On a bright Sunday this spring, a family set out on a drive to the mountains southeast of Los Angeles. After dancing the freeways through sprawling suburbs toward a notch called Crystal Canyon, where the road narrowed, the family's car came to a halt amid a pileup of traffic.

"Must be an accident," said papa as he inched the car forward and spied California Highway Patrol officers directing traffic.

But it was no accident. "The canyon is full," said an officer. "You'll have to go back. The canyon is closed."

The canyon is closed: A concept to live by in the 1990s?

The latest troubling news locally is that the National Park Service has withdrawn objections to a four-lane highway smack through the middle of Seneca State Park in Gaithersburg.

Great Seneca Highway, due for completion in 1990, thus has cleared its last apparent hurdle on the way to carrying 40,000 cars a day between burgeoning housing subdivisions in Germantown and burgeoning office projects in Gaithersburg/Rockville.

Seneca State Park Superintendent Cliff Denney, who pushed for improving existing roads rather than bulldozing the woods to make a new crossing over Seneca Creek, shook his head when asked about it.

"I work for the state, and the state backs the road," he said. "I have to support the state position, and we'll do our best to live with it."

Still, "I don't know what the purpose of that road is," he added. "I probably shouldn't say that, but . . . "

Denney is a slow-talking, nature-loving refugee from the Eastern Shore who appreciates the deer, quail, rabbits, raccoons and songbirds that inhabit his 6,700-acre refuge. Recently, he watched an immature bald eagle dive from its perch and snatch a fish out of the park's 90-acre Clopper Lake.

The county won federal approval for the new road only after promising to mitigate the effects of the creek crossing by buying replacement land for the mile of woods the road will eat up, erecting sound barriers and building bridges so wildlife can pass freely underneath.

Lovely. But can you mitigate 40,000 cars a day through the heart of a sanctuary? Will eagles still come to this place? Will people still come to relax with commuter traffic roaring by?

At Crystal Canyon and Seneca Creek, from West Coast to East, the problem is the same: Too many people.

In a speech at the Outdoor Writers Association of America's annual meeting in Montana last week, Rupert Cutler of Population-Environment Balance, a Washington group lobbying for population growth policy, addressed the problem and posed a troubling question:

"Will the end of the 20th century be seen by historians as the end of the golden age of ecological diversity and stability?"

Cutler's concern is simple. When the Pilgrims settled in North America, the world's human population was a half-billion, he said. By Teddy Roosevelt's time, it was 1 1/2 billion; by 1972, it was 4 billion and today it's nearing 5 billion.

In some respects, that's fine; we're the dominant species, and it's normal to expand in numbers and range.

The danger comes when one species so dominates it starts squeezing out other life forms, diminishing ecological diversity, which is the key to any environmental system's long-term health.

Take a nearby example: In Back Bay near Norfolk, Eurasian milfoil, an aquatic grass introduced there by man, took off some years back and squeezed out other grasses. Then, when devastating disease later struck milfoil, there was nothing left to take its place. Back Bay, once one of the richest waterways in Virginia, became a barren wasteland, a milfoil graveyard.

Cutler sees ecological diversity similarly jeopardized around the world by man's unchecked expansion.

Example: "According to the World Wildlife Fund," he said, "we're losing {Latin American} tropical forests equal to the size of the state of New York every year, and the result may be that . . . between 500,000 and 1 million species may become extinct by the end of this century. The forests are being converted to feed an exploding human population."

He contends the United States, with the highest population growth rate of any industrialized nation, ought to be paying attention to the consequences of excessive population growth and looking at solutions.

He cited this observation, made by the Rockefeller Commission on Population Growth and the American Future 15 years ago:

"It is clear that the land-use explosion of 'spread city' is currently in full bloom . . .

"Without proper efforts to plan where and how future urban growth should occur, and without strong government leadership to implement the plans, the problems of sprawl, congestion, inadequate open space and environmental deterioration will grow on an ever-increasing scale."

Cutler's organization is working to encourage people and governments to stem population growth by lowering birth rates around the world, a tall order.

Meantime, will Americans bother to plan for growth at the local level and assure that it doesn't destroy their surroundings?

Will they find better places to put highways than through the middle of a park 15 miles from downtown? Will they concentrate on making blighted urban areas livable instead of bulldozing rich forests and whacking away at farmland to build endless rows of vinyl-sided townhouses?

Will they set aside enough recreation land around places like Los Angeles so the canyons aren't closed at noon on Sunday?

One wonders. "As far as I can tell," said a Maryland environmentalist during a recent discussion of the Great Seneca Highway and other dubious road projects around the state, "the development policy in this state can by summed up in three words:

"Build, build, build."