Environmentalists are threatening to strike down in a matter of months a proposed $25 million state park that's been in the works for 17 years.

The state Department of Natural Resources has been developing plans since 1966 to dam up the St. Mary's River above Great Mills, creating a 300-acre man-made swimming lake as the focus of a 2,500-acre recreation area.

Land was bought, homeowners and farmers were relocated from the countryside along the river. A consultant drew up plans. Last month the linchpin went in place as the Army Corps of Engineers, after years of delay, issued a permit for a 45-foot-high, federally financed earthen dam across the main stem of the St. Mary's to create the lake.

Just when everything seemed set, the Chesapeake Bay Foundation filed suit in U.S. District Court Feb. 16 protesting destruction of protected wetlands if the dam were built and bringing the project to a halt, at least temporarily. Faced with prospects of a court injunction and a chorus of environmental complaints if they signed the dam permit, state officials asked the corps for 90 days to review the project, said natural resources counsel Tom Deming.

Deming said his and other state agencies now are restudying the proposal to see "whether the state is really interested in a project of this scope at this time." Proponents of the project within state government declined further comment, citing the pending litigation.

The turn-around infuriated St. Mary's County officials, who were banking on recreational and financial rewards from the project and who wonder where the environmentalists were for the last 17 years.

"Isn't it strange?" asked County Administrator Ed Cox. "Out of the blue come these opponents from outside the county. What do you suppose caused this sudden, great concern?"

The environmentalists say the concern was occasioned because the St. Mary's, an unspoiled, meandering tributary of the Potomac 80 miles from Washington, is on an inventory of streams being considered for National Wild and Scenic Rivers status. Opponents of the project, including the Chesapeake Bay Foundation, Audubon Society and American Rivers Conservation Council, say that a dam there would be an unnecessary eyesore.

But the legal focus of the case is the Bay Foundation's contention, supported by the U.S. Environmental Protection Agency and Fish and Wildlife Service, that in all the years of planning no one bothered to study the effects of the dam on 170 acres of nutrient-rich wetlands that would be inundated. The Bay Foundation maintains such a study is required by a federal law protecting wetlands.

Foundation attorney Dick Gardner said dam proponents "fouled up so badly on procedural matters that we never even had to address the question of the desirability of a lake. We just questioned whether there was adequate environmental evaluation, cost-benefit analysis and consideration of alternatives."

Gardner and foundation Director Will Baker nonetheless do not conceal their distaste for a man-made lake plunked down in the middle of a free-flowing river. They also questioned the need for additional water recreation in St. Mary's, a county bounded on two sides by the Patuxent and Potomac rivers and on a third by the Chesapeake Bay.

The dam originally was one of five proposed on the St. Mary's by the federal Soil Conservation Service. One dam was built on the Eastern Branch, a tributary, in 1976, creating a lightly-used, 300-acre fishing lake near Great Mills. Plans for three smaller dams eventually were dropped, leaving only the St. Mary's dam still under consideration.

The original stated purpose of the dams was sediment control and flood protection for a few buildings in Great Mills. But over the years estimates of those benefits were downgraded and today 94 percent of prospective benefits claimed for the proposed dam on the St. Mary's are recreational advantages from the planned park.

The issue now under consideration by the state is whether the recreational benefits are worth the costs.

Gov. Harry Hughes "has some reservations" about making a major investment for another state park in St. Mary's during a time of severe budgetary constraints, said his environmental aide, John Griffin.

He said that while county politicians and natural resources officials may feel betrayed by the possibility of their long-term efforts coming to naught, "we get no strong feeling either in favor of or against the lake from the constituency" in the county.

To draw attention to the value of the wetlands that would be inundated, Chesapeake Bay Foundation staff assistant Stuart Lehman led a two-man exploration of the area last week.

The wetlands of the upper St. Mary's watershed are rich in plant and animal life. Five feet from the edge of a large beaver pond, Lehman's footsteps started a cottontail rabbit running. Five steps later he startled a flock of 60 black ducks, which whooshed off the water.

He explored beaver dams and a beaver mound; muskrat runs; watched herons fly off their roosts, croaking eerily, and found deer, fox and raccoon sign, but except for an overhead power line, no hint of man.