The past four years have seen an explosion in visitors to national parks in the West, prompting concern about environmental impacts – from air pollution and erosion in the parks to traffic congestion in nearby towns.

But the effects on Zion National Park in southwest Utah are unique, thanks to its star attraction. The most popular trail in the park takes visitors right into the streambed of the Virgin River, through a scenic section called the Narrows, where hikers wade in the water between steep redrock cliff walls.

As many as 4,000 people a day tramp through the streambed in the Narrows, double the number who tried it just a dozen years ago, said Dave Sharrow, hydrologist for the park.

In total, 4.3 million people visited Zion in 2016, a 65 percent increase in five years.

“The biggest thing we deal with right now is the crowding issue,” said Sharrow. “Our rangers were once able to patrol the backcountry and borders. Now all they do is hassle with visitors. Our maintenance folks can’t keep up with keeping the toilet paper stocked and the trash cans emptied. They’re overwhelmed by the workload.”

Harder to measure are the impacts on the Virgin River. All those feet trampling the river have increased erosion and affected aquatic life.

There is also the problem of “more trash and more poop in the bushes,” as Sharrow puts it. Visitors who didn’t plan ahead for their bathroom needs before starting a hike often end up doing their “business” within the narrow canyon walls – behind a bush or somewhere along the streambank.

The result is not only unsightly, but it increases the risk of e. coli contamination in the water.

Sharrow explained that the Virgin River already has a serious e. coli problem from heavy livestock grazing upstream. Cattle waste washes into the national park and it inevitably concentrates in the Narrows.

As a result, he said, it’s difficult to distinguish the hiker-caused e. coli problem from the background levels produced upstream. Even so, he’s confident it’s a problem.

“Intuitively, we think there’s got to be an impact,” he said. “But we also think that you don’t have to capture bacteria in a sample bottle in order to say that lots of poop in the bushes is not a good thing.”

In an effort to address the problem, the park added a toilet at the upstream end of the Narrows. It also passes out waste disposal kits – so-called “wag bags” – to all hikers who take out a permit to hike the full length of the Narrows starting from the top. The point is to encourage visitors to carry out their poop.

But Sharrow acknowledged this practice misses all the hikers who hike through from the bottom, which is where the park has seen the greatest increase in hikers. And not everyone will use a wag bag.

“Some people are a little uncomfortable with the concept,” he said. “I’m certain we’re not getting 100 percent compliance.”

The heavy traffic in the Narrows also impacts aquatic health. In 2007, the park contracted with a graduate student, Andrea Caires, to research how hiker traffic in the river affects aquatic insect life. The work was done as her master’s thesis in ecology.

The resulting study found steep population declines in a class of mayflies (known as Baetis) following heavy hiking pressure in the river. The species is a good measure of ecological health because it is sensitive to disturbance and an important food for fish.

“Lower benthic abundance of Baetis … indicates that it does not recover from hiker disturbance in the North Fork of the Virgin River during the visitor use season,” Caires wrote. “Hiker disturbance occurs every day during the summer and, as the frequency of a disturbance increases, the recolonization ability of aquatic invertebrates decreases.”

The study found that in November, two months after the end of peak visitor season, the mayfly population still had not recovered. A flash flood at the end of July may have contributed to this.

Overall, Caires concluded the population decline was not an immediate concern, partly because Baetis mayflies are the most abundant invertebrate in the river. But she recommended the study should be repeated “if use increases substantially.”

That was 10 years ago, and annual visitation to the park has increased by more than 1.6 million people since then. The study has not been repeated.

The heavy visitor load has also impacted Springdale, the park’s “gateway” town. Springdale has a full-time population of fewer than 600 people, but its actual size during tourist season swells by thousands due to park visitors.

The town plans to expand its drinking water treatment plant. This is partly driven by the need for more capacity due to the heavy visitor load, and because the existing plant is 30 years old and simply needs updating, town manager Rick Wixom said.

“We’re adding capacity to the plant for sure,” Wixom said. “It’s driven by the idea that we’re going to be a tourist town for the next 40 years, and what is that going to look like. We have never had a time when we didn’t have something going on in hotel development. That’s just a constant state of being here.”

The cost of the new water treatment plant was estimated at $5.6 million, Wixom said, a price that was going to be paid largely by the state. But the lowest bid came in at more than $7 million. So the town council now has to decide whether to rebid on the job or request more money from the state. Even so, Wixom hopes to start construction in four months.

The National Park Service, meanwhile, is considering adopting a reservation system for popular attractions in the park to reduce crowding and environmental impacts. This would include the heavily used Virgin River Narrows. In short, visitors could only access certain parts of the park if they made a reservation in advance.

“We’re realizing there’s probably some limit to the number of people we can put in that canyon and still have people have a positive experience,” Sharrow said.

The park released a list of alternatives this summer. A final proposal is expected in Fall 2018.