Two weeks ago at 7th and S Streets NW, the city's newest marketplace for heroin, the police turned on the fire hydrants and let water flood the gutters. The idea was to let water wash away the drugs and needles that the small-time dealers and junkies hide next to the curb or under cars to keep the drug handy, in case they need it, but off their bodies, in case the police search them.

The water rushed out and created a scene. Drugs went down the sewer. Junkies went out of their minds, and the distributors who supply the small-time dealers went out of their way to avoid all the trouble at 7th and S.

The swarming tribe hovering on the corner was off balance for the moment. But the next day Police Chief Maurice Turner ordered the water strategy stopped. He said it was reminiscent of the days when southern policemen used water hoses on demonstrators.

The chief has his heart in the right place. But he doesn't live in the neighborhood. I do. With that crowd, that blight, in your neighborhood, near your wife and child, you lose all concern for appearances. If water gushing down the streets will stop the drug deals and make the small-time thieves, punks and crazies go elsewhere, then turn on the hydrant, chief.

In every direction from 7th and S, home owners grease their fences to keep addicts from leaning against them, and from nodding out in front of their homes. When a fire started late one night on S Street, from unknown causes, residents wondered if the junkies and hustlers were retaliating for the greased fences.

But the neighborhood can't retaliate for double-parked cars with people nodding out in them. The neighborhood can't retaliate for the dead people found in vacant buildings. And the neighborhood can't get even for the all-night happenings in the shooting galleries where the junkies go to inject. How do you retaliate for the junkies doing their act before the neighborhood kids, who are easily fascinated by money changing hands, white envelopes being secretly passed about and chatter about "getting fired-up," "copping a taste," and "killer dope."

Thursday night. It's raining on and off, and the rain seems to be washing every junkie, wino and hustler in Washington to the corner of 7th and S, making it the city's newest cesspool for low life.

By 6:00 p.m. the crowd of about 300 is swarming: swollen, scarred junkies waiting for dealers; minor-league drug dealers waiting for a drop from a bigger drug dealer, "the candy man"; and the part-time junkies, driving by, looking for a dealer they recognize so they can start their weekend of wasting away a little early.

The corner for this carnival of slow death used to be 14th and U streets. But things have been hot over there for the last few months since political pressure from the people who live in that neighborhood forced the mayor and the police to get addicts moving. The police started filming the beehive of deals at 14th and U and 14th and Wallach and walked up to people and snapped their pictures. Junkies don't like the spotlight, so the action moved south to 7th and S, where the sunken faces could remain anonymous.

Seventh and S has always had some drugs being sold nearby, as have 9th and O and other intersections in the middle of the city's poor downtown neighborhoods. With the flight from 14th and U, however, 7th and S now rivals 14th and U for billing as Drug Central. The police have followed the addicts to their new home, but it is the same no-win scene that the good guys and bad guys played on 14th Street: the junkies cluster on the corner, the cops bust some of them sometimes, but the corner stays crowded, no matter what the cops do.

Patrolman Roy Derr and his partner, Clarence Black, turn left off 7th Street and pull their blue and white patrol car into a vacant lot. The car crunches on old syringes and empty packages of Kool cigarettes--a junkie favorite--as it rolls to a stop at 7th and T. As Derr, a good, aggressive cop even after six years in this area, gets out, the edge of the stormy gathering on the corner begins to cross the street. When Black, the calmer partner, gets out and walks into the crowd with Derr, the swarm shifts.

As they walk, 300 people move in front of them, at the same pace, like a twilight zone version of Simon Says. A few stragglers break off, some stop at the ice cream truck. ("That guy makes some money," says Derr. "Junkies got to have sweets.") The crowd turns right, beginning to circle the block. The police follow behind them. By the time the policemen get to 7th and S, a block from where they started, there is already another swarm of punks, hustlers and junkies back at 7th and T. The crowd there is bigger than it was when the cops drove up.

Black and Derr are not the only police on the scene. In a five-minute period, a half- dozen police cars criss-cross 7th and S. Two pairs of police walk by. The cops go into the swarm, pinpointing people drinking beer on the street--illegal in the District--littering or loitering in cars. The idea is to let every one know the cops are here. Still, the blob that eats 7th and S grows.

"This stuff destroys the neighborhood," says Black, who grew up in the city. "They were starting to do some of the same thing dealing marijuana on the street over on Peabody Street where my grandmother lives. They'd just walk up to you with the stuff in their hands and ask you if you need some. . . . The District police got on it, and I did some off-duty policing. If you let that go on, you'll get muggings, purse-snatchings, break-ins, people leaving the neighborhood in a minute."

Since the police can no longer flood the area with water, they now flood 7th and S with police from the Third District, from the Special Operations Division and from the vice squad. But the beat of drug sales goes on, and the swarm grows.

"These houses," says Jim Spicer, who lives around the corner from 7th and S, "were torn up after the riots. Nobody lived in them from '68 to December of '76. Three contractors redid the whole block. I paid about $30,000 for mine. Now it's assessed at $70,000, but I would not sell for that little.... We've got a Metro stop coming at Rhode Island Avenue, we've got schools right here, Howard up the street. It is close to downtown. This area could be real nice if we could get rid of the junkies."

Get rid of the junkies, Chief Turner. You don't control the flow of heroin into the country or national policy on dealing with heroin. But if the city is to live, somehow the junkie carnival will have to be shut down. Moving the circus from 14th and U to 7th and S and maybe Anacostia next is not good enough. It may be time to think about what legalizing heroin would do, to ask city officials to get after the owners of the vacant buildings where the addicts get high, or to devise with the U.S. attorney new ways to enforce more laws that could help get the sick swarm off the corner.

The writer is a member of the edi torial page staff.