Irrigation is the method of supplying water to land to support plant growth. This technology has had a powerful role in the history of civilization. In arid regions sunshine is plentiful and soil is usually fertile, so irrigation supplies the critical factor needed for plant growth. Yields have been high, but not without costs. Historic problems include salinization and water logging; contemporary difficulties include immense costs, spread of water-borne diseases, and degraded aquatic environments.

One geographer described California's Sierra Nevada as the "mother nurse of the San Joaquin Valley." Its heavy winter snowpack provides abundant and extended runoff for the rich valley soils below. Numerous irrigation districts, formed to build diversion and storage dams, supply water through gravity-fed canals. The snow melt is low in nutrients, so salinization problems are minimal. Wealth from the lush fruit orchards has enriched the state.

By contrast, the Colorado River, like the...