With its parks, its wide tree-lined avenues and streets, and its formal monumental core, Washington is one of the greenest cities on the face of the Earth. And, thanks mainly to the height limitation and Maj. L'Enfant's foresighted plan, the city radiates a unique sense of spaciousness.

One hardly needs statistics to prove such assertions. But they exist. One study shows that Washington's streets, sidewalks and alleys take up nearly 43 percent of its total area, compared with 23 percent in London, 9 percent in Tokyo. Another set of figures, from the early '80s, tells that Washington has more than 45 square meters of park land per capita, compared with 30 in London, 2.7 in Tokyo.

Unlike conventional postwar American cities or, for that matter, unlike many contemporary cities worldwide, with their grid patterns and clusters of tall buildings, Washington's order of streets, buildings, parks and public monuments is strong, dynamic and clear. More than in most cities, one can measure distances accurately here with the naked eye. In Washington, more often than not, to take the measure of a place is to comprehend its symbolic import: the statue centered in the circular park, the domed pediment at the end of a street, the Capitol on its hill.

And in Washington, though there seems to be a general understanding that one tampers at peril with this balance, there's always change, and always something to argue about. One place where we're stumbling dangerously close to excessive tampering is in the monumental core, that central area encompassing our major political institutions and our principal national shrines. The danger is that we are in the process of significantly altering its character by peppering it with new memorials.

If the sheer number of proposed memorials is worrisome, far more so is the preposterous size of a few of them. The big offenders, so far, are the Franklin Delano Roosevelt Memorial near the Tidal Basin, the Korean War Veterans Memorial near the Lincoln Memorial and the National Peace Garden at Hains Point. Size is relative, of course, but given the limited amount of terrain available, these memorials and gardens are much, much too large. Furthermore, each in its way severely damages the sense of openness that is a rare attraction of its site.

The proposed Roosevelt Memorial, for instance, is longer than the Washington Monument is high. As currently configured it consists of about 800 feet of granite walkways and exterior "rooms," each ornamented with pools or massive falls of water and relief sculptures and inscriptions on walled enclosures. Designed in the mid-'70s by landscape architect Lawrence Halprin of San Francisco, the project suffered through a decade without much in the way of congressional funds beforere surfacing in an appropriations measure last year.

At the April meeting of the Commission of Fine Arts, various members objected to the amount and type of paving, the hardness of the edges and the obstruction of the view between the Tidal Basin and the Potomac River. Halprin will return next week with a revised design, but it's doubtful that it will change much in character or scope. Such changes to the memorial, he pointed out, would drastically harm its basic conception.

Make no mistake about it, Halprin is a master at creating high-voltage urban places where the sequence of activities is, as he often says, intensely choreographed. In Seattle he's transformed a stretch of land wasted by freeway construction into a multifaceted urban park connecting isolated fragments of the city. The splendid walkways and walk-on fountains he designed have brought new vitality to the heart of Portland, Ore. And he's performed similar magic in many different San Francisco places.

There is not much doubt that his Roosevelt Memorial would be successful, on its own terms -- its plantings are fine, its sculptures (by different, well-regarded artists) excellent, its spaces varied and attractive. A visitor would begin a trek on the south side, at a platform abutting Ohio Drive near the Potomac, and progress across a field on a paved, tree-lined walkway to the meandering sequence of "rooms" before emerging at the Tidal Basin's very edge.

But it is a forceful design plopped in the wrong place. There are locations in downtown Washington that might be improved dramatically by one of Halprin's patented sequences of hard-soft, closed-open spaces. Not this place, however. Here, it seems willfully out of sync; it would be a tremendous intrusion. It breaks the mold of white neoclassical buildings in open, green spaces and does so decisively -- this is an active urban park rather than a passive, contemplative setting. It would necessitate a sizable change in the roadway system of West Potomac Park, bring about an immense intensification of activity there and forever alter a quiet, open glade that has its own compelling allure.

