Plans to develop Little Seneca Regional Park, and use its lake as a reservoir for Montgomery and Prince George's counties in time of drought, will be given a public hearing tonight at 8 at Seneca Valley High School, Germantown.

The proposed 525-acre lake, which would be open for fishing and boating even if used as a reservoir, would supply water to the two counties about once every seven years during summer water shortages, according to Robert Young, associate director of parks for the Maryland National Capital Park and Planning Commission.

The park agency is proposing to spend $3.6 million to build roads, a visitors' center, a marina and picnic sites in the 1,400-acre park on Seneca Creek near Boyds.Most of the land already belongs to Montgomery County, but is undeveloped and closed to the public. The dam and reservoir would be built by the Washington Suburban Sanitary Commission at a cost of $25.5 million.

If the preliminary plans are approved, Young said the County Council would hold additional hearings on the project before final action is taken. The reservoir and park could be completed by the summer of 1983, Young said.

Until last year, county officials had planned a 452-acre flood-control and recreational lake. They have since realized that the lake's waters could be released into the Potomac, just above the county's intake pipes for drinking water, and thus could serve as an emergency water source. Studies also showed that a slightly larger lake, covering 525 acres, actually would cost less to build because of the terrain.

The reservoir would hold just over 3 billion gallons of water, about one-fourth the capacity of the WSSC's Triadelphia and Rocky Gorge reservoirs on the Patuxent River along the boundaries of Howard, Montgomery and Prince George's counties.

Those reservoirs and the 6,600 acres around them are open to the public, although the WSSCthreatened to close the reservoirs to the public this year to save money. The Patuxent and Potomac supply drinking water to Prince George's and Montgomery counties.

The Little Seneca Lake project would displace about 21 households, including four dairy farms, and three historic sites. It would inundate 525 acres and about six miles of trout streams.

Two of the historic buildings, a 19th century one-room schoolhouse and a former slave cabin, "can and probably will be moved," Young said last week. The third, a 19th century boarding house and hotel, which would be on the edge of the lake, probably will be restored and used by the park agency, Young said.

After informal meetings with area residents, park officials changed some of the development plans. The eastern portion of the park will be left largely undeveloped, except for picnic areas on the largest of the peninsulas that will extend into the lake.

On the westernmost peninsula, adjacent to the park's main entrance on Route 121 near I-270, plans call for a visitors' center and marina. Small sailboats, canoes, rowboats and electrically powered boats will be allowed on the lake, but gasoline-powered boats will be banned. As with the WSSC's other two reservoirs, swimming will not be permitted.