WITH THE CREATION of Great Basin National Park, Congress has placed a spectacular landscape in eastern Nevada's deserts and mountains under the special protection that covers the park system. Its opening last weekend was the culmination of a campaign going back to the 1920s to preserve it. Great Basin won't be the last national park, but the architects of the system are having increasing trouble finding large sites to which their neighbors and the country as a whole are prepared to make that irreversible commitment to keep their hands -- and their shovels -- off. Even at remote Great Basin, Congress found it expedient to avoid a collision with the ranchers by deeming their herds to belong to the park's natural ecology.

Where will the next park be opened? Rep. Bruce Vento, whose subcommittee handles park legislation, thinks that it might be in Samoa, where traditional land-holding practices are more conducive than elsewhere in this country. There's a plan to create a Tall Grass Prairie National Preserve in Oklahoma, but in the system's terminology a preserve is under less stringent rules than a park and would permit, for example, hunting and oil drilling on the land.

The park system is now operating without much sense of direction. For a decade beginning in the late 1960s, it vigorously pursued a strategy of acquiring recreational and historical sites that, unlike most of the big parks, would be easily accessible to the places where most Americans live. But the Reagan administration has resisted further expansion, arguing instead that money ought to be devoted to better maintenance of the present holdings. Except for Harry Truman's home in Independence, Mo., Great Basin is the first addition of federal park land in six years.The park system has a good deal of internal momentum -- and momentum was chiefly responsible for the enactment of the Great Basin leg-islation, in the face of the administration's indifference. But currently there's no controlling vision.

The national park system, from Independence Hall in Philadelphia to the great western parks and beyond to the Golden Gate, tells its millions of visitors a lot about this country and the meaning of being American. That's why it's important to keep working on the parks. Progress has been slow in this decade but, as Great Basin demonstrates, it hasn't been stoppe