The sometimes warring jurisdictions of the Washington area yesterday reached a long sought water-sharing agreement that local leaders claim will guarantee an adequate water supply for the region well into the next century.

The agreement, which includes the District of Columbia, suburban Maryland and northern Virginia, will not require construction of any expensive and controversial dams on the Potomac River. The only new impoundment would be built on Little Seneca Creek in upper Montgomery County.

The emphasis of the agreement is better management of present resources rather than massive new construction -- an approach that is beginning to attract attention up and down the water-short Eastern Seaboard.

"I think this agreement represents a major breakthrough," said John F. Herrity, chairman of the Fairfax County Board of Supervisors, who represented his county in the talks that have been taking place at the Metropolitan Washington Council of Governments. "We have insured an adequate water supply for the region until the year 2030."

For years, the U.S. Army Corps of Engineers had argued that the only way the region could take care of its long-term water needs was by constructing a series of dams upstream on the Potomac. But because of intense opposition from communities near the proposed sites, as well as the cost only one dam -- Bloomington, in western Maryland -- actually has been built.

While the region's govenements began to search, sometimes desperately, for alternative solutions, a planning engineer at the Interstate Commission on the Potomac River Basin, Daniel P. Sheer, came up with a scenario that would involve a minimum of new construction and yet take care of the area's long-term needs.

Sheer's plan became the basis for the agreement that was reached in principle yesterday.

Under the plan, if the Potomac's flow fails to a precariously low level, it would get a quick shot of water from the impoundment on Little Seneca, one of its tributaries. This would permit the District, suburban Maryland and northern Virginia to continue tapping the river.

If the drought persists, more water, for a longer term, would be released into the lower Potomac from the dam at Bloomington Lake.

The Bloomington dam is almost finished. Construction of the Little Seneca impoundment could be started relatively soon since Montgomery County owns most of the land that would be flooded by its lake.

As the region continues to grow, water-supply management under the agreement would include a range of restrictions on use to stretch out the supply at Little Seneca Lake and Bloomington.