The watershed of the Mississippi River, for example, includes almost half of the United States. Prevention of pollution alone will not insure a future water supply. To have enough water for the future, watersheds must be protected. The unregulated flow of many of the nation s rivers is highly variable. For example, the rate of flow may be thousands of times greater during floods than during drought.
Some streams cease flowing during drought.
Thus, reliance on surface water as a source of supply usually requires a dam, creating a reservoir to store excess water from wet periods for use during dry periods. If the reservoir is located upstream from the location where the water is to be used, the water stored during the wet periods may be released to the river system during dry periods to flow downstream to the point of use. In either situation, there usually are requirements concerning the minimum flows that must remain in the river either below the dam or below the point of diversion. These minimum flows are established to provide sufficient water for instream uses such as waste dilution, fish and wildlife habitat, and navigation. The purpose of the reservoir is to make up the difference between the demand for water and the available streamflow during the dry periods, when the available streamflow is less than the demand.