For Robert S. McGarry, a general manager of the Washington Suburban Sanitary Commission (WSSC), running a giant water and sewer agency is like fighting a war.

"War is a whole series of unexpected events," said McGarry, a retired Army general. "So responding to an emergency is what I've been prepared for in my whole life."

McGarry's most recent emergency occurred when a July 6 fire at a WSSC water pumping station led to a crisis that consumed suburban Maryland for three days, and threatened the water supply of 1.2 million residents of Montgomery and Prince George's County.

In that crisis, during his second month at the helm of the WSSC, McGarry took command as if he were in the field. He ordered a helicopter, flew to the scene, radioed instructions back to headquarters, formed teams to solve different problems and made emergency contracts to repair the damaged pumping facility.

His efforts worked, and now McGarry is helping prepare other regional officials for similar crises. At the same time, he is working on long-range solutions designed to prevent or ward off future water shortages.

McGarry, overseer of the water and sewer agency's $290 million budget, is a soft-spoken retired Army engineer who claims a penchant for compromise and quick action.

A native of Oak Park, Ill. and a West Point graduate, McGarry, 46, retired from the Army this year with the rank of brigadier general after 25 years of service.He lives with his wife, Marilyn, in a three-story townhouse in Potomac. They have two children, Melanie, 24, a nurse in Baltimore, and Paul, 21, a student at the University of North Carolina.

His Army experience included three years as district engineer of the U.S. Army Corps of Engineers in Baltimore - a job that included supervision of the Washington Aqueduct water system serving the District and parts of Virginia. He also managed major Army facilities in Berlin and Frankfurt, and served a tour in the Korean War and two in the Vietnam War.

In the assessment of the WSSC commissioners and county officials he serves, McGarry is meeting his goal of providing strong leadership for an agency that only a year ago was surrounded by scandal.

Five WSSC inspectors were indicted in January 1976 and charged with taking bribes. The indictments were later thrown out on technicalities, but they brought public dissatisfaction with the agency to a head last year. After longtime general manager Robert McLeod retired, McGarry was hired by the commission this year.

"He's a super, super guy. He just about does everything right," said WSSC chairman Vera Berkman, who originally preferred another candidate for McGarry's job. "My thinking about an Army general was that he would do things the way things were always done - and I was completely wrong."

William B. Amonett, chairman of the Prince George's County Council, said McGarry has impressed him as "the kind of man who's really taking the bull by the horns."

Marjorie Johnson, McGarry's staff coordinator, offered the most graphic description. She moved from the WSSC public affairs office to begin working as his chief staff assistant a week after he took office.

"I've found him to be a matter-of-fact person. No frills, no flourishes. Just one-two-three. He's a doer - underscored," she said. "He might say he wants so and so done today. It behooves a person to get onto it because tomorrow he's going to ask, 'How's it going?' not 'Did you do it?'

"I have never been in the military, but I am fully aware that he's the general and I am the executive," she said.

McGarry describes his job and his manner of working more modestly. In the agency's daily work, he wants to "try to prevent screwups," while in long-range policy, he wants to take decisions reached by the counties that the WSSC serves "and move the hell quickly on it."

A day in the life of the general manager of the WSSC generally lasts 12 hours, and might as easily involve a visit to a sewage treatment plant as an important government meeting.

On Monday July 25, a day McGarry picked as typical, he began his working early, leaving home at 7:15 a.m. and arriving at WSSC headquarters on Hamilton Street in Hyattsville in time to take care of some paperwork.

An agency like the WSSC of course works for the public and the ratepayers," he explained. "That means meeting a lot of people, seeing them, hearing their concerns. I've been devoting "my days to seeing people, and I've been doing paperwork at home at night."

McGarry said he likes to "get out and see the action," so he tries to set aside one day a week for special trips to WSSC installations. Sometimes he stops off at a plant on his way to or from work.

He said the field runs provide visible leadership for the agency's 1,800 employees, and leave employees with a picture of what the new boss looks like: "He's 6-foot-6 and smokes a lot."

McGarry also believes he can better understand and solve problems after seeing them firsthand.

"After 27 years in the business, I can walk through a project (like the Piscataway sewer plant) and get a pretty good feel for the quality of work, the quality of mangement and how the work is progressing," he said. "And many technical problems are better solved, I have found, just by standing there and looking at the thing or the proposed thing."

