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Snake River Plain Aquifer
Source	Annual Recharge	Annual Outflow
Irrigation diversions	5.1
Valley underflow	1.5
Precipitation	0.8
River seepage	1.3
Total	8.7
Pumping for irrigation		1.6
Springs & river gains		7.1
Total in millions of acre-feet		8.7
Estimated annual water budget for the Snake River Plain Aquifer (from Hackett and others, 1986). Numbers are in millions of acre-feet, that is enough water to cover a million acres one foot deep.

Snake River Plain Aquifer
The Snake River Plain Aquifer underlies about 10,000 square miles of eastern Idaho. Its annual recharge and discharge is estimated at 8 million acre-feet, and its storage capacity is estimated at 200 million acre-feet, enough to cover the entire state of Idaho with 4 feet of water. The aquifer is recharged by precipitation that falls on the ranges to the north and south and flows into rivers whose discharge is absorbed into porous sediment. Recharge by irrigation water is another important source for the aquifer.

More than 3 million acres of land on the Snake River Plain were irrigated in 1980, of which 1 million were supplied by ground water and 2 million by surface water from canals (Whitehead, 1986).

The Growth of Agricultural Idaho
In the 1870s, economic growth in southern Idaho awaited several things. A reliable and adequate transportation system, needed to carry produce to market and to bring back machinery and equipment and the people to operate them, arrived in the early 1880s with the railroads. The territory grew and became a state; the state grew and towns began to proliferate along the rail lines. Vast areas of southern Idaho lay unused and unoccupied, however, because of a lack of water. Irrigation projects were needed.

(left) Last Chance Canal at its headgate along the Bear River northeast of Grace, (august, 1991). (center) Sprinkler irrigation against the summer sun, Star Valley, Wyoming, (June, 1977). (right) Sign protesting Idaho Department of Water Resources policies in the lower Big Lost River drainage, (June, 1995). During drought years in 1993 and 1994 the water table dropped in this area because there was not enough recharge to keep up with irrigation withdrawal of both surface and ground water. Hundreds of cottonwood trees along newly dry watercourses died.

Early Irrigation Failures
Although visionary engineers like Arthur Foote, who surveyed and attempted to start canal systems in the Boise Valley in the 1880s, saw the potential in irrigated agriculture in Idaho, large irrigation systems on the Snake River Plain were stymied by lack of investor capital through the 1890s. Although the Mormon Pioneers had made the desert bloom in their corner of the state, central and western Idaho remained the territory of cattle and sheep grazers who neither needed irrigation nor welcomed the fences and restrictions which came with building of farms.

The Carey Act, named after a Wyoming senator, was passed by Congress in 1894 to stimulate state and private cooperation to develop canal systems. The Act gave western states up to a million acres of Federal land if the land could be irrigated or reclaimed. Idaho contains 3/5 of all land reclaimed under the Carey Act.