Figure 8.28 Reaching the limits of irrigated agriculture. In the mid-twentieth century, irrigation water from Lake Mead, the reservoir behind the Hoover Dam, enabled the conversion of millions of acres of arid lands to agriculture in the Southwest. The reservoir has not reached capacity for three decades, however (drought marks are clearly shown in this photo), and likely never will again, putting the future of some irrigated lands into question. (iStock/360/Getty Images.)