The study also found that fewer people visit national parks when ozone levels climb.
The lock was closed to navigation two years ago, but it still affords a great view of the falls.
Creating a national park facility at one of Minneapolis’ most popular spots would likely take years — and involve overcoming some major hurdles.
Rangers have cited some who have crossed into national parks, which are all closed by the government shutdown. Gate-jumpers could face up to six months in jail, although land policy experts raise some questions.
Some Americans are challenging government shutdown national park closures by leaping over barricades or tossing cones aside in acts they call civil disobedience, but which some authorities call just breaking the law.
Some Americans see the closure of national parks as politically motivated, but others say keeping the areas open during the government shutdown invites liability problems. For many, the parks strike a chord.