Creation of a 525-acre lake that will be both the Washington area's long-debated fourth reservoir and a major recreation facility for the region has been approved by the Montgomery County Council.

The $30.5 million lake, to be called Little Seneca Reservoir, will be created through damming near the community of Boyds, perhaps as early as this summer. Although primarily intended as an emergency source of drinking water, it will be open to boating, swimming and fishing

Most of the area's major recreational lakes are in Fairfax County: Lakes Anne, Fairfax, Barcroft, Burke, Occoquan, Thoreau and Audubon. Others are the Tridelphia and Rocky Gorge reservoirs on the Patuxent River, on the borders of Montgomery, Howard and Prince George's counties. Montgomery has two flood control lakes, Needwood and Bernard Frank.

The new, 4.25 billion gallon reservoir will be paid for by water and sewer users in the District of Columbia and Prince George's, Fairfax and Montgomery counties.

In a drought emergency, the reservoir could release a 20- to 30-day supply of water in 12 hours, said Washington Suburban Sanitary Commission officials. Without it, they say, it would take several days for water to flow here from another reservoir 60 miles northeast of Washington.

The council was the last of the four jurisdictions to approve the reservoir, which has been planned for 20 years. Detractors, led by County Councilwoman Rose Crenca, claimed there is little need for the reservoir because it will not be needed by county residents for many years.

"They haven't documented the need for this reservoir any time this century," said Crenca, the lone dissenter on a 6-to-1 council vote on Tuesday. "We just don't need it."

Montgomery officials became interested in the Little Seneca site when droughts struck the Washington area in the 1960s and '70s.

WSSC general manager Robert S. McGarry said that the planned reservoir will operate in conjunction with five other area reservoirs to supply Montgomery, the District of Columbia, Prince George's and Fairfax counties in emergencies.

Fifty percent of the cost of building Little Seneca will come from bonds sold by the WSSC. Water customers in Prince George's and Montgomery will share the facility's operating costs and debt service on the bonds. Water customers in the District of Columbia will pay 40 percent of the remaining costs and Fairfax County will assume the other 10 percent.

McGarry said that customers in Prince George's and Montgomery will see a one or two percent increase in their annual water bills, or an extra 1.5 to 1.7 cents per 1,000 gallons of water used.

Representatives of the Montgomery County Taxpayers League say that the cost will be five to seven cents per thousand gallons. They attacked the reservoir as a frivolous venture that will be used primarily for recreational purposes.

"County taxpayers won't see any benefit from this for at least 20 years," said Taxpayers League chairman Wylie Barrow, referring to a county council staff memorandum that said that water stored in Little Seneca "will not be needed until after (the year) 2000 according to present population and water usage projections."

Crenca, who unsuccessfully sought a public hearing on the project, added that she was concerned that "real estate interests are involved and really want to create another Lake Barcroft situation in Montgomery County," referring to major real estate developments under construction adjacent to Lake Barcroft, a man-made lake near Columbia Pike in Fairfax.

Stanton Ware, another taxpayers league member who once worked on water resource research for the Interior Department, argued that Little Seneca is unnecessary because the 11-billion gallon Bloomington Dam reservoir in Allegheny County, and the Tridelphia and Rocky Gorge reservoirs on the Patuxent River, could supply enough water in a drought.

But McGarry said that water from Bloomington could take from four to seven days to reach Washington.

Councilman Scott Fosler, who voted in favor of Little Seneca, agreed that it would not be needed for at least 20 years, but added "beyond that, we will need greater reservoir capacity. If we wait, will we be able to come up with the same land and the same regional agreement?"

Council President Neal Potter echoed Fosler's sentiments and said that the project would help unemployment in the construction industry and would complement the Bloomington Dam.

"Without it, in an emergency, . . . the world won't come to an end, but drastic actions would be necessary and there would be serious curtailment of individual water use," Potter said.

The county councils of Prince George's and Montgomery are expected to approve the reservoir's financing when they meet to consider the overall WSSC budget in May.