This glade, to be sure, is now used of an evening for softball games, and at the insistence of the National Park Service, Halprin took pains to reconfigure the space so the softball fields would remain. Nonetheless, the glade would be split in half by the memorial's tree-framed walkway, and a sizable chunk of its northern edge, now loosely defined by a roadway and plantings of elms and the Tidal Basin's cherry trees, would become impenetrable.

Unquestionably, from the outside this memorial would be perceived as a barrier. In this sense, the design is all too rigid. One would either be inside, or outside -- there's little here of the pleasant give-and-take between structure and setting that makes so many of the older memorials so memorable. And consider this: The Park Service estimates that the memorial would draw 10 million to 12 million visitors per year. All by itself this hyperactive new park-within-a-park would do much to upset the already fragile balance between national and local uses in the monumental core.

Many of the same criticisms apply to the proposed Peace Garden, which would fill up the entire 12 acres at the tip of Hains Point -- one of the city's most gloriously open of open spaces -- with yet another intrusive, self-enclosed park-within-a-park.

There are four primary elements in the competition-winning design by architect Eduardo Catalano of Cambridge, Mass.: a hemispherical, glass-sheathed entrance pavilion; a formal sequence of curved walkways tracing the pattern of a giant, seven-leaf olive branch; a natural amphitheater at the tip of Hains Point for up to 4,000 persons; and a tree-lined berm surrounding the entire composition and continuing the existing perimeter pathway.

Conceptually, this is a simplified, Orientalized version of a formal, 18th-century French parterre, ingeniously adapted to new symbolic ends. The symbolism of this elegant olive branch, however, will be most remarkable when seen from the air. From the five-foot height of the perimeter berm its impact will be much less intense, and from ground level -- that is, from the viewpoint of most visitors -- it will be thoroughly invisible.

In contrast to Halprin's busy design -- which promises at the very least a series of varied experiences to the visitor who would enter its prescribed path, Catalano's design seems static, even empty. But it is no more accessible than Halprin's -- one cannot easily enter or leave the central space from the longitudinal edges of this very long composition. Nor are the proposed materials all that promising -- grass for the interstices between the leaves, a variety of green ground covers for the slightly raised surfaces of the leaves themselves and white flowers (symbolizing those of the olive tree) in circular planters where the leaves touch the stem and each other.

The designer is wont to invoke Oriental mysticism in his explanations -- "in the Western world the profound spirit dwelling in gardens has not yet been appreciated" -- but, unlike great Japanese gardens, this one is neither a carefully calculated succession of interesting events nor a beautifully scaled contemplative expanse. Rather, it is more like a diagram, a disappointing labyrinth at once too large and too simple, sealed in a big meadow.

Not nearly so extensive as these two proposals, the Korean War Veterans Memorial is nonetheless rather absurd in its internal scale. Though not necessarily too big for its beautiful site in Ash Woods, southeast of the Lincoln Memorial, it remains too big for its own britches.

The design idea here (by a competition-winning team associated with Pennsylvania State University) is to tell a simple, patriotic story -- 38 soldiers file toward an American flag flying atop a standard in a spacious plaza. It's an evocative image but quite a long march -- 350 feet from west to east -- from which, for visitors, there is no escape: This too is an enclosed system imposed on a beautiful swath of open ground.

It also sets a troubling precedent -- if the generals behind the Korean War Veterans Memorial captured this much terrain, will not the generals of World War II, the memorial for which is yet to come, feel justified in demanding lots more? The likely answer is yes, and the next question is, where in the world -- or, to the point, where in the monumental core -- could it comfortably be fit?

This is not just a wiseacre question. Given the ancient propensity of memorial builders to overstatement, given recent history and given the precious quality of the capital city's green open space, the time has arrived to say no to the inappropriate and/or the merely big memorial design. Indeed, the time will come right soon when we'll have to say no to all major memorials in the monumental core. In the meantime, we must recognize and protect the splendid character of what we already have.