The rocky shorelines, shifting deserts and winding canyons of the country’s 59 national parks have been hallmarks of American vacations for generations. But the number of park visitors has reached an unprecedented level, leaving many tourists frustrated and many environmentalists concerned about the toll of overcrowding. In 2016, the National Park Service tracked a record 331 million visits, and after a busy summer, the system is likely to surpass that number this year. In August alone, some 40 million people came through park service gates. Shuttle buses at Zion National Park, in southwest Utah, filled like sweaty subway cars. Selfie-takers clogged the slender path through the Narrows slot canyon, one of the park’s best-known attractions. And at the top of Angels Landing, an iconic trail of switchbacks on the east side of the park, some portable toilets were marked off with a sign: “Due to extreme use, these toilets have reached capacity.” Zion is among the most visited parks in the system and is particularly prone to crowding because many of its most popular sites sit in a narrow six-mile canyon. In 2016, about 4.3 million people visited, up 60 percent from a decade ago.So this year, park managers announced they were considering a first for any national park: requiring reservations for entry. A final decision is expected in 2018. “We don’t have a choice,” said Jack Burns, who has worked in Zion since 1982. “We have to do something. If this going to remain a place of special importance for generations, we have to do something now.” The National Park Service was created in 1916 to protect the country’s growing system of parks and monuments. Its mandate is to conserve scenery and wildlife while also protecting visitor enjoyment for generations to come. For years, the lack of a reservation system for park entry aligned with the service’s ethos of democracy and discovery: Anyone could come, pretty much anytime. (The service has long required permits for hiking in more remote areas.) But lately, both visitors and nature are suffering. Mr. Burns, who is on a team that is considering a reservation system, said some people showed up for a vacation they had planned for months, spent a day in the gridlock and turned around. Rangers, stressed by the frustrated masses, have started a monthly meeting to discuss “visitor use” that some say has turned into a group-therapy session. And Zion’s delicate desert ecosystem has been battered by tourists, some of whom wash diapers in the Virgin River, scratch their names into boulders and fly drone cameras through once quiet skies. The park has about 25 miles of developed trails. But over time, rangers have mapped about 600 miles of visitor-made paths, which damage vegetation and soil and take a toll on wildlife. The story is similar at parks from Yosemite in California to Acadia in Maine. And the crowding problem comes as the system faces the dual threat of a funding shortage and climate change. The park system has a maintenance backlog of more than $11 billion and President Trump has proposed a 13 percent cut to the service.