As the fiery orange sun streaked Memorial Day's first rays across the dry, desert landscape, cars were already lined up to get into remote Zion National Park, located in the western U.S. state of Utah.

Zion National Park's Scenic Drive was so full Sunday that the park had to close its access by 6:30 a.m. and the whole park was full by 11 a.m., park officials said on Twitter, as visitors flocked to the popular park located 260 kilometers northeast of Las Vegas, where tumbleweed and tiny towns frequent the landscape.

It was the same scene at Zion on Monday - one of America's biggest national holidays, held since 1971 to honor the country's military dead - where citizens swarmed to stretch their limbs after weeks of COVID-19 stay-at-home orders.

With 4 million visitors a year, Zion and nearby Grand Canyon National Park, with 6.3 million guests, are two of the country's four most popular parks, along with Colorado's Rocky Mountain and Tennessee's Great Smoky Mountains, in the eastern United States, according to 2018 National Geographic statistics.

Both Grand Canyon and Zion opened this past week, despite reservations and opposition - both located in parsley-populated, high-elevation desert areas of southern Utah and northern Arizona in states run by Republican governors.

In neighboring Colorado, run by Democrat Jared Polis, citizens will wait until Wednesday, when a phased reopening of Rocky Mountain will begin.

Meanwhile, the number of infections and deaths continued to grow each week. The United States has so far had more than 1.6 million cases and more than 97,000 deaths, according to Johns Hopkins University.

ALARMED

A USA Today article last week said COVID-19 cases are still peaking in more than a dozen American states, and noted that "several states that relaxed social distancing measures - or never implemented them at all - now have among the fastest growing rates of coronavirus cases per capita."

U.S. deaths from the 90-day pandemic are projected to crest 100,000 in the next few days.

With America's national parks hosting some 320 million visitors a year, and that traffic occurring in the summer months, health experts and politicians questioned the rush to reopen, and especially, the absence of direction or a cohesive plan from the federal government.

"Ensuring the safety of NPS (National Park Service) employees, visitors, and gateway communities is your responsibility, and human safety must take precedence over any politically motivated decisions to reopen national park sites," House Natural Resources Chairman Raul M. Grijalva said in a statement last week.

Grijalva, an Arizona Democrat, has been critical of Trump administration's call for reopening parks, including Grand Canyon, that reopened for day traffic on Friday, where photographs on Twitter Sunday showed visitors observing social-distancing guidelines at the park's scenic overlook sites.

"I recognize the benefits of reopening national parks and other public land sites when appropriate," Grijalva wrote. "But rushing to reopen national parks prematurely and in the absence of stringent safeguards threatens public health and puts lives in jeopardy," he added.

To access Grand Canyon, visitors from the populous nearby city of Phoenix must travel through the Native American Navajo reservation, where COVID-19 cases have spiked.

Alicyn Gitlin, Grand Canyon program manager for the Sierra Club's Grand Canyon Chapter, told Roll Call last week it was a "terrible time" to encourage wide scale travel through the Navajo Nation and northern Arizona, where cases have continued to rise in Coconino County.

"The large population that lives at Grand Canyon and all nearby communities are put at risk by this move," Gitlin said in response to the partial reopening of the Grand Canyon.

Last week there had been 4,153 confirmed positive cases and 144 deaths in the Navajo Nation, according to the Navajo Department of Health.

Navajo Nation President Jonathan Nez told MSNBC on Friday "the tribe was forced to temporarily suspend tourism in order to protect tribal members," and asked for funding to mount a formal "opposition to reopening of Grand Canyon."

On the Navajo Nation's Facebook page Friday, tribal leaders asked visitors to stay away from the Grand Canyon for the time being.

"Unconscionable," said Sandy Bahr, director of Sierra Club's Grand Canyon Chapter of the park's reopening. "National Parks reopen without release of plan or infection data," she posted on Twitter Friday.

CONSERVATIVE COLORADO

In neighboring Colorado, Rocky Mountain will reopen later than most parks in the west, except for those in the state of California. Memorial Day saw "Closed" signs at Colorado's most visited park, and barred gates kept the park vacant, except for wild animals that park rangers say have proliferated in the absence of humans.

Rocky Mountain will begin a phased reopening on May 27, when the park will start issuing wilderness camping permits and the primary shuttle bus will start running again.

But restrictions on travel and permits inside the park will continue, officials said.

To the south in Colorado, Great Sand Dunes National Park "will open campgrounds, picnic areas and hiking trails on June 3, although backcountry permits will not start until mid-June," the park's website said.

Meanwhile, Colorado's fourth national park, Black Canyon of the Gunnison opened limited access to trails and day hikes this past week.

Near the state's "Four Corners," Mesa Verde National Park began a phased re-opening Sunday, and reported a steady stream of visitors, who also enjoyed access to the park's concession facilities.

In California, famous Yosemite National Park announced this week it will reopen "sometime in June." The Golden State is home to 10 of the country's 62 national parks, more than any other state.