Triadelphia and Rocky Gorge reservoirs, which attract more than 200,000 visitors a year, may not be closed to the public after all, Robert McGarry, general manager of the Washington Suburban Sanitary Commission, told the commissoners at a meeting last week.

The suburban Maryland sewer and water agency announced plans earlier this fall to close the 25- and 35-year-old reservoirs to the public as part of $1 million in budget reductions that include eliminating 104 WSSC jobs, six of them guard or maintenance positions at the reservoirs.

The WSSC's 1.2 million water customers have been subsidizing recreational use of the reservoirs, although "many of these customers do not visit the WSSC reservoirs . . . and receive no direct benefit" from them, McGarry said in a press release issued at last week's meeting.

"The basic goal" will be to keep the reservoirs open "with zero impact on the customer water rates . . . and without detriment ot the basic water supply function" of the two lakes, he said.

The WSSC is now proposing that the state or counties operate the reservoirs, which lie along the Patuxent River on the border between Montgomery and Howard counties and in part of Prince George's County.

The agency also is considering raising the fees charged for boating, fishing and riding and instituting fees for the thousands of picnickers who now have free use of parking areas and picnic grounds. In addition, the commission is considering use of volunteer help -- members of the Izaak Walton League's six Montgomery County chapters have volunteered. Another proposal calls for contracting with a commercial firm to operate the reservoirs and the 6,000 acres of woods and fields around them.

McGarry said his staff will evaluate the alternatives and report to the commission no later than Feb. 15, a month before the reservoirs usually are opened to the public for fishng and boating. The lakes were closed Saturday for the winter, but the WSSC had announced that they would not be reopened next spring.

WSSC staff members have already met with Maryland wildlife officials, who have offered to take over the job of patrolling the small areas around the reservoirs that have been open to hunters, and have asked for a meeting with officials of the Maryland State Department of Natural Resources.

McGarry told the commission, whose six Prince George's and Montgomery County members said they have been deluged with calls and letters protesting the closing of the reservoirs, that he was "fairly confident" of finding a way to reopen them next spring.

The WSSC has been charging $10 for seasonal boating and fishing permits at the lakes and $10 for use of the bridle trails around them. In 1978 more than 3,000 season fishing and boating permits were issued, another 400 mooring permits, costing $10 each, and almost 7,000 one-day $1 fishing and boating permits were issued.

The fees brought in about $42,000 last year. The annual cost of operating the reservoirs has been about $200,000 -- including about $15,000 a year to repair damage from vandalism -- but Mc Garry told the commission last week he thought the lakes could be operated for less.