ground1mter, flO1v pattern radiates out1mrd from Yuma <br />Mesa in all directions. Th~ underflow is northward to the south <br />Gila Valley, soutlnmrd across the Axizona-Sonora boundary and <br />lvest1\'ard from the mesa to beneath Yuma Valley. A water budget <br />analysis, indicates a total groundwater underflo1v, across the <br />International Boundary, of about 70,000 acre-feet per year <br />(Figure A-ll). <br /> <br />The findings of the geologic and hydrologic investigations and <br />analog model studies indicate the following conclusions: <br /> <br />1. There is no longer any large quantity of natural recharge <br />to the groundwater basin along the Lower Colorado River, <br />because the flow of the river in the boundary section - <br />natural source of recharge - has been reduced to relatively <br />small amounts, lvi th the possible exception of infrequcnt <br />years of high flows. <br /> <br />2. The dominant source of recharge to the basin under present <br />conditions is "man-made" from irrigation of lands and per- <br />colation from canals. The principal contribution to the <br />basin in the Yuma area is from the irrigation on the Yuma <br />Hesa in the United States which accelerated in 1945 and <br />by 1965 resulted ~n an accumulation in groundwater storage <br />ull(ler the mesa of about 1.5 to 2.0 million aere-fcet to <br />form a groundwater "mound", ld th outflo1r radiating in all <br />directions (Figure "\-11). The groundl,a tcr flo1V south1,ard <br />across the Arizona-Sonora boundary east of San Luis 1ms <br />about 45,000 Al'h-r as compared to the 20,000 lIF/yr undcr <br />~.- --"-~' "- <br />virgin conditions, due to an increase in the hydraulic <br />gradient. <br />