More than a third of all international visitors to the United States find time to visit the National Park System, according to the U.S. Travel Association, which notes that the numbers have been slowly, but steadily, increasing. Of the 38.4 million overseas visitors who came to the United States in 2015, 35.4 percent, or 13.6 million, traveled to a park destination, the association said after analyzing Commerce Department data.

"Even though a number of factors are exerting downward pressure on inbound travel volume, we continue to see pretty healthy trends in numbers of overseas visitors," said U.S. Travel Association President and CEO Roger Dow in a release. "There are a number of good theories as to why—one is that many of the country's prime attractions, like the Grand Canyon or Yellowstone, can't be replicated just by going to a cheaper country; another is that the U.S. offers world-class activities at the full range of price points, from the tip-top of luxury to the freest of the free. Both of those are born out by the growth we are seeing in international interest in our national parks."

U.S. Travel economists expect the share of international visitors stopping at U.S. national parks during their trips to steadily increase. The total number of overseas arrivals to the United States is expected to reach 40 million in 2017, and the share of these travelers that visit a national park or monument is projected to increase to 36.5 percent. As a result, national parks and monuments will likely receive about 14.6 million overseas travelers in 2017, up 7.3 percent from 2015.

Publicity around the National Park Service's 100th anniversary this year, supported by Brand USA and other organizations, is a likely contributor to the bump, the association said. But so, too, is the U.S. dollar's continued strong performance against other currencies—meaning that as overseas travelers come to the U.S., they are likely looking at more cost-effective destination options.

In 2015, the United Kingdom, China, Germany, Australia and France were the top five sources of overseas visitors to national parks and monuments, accounting for nearly 43 percent of total overseas visitation to national parks and monuments.

Share of Overseas Arrivals Visiting NPS

The National Park Service reports that visitors to national parks and monuments last year spent a combined total of $16.9 billion during their trips. That spending, which largely occurred in local "gateway regions," or the communities within 60 miles of a park, supported 295,000 jobs and $11.1 billion in income for American workers. This in turn resulted in $32 billion in economic output for the entire U.S. economy.

"U.S. national parks house some of our country's most breathtaking natural and cultural treasures. As the U.S. dollar continues its strong run, promoting our national parks and monuments as the world-class attractions they are is imperative to reaching the U.S. government's official goal of 100 million international visitors to the U.S. by 2021—and for fueling economies in every corner of the nation," said Mr. Dow.