Park officials say the drop in campers may be due to a slumping economy, higher gas prices, more competition for people's time, and changing demographics.
"The long weekend is replacing the two-week time off," Jim Gramann, a professor at Texas A&M University and a visiting social scientist for the National Park Service, told the AP. "That means fewer overnight stays in the national parks."
Gramann said population changes may also have an impact because of the growth among some groups that are not traditional parkgoers.
The Park Service reported that overnight stays in national parks fell by 13.8 million, or 20 percent, between 1995 and 2005, and have fallen an additional 4.3 percent in the first eight months of this year.
Visits to “gem parks” in the intermountain region, which include Rocky Mountain, dipped between 2 percent and 15 percent during that time.