YOSEMITE NATIONAL PARK, Calif.—When Christine Romalewski imagined her first trip to this national idyll, the New Orleans resident pictured remote lodges in a majestic wilderness. Deer and squirrels would be the only other creatures she and her husband, Rob, might encounter.

“We didn’t realize we were going to see folks in bathing suits, tubing, and people walking through meadows and people walking along the sides of the highway,” said Mrs. Romalewski, 67 years old, surrounded by throngs of picture-taking tourists in late June....