The Department of Interior has approved a $1.8 million matching grant to Maryland to expand Seneca State Park by 1,000 acres and is expected to approve another $1.25 million in matching federal funds within the next few months to develop the park.

Seneca already is Montgomery County's largest park but one of its least known because it has been closed to the public since the state began acquiring land for it in 1955.

The stream valley park's existing 4,300 acres will be opened for the first time this fall or early next spring. In anticipation of the federal development grant, the state last month began constructing buildings and boat docks at the park's large, fish-stoked lake west of Gaithersburg.

When all of Seneca's 6,609 acres are purchased - and connected to several county parks also being created now - it will provide a large green belt of open space across Montgomery County's raid-section from the Potamac River to beyond Montgomery Village and Gaithersburg to Route 108 near Lylonsville. The parkland follows the Great and Little Seneca creeks and their tributaries.

The $1.8 million grant brings to almost $32 million Maryland's share of federal Land and Water Conservation Funds since the matching grant program began in 1965. The program funnels revenue from the leasing of federal lands - such as off-shore oil leases - back into the purchase and development of parkland.

The National Park Service also hopes to get $20 million in Land and Water Conservation funds during the next fiscal year to buy additional federal parkland in the greater Washington area, including $7.9 million to purchase 1,800 acres for the C&-O Canal National Historical Park. Most of the canal parkland remaining to be bought is west of Montgomery County.

Announcement of the Seneca State Park grant was hailed by state and county officials as a boon to Montgomery County residents and as a buffer of open space separating fast-growing sections of the county such as Germantown and Gaithersburg.

"We're looking forward to Seneca since it will complement our stream valley park system," said Tom Pearce, principal park planner for the Maryland National Capital Park and Planning Commission (MNCPPC).

The bi-county park agency has at least four parks that soon will contain more than 5,000 additional acres of open space connecting them directly with Seneca State Park. Most of these county parks are undeveloped and relatively unknown to county residents, said Pearce, although residents can hike through them or fish in the streams if they can find the unmarked parkland.

Seneca State Park originally was proposed in the early 1950s as a 1,000-acre park to protect some of the vast Seneca Creek watershed, the only stream valley and flood plain in the county that it still unprotected. The park's planned boundaries were expanded in 1966 to 3,500 acres, when the first significant land purchases were made, and in 1972 the legislature enlarged the ultimate boundaries to include 6,609 acres, or about 11 square miles.

Much of the current development in the park will be around the lake on Long Draft Stream, with boat docks and ramps, park maintenance and administration buildings and picnic areas and shelters. Although the lake was stocked with fish two years ago, fishing will be prohibited until the park is officially opened since the only access is through the busy construction site.

Seneca will be the largest park within the county. Patuxent State Park, with 7,960 acres when it is complete, will be somewhat larger but it is in both Montgomery and Howard counties.

Maryland has two other state parks in Montgomery County - the McKee-Beshers wildlife are, which contains 1,500 acres adjoining the C&O Canal and Seneca State Park, and the new Monocacy Natural Resource Area, which was acquired last year. The Monocacy area presently contains 1,475 acres and ultimately will contain 2,000 acres.

Adding to the open space around Seneca State Park are the four parks of the Maryland National Capital Park and Planning Commission:

Great Seneca Extension, as the MNCPPC park near Montgomery Village is called, consists of only 692 acres now but will have 2,323 acres when the remaining land within the park's boundaries is bought over the next six or seven years.

Little Seneca Park now consists of 417 acres off White Ground Road north of Dawsonville, but ultimately will contain 1,172 acres,. It is not officially open to public use but residents can tramp across it.

Little Seneca Regional Park (regional parks are more intensively developed for recreation) is farther up-stream and almost all the land for the MNCPPC owning 789 of the planned 853 acres. Although not officially open to the public, it is accessible off Clarksburg Road (Rte. 121) less than a mile north of the Baltimore and Ohio Railroad tracks.

South Germantown Regional Park, near Clopper and Schaeffer roads, already has some ballfields on its 511 acres. The park ultimately will be expanded to 773 acres over the next six years.