Within the next two years, federal officials may be forced to declare a water shortage on the Colorado River, an unprecedented situation that would reduce the amount of water available to parts of the southwestern United States and Mexico. The thermometer of the river’s health is Lake Mead, formed behind Hoover Dam. When the lake falls below a certain level, a shortage is declared and people begin to lose access to water based on an arcane priority system. The first to receive cuts are Arizona, Nevada and Mexico.