As the Mormon settlements expanded into other parts of southeastern Idaho (beginning with Bear Lake Valley in 1863) irrigated farms spread over that part of the territory. Gold discoveries in the Boise Basin in 1862 brought irrigated farming to the nearby valleys the next year. In 1863, all the easily watered lands immediately adjacent to the Boise River were taken up and farmed, and by 1864 some important canal companies were getting started. Many early ditches were extended cooperatively by a group of interested farmers whose organization was informal at best. In order to get water onto more difficult bench lands, extensive capital investment was required for the large canals which had to be engineered and dug. Enterprises of this scale could be managed in the upper Snake River Valley by the Mormons who already were organized for community cooperative action. But in southwestern, Idaho, privately incorporated canal companies began to do the job. The Ridenbaugh Canal is a good example of this kind of construction, and the New York Canal started this way.

Failures of canal companies, not only in Idaho, but over the West in general, to manage large-scale reclamation led to arrangements for state assistance. Under the Desert Land Act of 1894 (generally known as the Carey Act) the President of the United States was authorized to transfer up to one million acres of arid land to each of the reclamation states with public lands; the states then could sell the lands in 160 acre lots to the farmers who would be served by canal companies under arrangements approved by the state reclamation engineer. To qualify for lands under the Carey Act, Idaho immediately established the office of reclamation engineer and provided for the organization of irrigation districts of interested farmers. (Irrigation districts are special districts somewhat like school districts, only providing for reclamation rather than education.) Some years went by before the law was perfected to the point that an irrigation district could be set up under it. But by 1900 the Pioneer Irrigation District was established around Caldwell, and others soon followed. Meanwhile, steps were being taken to irrigate a large tract of land in the Twin Falls country under the Carey Act. By 1905 a large canal from Milner Dam was bringing water onto new farms in the desert lands south of the Snake River, and a few years later, Carey Act lands on the north side were receiving water from Milner.

Idaho benefited from the Carey Act far more than did any other state: about 60 percent of all Carey Act lands irrigated in the United States are in Idaho. Since most of the reclamation states could not manage to use the Carey Act to advantage, another reclamation act of 1902 provided for a United States Reclamation Service (now the Bureau of Reclamation) to work through irrigation districts in the western states. Capital and engineering necessary for great dams and distribution systems were provided from land revenues by the United States on the condition that the irrigation districts repay the costs of their projects. The Minidoka projects around Rupert and Gooding and the Boise Project in southwestern Idaho grew out of the 1902 act. For the Boise area, such enterprises as the New York Canal (which had commenced as a private canal company venture) were brought to completion under federal reclamation ; great new reservoirs and dams--the major early one being Arrowrock, which was highest in the world for about twenty years after its completion in 1915--were added shortly. Later dam construction by the Bureau of Reclamation served major Carey Act, as well as Reclamation Act, projects: American Falls (1927), Cascade (1948), Anderson Ranch (1950), and Palisades (1956), being the more notable.