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
Flora of the Arizona Upland Flora of the Arizona Upland Flora of the Arizona Upland' includes higher elevation Sonora Desert plants that require more moisture and cooler climates than those of the adjacent Sonoran Desert areas in the Colorado Desert of the lower Colorado River valley area, and which can withstand frost, unlike plants of the Sonoran Desert south of the border between the United States and Mexico. The Arizona Upland receives between 12 and 15 inches of annual precipitation, of which half falls in the summer monsoons. Cold air collects in the valleys. Less cold tolerant species grow on the warmer slopes above. The