Extract: Beginning with the Mormon colonization of Salt Lake Valley, Utah, in 1847, irrigation has been a significant part of agriculture in the United States. Most of the Nation's irrigated acreage has always been located in the arid and semiarid regions of the West, but in recent years supplemental irrigation has been steadily increasing in the humid East. More farmers in eastern regions are finding that supplemental irrigation is profitable, especially as insurance against crop failures in drought years, and also by increasing average yields. Water supply is apparently the principal constraint to irrigation expansion in the West. Conservation of available water resources is apparently the best measure for expanding irrigation there.