Like millions before him, President Carter made the tourists' pilgrimage to Old Faithful yesterday.

The Carters helicoptered into the park, avoiding the traffic that sometimes congests the area.

About 2 1/2 million people come here each year, almost all in the summer. Parking lots are clogged with out-of-state vehicles.

While there is still a vast wilderness of rivers, canyons and forests in Yellowstone and in neighboring Grand Teton, vandalous visitors have wreaked a kind of havoc of nature in some areas.

Rangers say, for example, that Morning Glory Pool, one of the famous hot springs here, has become 20 degrees cooler because of the trash thrown into it. Once blue, the water is now orange.

Carter is in the second week of his western vacation. He looks remarkably relaxed and sunburned and hasn't been seen in coat and tie for a week.

Yet the president spent much of Friday on the phone to members of Congress discussing all the administration legislation backed up. Then he took a break to go sailing on Jackson Lake in the Grand Tetons Friday, he returned to a harrage of reporters' questions on the natural gas issue.

Old Faithful is neither the most regular nor the highest geyser in the United States. It is, however, the geyser that has changed the least over the generations.

That is because it is self-contained, with its own underground water and heat supply. Other geysers here are linked to vast underground channels throughout the park.

The springs here are kept hot by molten magma five miles beneath the surface. Yellowstone Park is described as the closest surface point to that layer of magma other than volcanoes.