Alaska's national parks are seeing a boom in tourism, despite a slightly downward trend nationally. Last year alone, parks in the state brought in nearly $2 billion. National Park Service officials say a new set of Postal Service stamps that highlights two Alaska rivers can only help.

"It's free advertising," said Bert Frost, regional director for the National Park Service in Alaska. 

Alaska's Koyukuk and Tlikakila Rivers are featured in the postal service's new, wild and scenic rivers collection. 

"People are going to buy the stamps and say well, first, they won't be able to say the name of the Tlikakila River, and then they'll go, 'Maybe I should find out where that is and why it's so special. Why is it on this stamp?'" Frost said. 

While Frost said he doesn't know about a direct correlation between the stamps and tourism, overall, he says Alaska has seen a stable, upward trend in national park visitors. 

"We have been around 2 million, 2.5 million. This year we're up to 2.9 million," Frost said. He notes that the increase is likely partly tied to the increasing size of cruise ships and their ability to carry more passengers to Alaska. 

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