Thousands of acres of prime farmland have transformed the desert into one of the most productive farming regions in California with an annual crop production of over $1 billion. Agriculture is the largest industry in Imperial County and accounts for 48% of all employment. Although this region is a desert, with high temperatures and low average rainfall of three inches (75 mm) per year, the economy is heavily based on agriculture due to irrigation, which is supplied wholly from the Colorado River via the All-American Canal. A vast system of canals, check dams, and pipelines carry the water all over the valley, a system which forms the Imperial Irrigation District, or IID. The water distribution system includes over of canal and with of pipeline. The number of canal and pipeline branches number roughly over a hundred. Imported water and a long growing season allow two crop cycles each year, and the Imperial Valley is a major source of winter fruits and vegetables, cotton, and grain for U.S. and international markets. Alfalfa is another major crop produced in the Imperial Valley. The agricultural lands are served by a constructed agricultural drain system, which conveys surface runoff and subsurface drainage from fields to the Salton Sea, which is a designated repository for agricultural runoff. Imperial County produces nearly 2/3 of all vegetables consumed by Americans during the winter.