BECAUSE YOSEMITE is a flagship of the National Park System, the plans being completed by the Park Service for its future are critical. Policies adopted there are likely to be copied or adapted elsewhere. As Yosemite goes, so may go Yellowstone, the Grand Canyon, the Smokies, the Tetons and dozens of other national treasures.

The basic problem at Yosemite, as in many other parks, is too many people in too many automobiles. On a beautiful summer weekend or holiday, the valley of Yosemite Creek takes on the appearance of a shopping center; 50,000 people and 15,000 cars have been logged in one day. The impact of that much congestion in a narrow valley is obvious.

National Park Service Director William J. Whalen recently took the first step toward unclogging Yosemite. He announced a plan to move out of the valley the housing and other facilities used by about 1,400 park and concessionaire employees. The plan calls for a nonprofit group, Yosemite Institute, to build houses on the park's perimeter and sell them to employees.

This is an innovative idea that Congress ought to consider seriously. It would take the burden of financing this move off the taxpayers while giving Park Service employees an opportunity to buy homes. By clearing out what is in effect a small town, the plan would reduce congestion.

Unfortunately, remarks made by Mr. Whalen when he announced this plan indicate that the Park Service is backing away from other proposals to reduce congestion.A draft management plan for the park released in 1978 called for reducing slightly the number of overnight accommodations in the valley and for curtailing sharply the use of automobiles. Another proposal would have required most visitors to leave their cars in parking lots near the park boundaries and do their sightseeing on foot or in shuttle buses.

Mr. Whalen evidently believes that the experimental bus system used last year was not successful and that the costs of ridding the park of automobiles are higher than the Park Service can persuade Congress to pay. But unless his final blueprint for Yosemite's future at least moves toward a solution to the auto problem, Congress -- and the rest of us -- will find it a serious disappointment. The valley is much too precious to be left at the mercy of the automobile.