Ariz. Rev. Stat. § 11-116
Yuma county, the county seat of which is Yuma, is bounded as follows:
Commencing at the point where a meridian line one hundred thirteen degrees twenty minutes west longitude, as defined by the Atwood survey of 1918, intersects the southern boundary line of the state of Arizona, being the southwestern corner of Pima county; thence north on the meridian, as defined, and along the western boundaries of Pima and Maricopa counties to the Gila and Salt river base line; thence west along said base line to a line common to ranges 16 and 17 west, Gila and Salt river base and meridian; thence north along said range line to the line common to townships 1 and 2 north; thence west along said township line to the line common to ranges 19 and 20 west; thence south along said range line to the line common to townships 4 and 5 south; thence west along said line to the west boundary of the state of Arizona; thence south along the west boundary of the state of Arizona to the southwest corner of the state of Arizona; thence easterly and southerly along the southern boundary line of the state of Arizona to the point of beginning.
A.R.S. § 11-116