Unlike previous shutdowns, the Trump administration was leaving parks open to visitors, said John Garder, senior budget director of the non-profit National Parks Conservation Association.

“We’re afraid that we’re going to start seeing significant damage to the natural resources in parks and potentially to historic and other cultural artefacts,” he said. “We’re concerned there’ll be impacts to visitors’ safety. It’s really a nightmare scenario.”

Under the park service’s shutdown plan, authorities have to close any area where rubbish or other problems become threats to health and safety or to wildlife, spokesman Jeremy Barnum said.

“At the superintendent’s discretion, parks may close grounds/areas with sensitive natural, cultural, historic, or archaeological resources vulnerable to destruction, looting, or other damage that cannot be adequately protected by the excepted law enforcement staff that remain on duty,” he said.

In the southern Sierra Nevada in Central California, some areas of the Sequoia and Kings Canyon national parks were closed on Monday evening. In Sequoia, home to immense and ancient giant sequoias, General Highway was closed because overflowing bins were spreading litter and posed a threat to wildlife and the icy, jammed road was seeing up to three-hour delays, according to the National Park Service.

Also closed was the Grant Tree Trail, a popular hiking spot, because the government shutdown stopped maintenance and left the path dangerously slick from ice and snow, with at least one injury reported, the park service said.

Campers at Joshua Tree National Park in Southern California’s deserts were reporting squabbles as different families laid claims to sites, with no rangers on hand to adjudicate, said Ethan Feltges, who operates the Coyote Corner gift shop.

Feltges and other business owners around Joshua Tree stepped into the gap as much as possible.

He set up a portable toilet at his shop to help visitors streaming in and out of the park. He was spending his days standing outside, offering tips about the park in place of the rangers.

“The whole community has come together,” Feltges said. “Everyone loves the park. And there’s a lot of businesses that actually need the park.”

Most visitors were being respectful of the desert wilderness and park facilities, said Joshua Tree superintendent David Smith.

But some are seizing on the shortage of park staff to off-road illegally and otherwise damage the park, as well as relieving themselves in the open, a park statement said. Joshua Tree said it would begin closing some campgrounds.

At Yosemite, Snider said crowds of visitors were driving into the park to take advantage of free admission, with only a few park rangers working and a limited number of toilets open.

Visitors were allowing their dogs off their leads in an area rich with bears and other wildlife, and scattering rubbish along the roads, Snider said.

“You’re looking at Yosemite Falls and in front of you is plastic bottles and trash bags,” he said.

Officials at Rocky Mountain National Park in Colorado said Monday they were closing restrooms and locking up trash bins in many locations.

In Yellowstone National Park, private companies have picked up some of the maintenance normally done by federal workers. The contractors that operate park tours by snowmobile, buses and vans are grooming trails, hauling rubbish and replacing toilet paper at toilets along their routes.

Roads inside Yellowstone are normally closed for winter, meaning most visitors get to attractions like Old Faithful or the Grand Canyon through guides. Those guides are splitting the cost of grooming the trails used by their vehicles to keep their operations going, said Travis Watt, general manager of See Yellowstone Alpen Guides.

The tour companies can probably keep this system going through the entire winter season if they need to, Watt said.

“It’s definitely not our preference – the park service does a good job doing their thing and we hate to see them out of work,” Watt said. “But it’s something we can handle.”