Not very many years ago, the Army Corps of Engineers' answer to any water-supply problem was likely to be: build a dam. That policy reached its high point locally in the early 1960s, when the corps proposed coping with the Potomac's erratic flow by building -- remember? -- 17 dams. Now times and the corps have changed. In a major report coming out this week, the engineers conclude that the Washington region will not need any more big dams at all for at least 50 years. Instead, the corps has found that area governments can get enough water by curbing consumption and managing their existing reservoirs more effectively.

The corps has reached this sound view through evolution, not a sudden turnabout. Since 1963, its controversial Potomac dam proposals have been dwindling from 17 to six to two to one, on the North Fork of the Shenandoah, which was shelved two years ago. At first, that was largely a retreat under public fire; the corps still maintained that damming the Potomac would be desirable -- and warned that the Washington area, by resisting all dams, was inviting trouble in the next big drought.

The real change in the corps' approach has come since 1977, when President Carter started overhauling federal water policies to favor more conservation and less construction. Coincidentally, that was also the year that Wasington area officials were jolted by two ominous events: the pumping-station fire that interrupted water service in some Maryland suburbs, and the Occoquan Reservoir shortage that made Fairfax County impose water-use curbs and seek a new Potomac pipeline. That's when water-saving and cooperation really caught on as regional policies. And the corps, to its credit, came around -- partly because its nationwide outlook changed, and partly because it realized that regional contingency plans are good insurance for its own Washington aqueduct customers in the District and Arlington.

In one sense, this is just the start. The corps' draft will now be subject to public review; some of its particulars are bound to be controversial. Even after a plan has been agreed on, new pipelines and political agreements will have to be built before the area's separate water systems can be effectively linked. But federal and local policies are now on the same general course -- and on a course that promises to be much more economical and environmentally sound than damming up the river any more.