Study co-author Ivan Rudik said air quality warnings issued by parks and other government agencies may be causing the visitation drop.
Rocky Mountain National Park (RMNP)
Park officials said the decision was based on the extreme fire danger and the large number of visitors expected this month.
More than 300 million people visit national parks every year -- but only 7 percent of them are African-Americans.
From strenuous peak-topping climbs to peaceful walks on the Eastern Plains, Colorado offers plenty of wildflower views.
The $35 fee applies mostly in the West: Yellowstone, Zion, Bryce Canyon, Mount Rainier, Rocky Mountain and Grand Teton parks, among others.
The plan would force visitors to pay $70 per vehicle during the peak summer season.
The proposal comes not long after many of the parks that charge entrance fees raised them. The rationale is the same — to address a backlog of maintenance and infrastructure.
Secretary Zinke has mentioned public-private partnerships for campgrounds multiple times, including with reporters and at a Recreational Vehicle Industry Association meeting.
In Rocky Mountain National Park, $200,000 matched by $200,000 from the Rocky Mountain Conservancy will go to trail maintenance.
Let's face it: there's often a gap between what the pros shoot and what the rest of us come away with at Rocky Mountain National Park.