Colorado River too much of the lower basin allotment; in 1944 a compromise was reached in which Arizona would get a firm allocation of , but only if California's allocation was prioritized during drought years. These and nine other decisions, compacts, federal acts and agreements made between 1922 and 1973 form what is now known as the Law of the River. On September 30, 1935, the United States Bureau of Reclamation (USBR) completed Hoover Dam in the Black Canyon of the Colorado River. Behind the dam rose Lake Mead, the largest artificial lake in the U.S., capable of holding more than two
Transportation in Arizona large-scale reconstruction and expansion of I-10 was supported and completed in mid-2009. Yuma is served by I-8, while Casa Grande served by I-8 and I-10; Flagstaff is served by I-17 and I-40. US 95 parallels the Colorado River, from Las Vegas to the Mexico–United States border near Yuma. Historic US 66, a major route for Midwestern emigrants prior to the advent of the interstate highway system, traversed the northern part of the state, passing through Flagstaff and Kingman. US 66 in Arizona closely followed the route of what is now I-40 except for an stretch between Seligman and Kingman now