Reservoir water levels are important because reservoirs supply water for people, agriculture and other needs. Low reservoir water levels mean water restrictions and trucking in water to affected communities. In extreme cases, this means establishing expensive, mobile desalination plants.
Reservoirs are not only important to communities, they are also critical for crop production. For instance, the San Luis Reservoir provides water to farmers in the San Joaquin Valley. Two-thirds of all people who live in Kansas get their water from reservoirs.
Snowpack and rainfall both affect reservoir levels. In California, the Sierra Mountains contain this snowpack, which is a mass of hardened snow that melts slowly over the warmer months. If it snows less during the winter or winter temperatures are warmer and snow doesn't form, the snowpack decreases and less water is available during the summer.
Two important reservoirs are Lake Mead and Lake Powell. Combined, they can hold over 50 million acre-feet or AF, which is 325,900 gallons per AF. As of 2015, the lakes are only at 44 percent capacity. Lake Mead, the largest reservoir in the United States, receives 96 percent of its water from snow melt in Utah, Colorado, Wyoming and New Mexico with 4 percent coming from the Colorado River. In turn, People from California, Nevada and Arizona receive water from Lake Mead.