The new Leesylvania State Park along the Potomac River near Woodbridge, which includes plans for the first public swimming beaches on the upper Potomac in more than three decades, could begin the first phase of its development next year, according to State Del. Floyd C. Bagley.

Bagley (D-Prince William) said last week he hopes the first $2.5 million phase of development, including park roads, utilities, beach development and some marina facilities, could start next year, two years before dates announced by Gov. Charles S. Robb when he discussed plans for the park last month.

Bagley, who lives near the park and is a member of the Virginia House Appropriations Committee, has vowed to fight for earlier funding and earlier opening for Leesylvania.

The 505-acre park, on a peninsula known locally as Freestone Point near Rte. 1 and I-95 south of Mason Neck peninsula, is the second major state park to be created in Northern Virginia in the past 15 years. It is scheduled to undergo a $6.7 million development between 1986 and 1991.

The master plan for development of Leesylvania, released by state officials last month, pictures "swimmers and bathers" on sandy beaches along the park's 3.5 miles of shoreline but also proposes a $1.2 million swimming pool, although it is not scheduled to be built until the end of the park's six- to eight-year development period.

Plans also include sailboat and motorboat marinas, an athletic complex to be built by Prince William County, and preservation of historic Lee and Fairfax family plantation ruins.

Although Leesylvania is still officially closed, the naturally sandy beaches are used regularly for picnicking and swimming by boaters and the owners of four-wheel drive vehicles who follow abandoned roads onto the peninsula, said Ranger Kathy Schutte, park manager.

Last week, the regional director of the Virginia Water Control Board, Thomas Schwarberg, in a telephone interview called the Potomac around Woodbridge one of the more polluted sections of the river in Northern Virginia and questioned state plans to allow swimming there.

"I certainly wouldn't want my children to swim there," Schwarberg said.

Despite dramatic improvements in the quality of Potomac River water in the past decade, the District still prohibits water-contact sports in its portion of the river, between Chain Bridge and Woodrow Wilson Bridge, but Maryland and Virginia have no similar prohibitions.

But even if splashing in the Potomac is considered dangerous to health and prohibited when Leesylvania opens, the new state park still is expected to become a major attraction for Northern Virginia residents as a boating and scenic hiking center, Bagley said.

The slow development of the park was criticized last week by Donald Curtis, the unofficial historian of Leesylvania State Park, who, with a Lee family descendant, Eleanor Lee Templeman, worked for almost a decade to help create the park.

"It took forever to get the park, and now it looks like it will take forever to develop it," said Curtis, whose family has lived nearby since 1701. "The people of Northern Virginia are lusting for just this kind of recreation and waiting until the 1990s is ridiculous."

Robert E. Lee's father, Light Horse Harry Lee, was born on his family's 3,000-acre Leesylvania tobacco plantation, established on the peninsula in the mid-1660s, and the ruins of their home, a 19th century Fairfax family mansion and a graveyard for both families, still stand in the jungle of trees and vines that now almost covers the peninsula.

A gambling casino and resort were built on the point after World War II, with boats full of slot machines moored to the end of a long pier--in Maryland waters where gambling was legal, Curtis said. The casino, swimming pools and boardwalks were abandoned in the late 1950s.

Curtis and Templeman persuaded the property's owner, Daniel K. Ludwig, to donate to the state half the value of the land, then appraised at $4 million. Virginia secured a $2 million matching grant from federal Land and Water Conservation Funds to buy the park in 1978.

One constriction on the park is the heavily used railroad tracks that carry major Amtrak and freight traffic and bisect the property. It will be fenced off. Access to the river sections of the park is under a railroad trestle.