Under a regional water agreement signed Friday, Montgomery County will get a new 500-acre recreational lake in Little Seneca Regional Park north of Gaithersburg, giving the Washington area a 4-billion-gallon reserve water supply in case of a drought.

The planned $31 million Little Seneca Lake, approved in principle by local jurisdictions but yet to be funded, could release a 20- to 30-day supply of water into the Potomac River in times of low river flow.

Under the agreement, the Seneca water would be shared in roughly the same way local jurisdictions will share water from the huge new $173 million Bloomington Dam. Completed last fall by the U.S. Army Corps of Engineers, the dam is 50 miles up the Potomac River from Washington on the Maryland-West Virginia border.

The Bloomington and Seneca dams, which officials predict will end the threat of drought in the Washington area until well into the 21st century, are the last major dams proposed for the Potomac and its nearby tributaries. At one time, a giant dam on the Potomac just above Great Falls was proposed, as were dams in the Seneca and Rock Creek watersheds.

Although no federal funds would be used to construct the lake, the U.S. Department of Agriculture has authorized a $427,000 soil conservation grant for Montgomery County farmers to help eliminate sediment and agricultural pollution--largely from dairy herds polluting the streams--in the watershed that feeds the lake.

The federal grants will be available to farmers who put up 25 percent matching money for projects that will prevent soil erosion by controlling run-off from more than 3,000 acres of cropland. The animal waste control projects on eight dairy farms will reduce stream pollution and provide farmers with greater quantities of manure fertilizer, according to the USDA. The projects are expected to cut sediment erosion by 60 percent and pollution from animal wastes by at least 50 percent, according to the USDA.

The Little Seneca Lake water would be released into the Potomac at Seneca Creek, just above the fresh water intake pipes of the District, Northern Virginia and suburban Maryland. This would provide an immediate emergency water supply for the Washington area. Water from the Bloomington Dam, the third-largest east of the Mississippi River, would take several days to reach Washington after it is released.

Under the agreement signed Friday, 50 percent of Little Seneca's costs would be paid by Montgomery and Prince George's counties through the bicounty Washington Suburban Sanitary Commission. The District would pay 40 percent and Fairfax County 10 percent.

The Maryland suburbs would pay the entire $3 million cost of acquiring the land for the reservoir and a park buffer. The five-acre Little Seneca Lake will be in the center of Montgomery's long-planned, 1,800-acre Little Seneca Regional Park just north of Boyds and west of I-270.

Construction of the dam could begin this summer and be completed in two years, according to Robert L. Young, Montgomery's associate director of parks. Most of the land for the park already has been purchased and plans for the dam are complete.

The Montgomery County Council last summer postponed action on the dam until a regional agreement was reached on water and cost sharing. The council is expected to approve funds for the dam this spring as part of the WSSC's capital improvements budget.

The dam will give Montgomery its largest county lake, larger than lakes Needwood and Frank, built with federal funds in the mid-1960s as flood-control projects for upper Rock Creek. It would be smaller than the two 800-acre Patuxent River reservoirs--Triadelphia and Duckett--owned by the WSSC.

Just as at the Patuxent reservoirs, swimming and the use of gasoline motors will be prohibited at Little Seneca, Young said. But the county plans to stock the lake with fish and encourage fishing, boating and picnicking. Sailboats, canoes and boats that can be powered by electric motors will be rented at the lake.

The Little Seneca Lake is not to be confused with the 90-acre lake at newly opened Seneca State Park just west of Gaithersburg. Like Needwood and Frank, it is a flood-control lake and its waters cannot be released in times of drought.

Any confusion over Seneca dams and parks would not be surprising, says Young. Since the 1930s, dams have been proposed on the Seneca and on the Potomac at Seneca, called Seneca dams, and sections of park land called Seneca regional or state park have been appearing on maps of the county for decades, as the parks have been bought piecemeal. Neither park is yet complete.

The federal government first proposed flood control dams in the Seneca and Rock Creek watersheds under the 1962 Small Watershed Protection Act. Just before construction of Needwood and Frank, Congress amended the act to include recreation as an added feature of such dam projects, said Young.

The escalating cost of Montgomery County land forced the federal government to drop plans for any flood-control/recreation dams in the Seneca watershed. But the county suddenly became interested in a Seneca dam and reservoir as a water supply when successive droughts hit the Washington area in the late 1960s and 1970s. The long-proposed Seneca dam is now a flood- and sediment-control, recreation and water supply dam