Rocky Mountain National Park's spectacular backcountry scenery helped draw a record number of visitors to the park in 2015/Kurt Repanshek

Add Rocky Mountain National Park to the 4 million club. Four million visitors, that is.

The park had already set a visitation record for a single year by the end of September 2015, and by year's end the total had grown to 4,155,916 visitors, which was a 21 percent increase over 2014. The highest visitation prior to 2015 was in 2014, when the park received 3,434,751 visitors. All months in 2015 set visitation records except for February and April.

That said, determining visitation is a difficult and imprecise effort, park officials note.

"Visitation statistics are reliably accurate estimates and help park managers see overall trends. Fall visitation, particularly on weekends, continues to increase at Rocky Mountain National Park," noted park spokeswoman Kyle Patterson. "The top ten busiest days in 2015, in order from first to tenth, were: September 26, October 3, September 6, September 27, September 20, September 19, July 12, August 8, July 3 and July 26.

Many other national parks in the Rocky Mountain West also had increases in visitation last year. Rocky Mountain began its Centennial Celebration in September of 2014, which continued through September of 2015. Other national parks celebrating their Centennial Anniversaries have experienced growth in visitation as well. Additional contributing factors of the rise in visitation include lower gas prices and increased population along the Front Range of Colorado, park officials said.

Park managers will continue to address what effect this level of visitation will have on visitor and staff safety, resource protection, visitor experiences and operational capacity.