In the American West, the task of securing sustainable water supplies for agricultural, industrial, or municipal use without adverse effect to the environment has become a daunting challenge. A demographic shift is occurring in the population of the United States where people are migrating in record numbers to the Southwest-the most arid region of the continent. Aquifers near many population centers are fully developed and, in some cases, being rapidly depleted. Many people and communities in the Southwest are dependent upon water drawn from ground-water storage, which cannot be sustained. Surface water supplies near population centers are fully or over allocated leaving little or no water for stream ecosystems. Science plays an important role in the solution of such problems by first raising the awareness of problems and, as alternative solutions are proposed, describing the potential hydrologic and environmental consequences.

The development of water supplies seems to progress through predictable phases: early development using gravity, construction and dependence upon technology, conjunctive use, and ultimately to some new level of sustainable use. The scientific data and information needed to support each phase of water development have changed and grown in complexity and sophistication. For example, conjunctive use is the recognition that surface water and ground water are in hydrologic connection and must be managed more holistically. This awareness is relatively recent, and the analytical tools necessary to construct unified water budgets and predictive models are only now being developed. To achieve sustainability, integrated science is being called upon to better quantify and monitor changes in the hydrologic system, the physical habitat requirements of stream and riparian ecosystems, and the life-sustaining needs of individual species being affected or in danger of extinction. The use of scientific information will be examined for the development and management of western water supplies, and the needs, as yet unmet, to achieve sustainable use and protection of sensitive riparian ecosystems.