On July 25, a heavy meeting schedule precluded field trips. If a problem arose in the field, McGarry would hear about it quickly anyway. The WSSC's control room, which receives all emergency calls and monitors the operations of the entire system on more than 200 dials and gauges, is only a few steps away from his office on the third floor of the WSSC headquarters.

At 9:30 a.m. McGarry arrived promptly at the WSSC's auditorium for a meeting of the Bi-county Water Supply Task Force, a group of officials from Montgomery and Prince George's Counties and the regional park and planning agencies. He waited patiently for the meeting to begin, a half hour late.

While he often lets subordinates take his place at task force meetings, he considered this one important. The task force was to hash out two key questions concerning water supply planning: What is the demand?; What alternatives are acceptable for supplying that demand?

A consultant gave public officials on the task force a picture of why these are what McGarry calls policy questions: "The amount of water you need is not written in stone," he said. "It's a trade off between how much you are willing to accept those sort of constraints (periodic water crises) and how much you want to pay to avoid it."

Montgomery County Council President John Menke urged that local governments plan to accept periodic water crises and limitations rather than pay much more for a nearly crisis-proof water system. McGarry, drawing on his recent experience, warned of the management and equity problems that would arise with periodic water shortages.

McGarry generally sat back at the task force meeting, letting the local officials work out solutions. When they deadlocked or seemed ready to discard alternatives he thought should be left open, he made suggestions that were readily accepted. He left satisfied, he said with the solutions accepted and the process of compromise by which they were achieved.

"One of the things I feel strongly about are that the decisions with water and sewer are with the counties. The old days of the WSSC planning a sewer plant here and just telling everybody about it are long gone," he said.

"I know of nothing we're doing that doesn't have the support of both counties and their rather strong support. We're extremely sensitive to the need to keep both counties fully informed of what we're doing and fully involved with the planning process."

McGarry left the task force meeting as it closed at 1 p.m., for a drive to a meeting in Annapolis and a quick sandwich in the car.

The Annapolis meeting concerned the Potomac River Low Flow Agreement, by which the various government agencies and water supply authorities in the Washington region - with the exception of the Corps of Engineers - have agreed on how they will withdraw water from the Potomac in times of low flow.

McGarry initiated the low flow talks when he was distrct engineer for the Corps. Now he is keenly interested as WSSC, manager, because an agreement signed by all parties is needed before the SWCC can construct a permanent weir in the Potomac to help the flow of water into its main water statioN.

As an often-heated discussion proceeded among officials of Maryland, Virginia and the Corps in a small room at the Tawes State Office Building, McGarry was looked to as a compromiser. He compiled, and the meeting adjourned with his compromise on the table.

On the way back to Hyattsville, McGarry explained, "We've got to keep 'em talking and out of the courts." While officials from Virginia and Maryland seemed eager to invite a court fight over rights to water in the Potomac, McGarry called that the worst way to proceed.

So far there has always been enough water in the Potomac for everyone: There has never been more demand than water on any one day.But the low flow agreement in crucial for the day when demand exceeds flow, he said.

"It's a possible event - if we had a hot, dry summer," McGarry said. "The Potomac River is flowing less now than it did in the 1966 and 1933 droughts. And our demand is high. We'd be divvying up a shortage (if demand exceeded water flow) . . . and everybody rightly wants to get their share.

"I really believe we can compromise and protect everybody's interests. If we get into the courts, we could all be delayed. Virginia has an urgent problem in that they need more water, and we need a weir. If you go to court, it could take years."

McGarry arrived back in Hyattsville late for a meeting of his technical advisory committee. He settled down with a 5 p.m. cup of coffee and talked over emergency regional planning with Frank Lamm of the Washington Area Council of Governments and, later, a WSSC contract matter with assistant general counsel, J. Lloyd Niles.

It was 7 p.m. by the time McGarry was ready to leave his office. He had already emptied the evening office coffee pot - he even drinks coffee before going to bed at night - and packed a bulging briefcase full of reports and paperwork.

He ruminated on his talk with Lamm on emergency planning, and "how fast you can respond when you relax your normal ways of doing business," as he did in the hours and days following the early July water crisis.

And he applauded the water supply task force for its accomplishments that morning: "I jokingly say the best Christmas present I could get would be for the task force to submit a report."

He acts as if he might really mean it.