The reservoir looks curiously like an acre or so of subterranean greenhouses.

It was a simple announcement, put out so quietly by the Department of Water and Power last week that it nearly obscured one small triumph of diplomacy in the agency’s greatly maligned program to upgrade its reservoirs.

The notice said that the Highland Reservoir, one of the oldest in the city, is about to be drained and replaced with a steel tank.

There was reason for the department to downplay the news. Almost every neighborhood which has a reservoir is mobilized these days to prevent the DWP from either covering or tanking it.

Reservoirs were originally made to store water for distribution to the city. But several of them offered such splendid views they came to be regarded as essential to the character of neighborhoods around them, most of which also happened to be quite wealthy.

Silver Lake in particular, taking its name from the man-made gem in its midst, faced a kind of extinction if the DWP proceeded with a plan to cover part of the blue surface with gray, synthetic rubber.

Collectively, homeowners from Silver Lake to Pacific Palisades have put the DWP through a public lashing that seems to have drawn the mammoth agency to a stalemate. Its reservoir program has largely been on hold for re-evaluation.

Meanwhile, just a hop over the hills from Silver Lake, a scrappy little community group called the Highland Park Heritage Trust took on the DWP all by itself in a more traditional game of hardball politics. It produced what both sides agree is an acceptable, if rather odd, compromise.

The DWP will tear out the existing 20-million-gallon reservoir and replace it with a 3-million-gallon tank. It will retain a 1907 Mission Revival pump house in its original condition. The keystone of the deal is that the agency will cover the new tank with a simulated old wooden roof.

To appreciate why that is a deal, you must know that Highland Park, though now bursting with immigrant people, still retains the fierce loyalty of old-stock residents whose hobby it is to trace every building back to the original tract map.

They well remember, for example, that the north end was once the township of Garvanza, whose name is kept alive today only by Garvanza Elementary School and Garvanza Hardware.

The town was founded in 1886 by developer Ralph Rogers and annexed into Los Angeles in 1899. Its water came from what was then the Garvanza reservoir, purchased by the city in 1902. A tract map of that year shows it had a wooden roof.

The city expanded the reservoir in 1909, building a field of interconnected slat gables to cover the water. It is still there today, warped and splintered by time.

From the street, the reservoir looks curiously like an acre or so of subterranean greenhouses. Tucked away behind Luther Burbank Junior High School, it attracts little notice.

But it has been carefully monitored over the years by DWP engineers and by Charlie Fisher of the Highland Park Heritage Trust.

The DWP became alarmed recently by numerous breaches in the roof, which allowed sunlight to strike the water. Fisher, in turn, became alarmed that a link to the original Garvanza would be lost.

Fisher, who owns an old house in the center of Highland Park, is an expert at digging up historical portfolios. He and his associates in the trust have presented dozens of homes to the Los Angeles Cultural Heritage Commission for historic cultural monument status.

Fearful of losing one of the last relics of Garvanza, Fisher struck preemptively, nominating the reservoir as a monument. Los Angeles Councilman Michael Woo borrowed this technique several months later in nominating Silver Lake as a monument.

A few days before Fisher’s nomination was to be heard, the DWP sent someone to negotiate with him. They met at the Eagle Rock Municipal Building in January, 1989.

“They said the roof is deteriorated,” Fisher said. “My comment was, ‘Replace it.’ ”

The DWP isn’t used to such ultimatums. But it had a good comeback. The dam itself is seismically inadequate and could fail in an earthquake, it said.

“We decided that was something we didn’t want to deal with,” Fisher said.

He proposed nominating the reservoir as a historic site, rather than a structure.

That subtle distinction would allow the DWP to retire the lake. All Fisher asked in return was a new wooden roof to conceal the tank.

Since the tank will stow away in one corner of the lake bed, the new roof should actually more closely resemble the original, smaller reservoir of the days of Garvanza, not an unhappy circumstance for a group that prefers things as they were.

“That was our goal, to get the tank taken out of view and leave something looking like the roof,” Fisher said.

In its announcement last week, the agency mentioned nothing about a roof. It merely said the unused portion of the existing lake would be landscaped.

Fisher said he still hasn’t seen any plans for the new roof.

“At this point, I’m taking their word for it,” he said.

He isn’t especially worried. Because the reservoir site is a historic cultural monument, the tank plans will have to be approved by the Cultural Heritage Commission.

And that’s his arena.