Grand Canyon on the Precipice.

Today the threats to the canyon’s future are “real and ongoing,” Uberuaga says. Despite a 2012 ban on new uranium mines on a million acres of public land around the park, the canyon’s waterways are still at risk of pollution from grandfathered mines and those on nearby state land, he says.

Meanwhile, the wilderness experience on the canyon floor, where the park limits river trips to 25,000 individuals a year, is being disrupted in some parts by noise overhead. An estimated 300,000 flights annually now cross over the canyon, Uberuaga says.

Air traffic could increase even more with proposals to expand the state of Arizona’s Grand Canyon Airport and flights at the Hualapai Tribe’s Grand Canyon West. The seven-year-old development on the west side of the canyon, home of the popular glass Skywalk attraction, became accessible to tourists by a paved road in early August. Up to a thousand people a day take helicopter trips to Hualapai land in the canyon, then embark on short boat trips on the river, Uberuaga says.

But the planned developments to the south and on Navajo land to the east are raising the most concern because of their scale and location.