The T rans Arizona Canal or the water supply challenge of the sun belt. After a rapid presentation of the arid climate of the Gila basin, the paper emphasizes the increasing disproportion between water resources and the almost exponential development of agriculture and moreover urbanization ; facing depletion of the fossile ground water supplies, the State of Arizona managed togetfromthe Federal Congress funds for the construction of a canal carrying water from the Colorado River to Phoenix and Tucson. It is the Trans Arizona Canal (536km) which has been partially built in 1985. It will divert each year 1,8 Billion m3 of water when completed, that is the totality of the State rights over the Colorado River, when added to the water already used.