A FIRST ENCOUNTER with Lake Tahoe is a breathtaking experience. The huge crystalline lake surrounded by high mountains is a place of magnificent natural beauty, unique in this country, and the air is, or used to be, clear, scented slightly by great numbers of pine trees. But now, unfortunately for everyone, it can be a "breathtaking" experience in other, less attractive ways. Pollution, has struck, and because the lake straddles the California-Nevada border, effective local management of its development is impossible.

A recent federal study documents the saddening trends at Lake Tahoe. The tourist trade is up nearly 90 percent in eight years, and to serve that influx the permanent population has grown from 3,000 in 1960 to 75,000 today. With that growth have come smog, traffic jams, air pollution, water pollution in the lake itself, threatened wildlife -- in short, an environment "strained to the breaking point." It is becoming, continues the federal study, "the very kind of urban environment which many [visitors] seek to escape."

In recent years as many as a quarter of a million visitors have jammed the area on summer days. What do most of them come for? Not, as you might have guessed, to hike, mountain climb, swim, sail, bird watch or, in winter, ski. Two thirds of them come for blackjack, roulette and the slot machines -- to gamble at the casinos on the Nevada shore. According to The Los Angeles Times, activity at the gaming tables is growing 17 times faster than outdoor recreation.

Some years ago, California and Nevada formed a joint agency to manage Tahoe's development. But the agency lacks the necessary authority, and decisions are left in the hands of the developers and the gambling industry. The best -- and perhaps the only -- hope for the future lies in designating Lake Tahoe a National Scenic Area. This would place the area under the management of the U.S. Forest Service, which would manage it as a national resource and have the authority to prevent further deterioration.

Without getting into the rights and wrongs of legalized gambling, this is beyond question not an enterprise that is enriched by a beautiful environment: it can be pursued just as happily in downtown Anywhere. Or, say, in Las Vegas. If the fragile environment at Lake Tahoe cannot support the thousands who want to gamble, it would make sense to put the casinos somewhere else. Tahoe's scenic qualities are on a par with those of Yosemite, Yellowstone and the Grand Canyon. They should have the same protection.