If the collectible wastes were diverted out of the upper estuary and if it proved possible to cope quickly with other pollution or to ignore it, during prolonged use salt water penetration from downstream would take place as fresh water was withdrawn above and not replaced. Studies on a mathematical model of the estuary indicate that under conditions that could materialize, this would make the water at the intake too salty for use. A barrier dam across the entire estuary at one or another point in the freshwater section could prevent such penetration, but would be hugely expensive and undoubtedly more obtrusive on a much-used part of the riverscape than most upstream reservoirs could possibly be. Furthermore, even if all these doubts and areas of ignorance were to be easily resolved, insistence that the upper estuary is the only logical answer to metropolitan Washington's water problem ignores the fact that major water demands are building fast in certain already-mentioned areas of the upper Basin, and that, since the Basin is a hydrological unit, measures to satisfy these demands can easily, economically, and quite logically be designed to furnish a good part of the metropolis' near-future safe margin of water supply as well. [Illustration] A need for vigorous research specifically directed toward exploring all these alternative means of supply is evident. If it moves fast enough and the knowledge that comes out of it is made available to planners, it may very quickly make a great difference in the kinds of sources of water they can turn to for the solution of problems, just as studies since the early 1960's, when the Army work on the Potomac was completed, have altered prevalent ideas about pollution control through flow augmentation, and have therefore greatly diminished the overall amount of water considered necessary to meet the Basin's demands. In the crucial meantime, the established certainty of storage in reservoirs is available. In river basins with reasonable annual amounts of precipitation but with human demands on streams that sometimes exceed the rate at which water flows down, such reservoirs are still usually the most dependable and efficient item in the present technology of water supply. And since they generally have other purposes to which proportionate shares of construction costs are assigned in individual cases--flood protection, water quality control, navigation, hydroelectric power, recreation, silt detention, etcetera--they tend often to be the most economic sources of big quantities of water. In one form or another they have been built from very ancient times, and they have been indispensable to the useful development of water resources in our expansive economy.