7. Las Vegas
Major Water Supply: Lake Mead/Colorado River
Population (U.S. rank): 567,000 (28th)
Population Growth Rate: 18.6 percent since 2000
Average Annual Rainfall: 4.5 in.
In the middle of the Mojave Desert, with an annual precipitation rate of only 10 cm, Las Vegas must rely on distant sources for its fresh water. The city’s main source is Lake Mead, which supplies 85% of the water used in the Las Vegas Valley. Unfortunately, the lake is 59% empty and is approaching its first water shortage ever. In addition to Las Vegas, it would affect other areas of Nevada and Arizona. Moreover, it could potentially stop the Hoover dam from producing electricity – as soon as 2013. This would affect many big California cities that receive hydro-electric power through the dam.