A sheriff on Tuesday lifted a mandatory evacuation order in northern California, which had impacted nearly 200,000 people in an area under threat of catastrophic failure at the tallest dam in the United States.

The Butte County Sheriff's Department announced it had reduced the evacuation order to a warning, allowing people in downstream communities from Oroville Dam -- 75 miles (120 kilometers) north of Sacramento -- to return home.

The sheriff's department said it had reduced the order to a warning thanks to "lower lake levels, further inspections, ongoing work to shore up the Oroville dam emergency spillway and updated weather forecasts."

The department urged some 188,000 impacted residents to "remain vigilant and prepared as conditions can rapidly change" -- saying that authorities could issue another evacuation order if necessary.

The area was bracing for another onslaught of storms forecast to begin Thursday, but the temperature is now projected to drop, which will diminish rains and lessen the flow into the reservoir.

President Donald Trump cleared the path to release federal aid after a request from California Governor Jerry Brown, who had already had declared a state of emergency to mobilize local resources.

The Defense Department has said it would send in federal troops to provide air transport, water rescue, medical care and shelter if asked.

Calling the situation a "textbook example of why we need to pursue a major infrastructure package in Congress," White House spokesman Sean Spicer stressed that the nation's dams, bridges, roads and ports have "fallen into disrepair."

"In order to prevent the next disaster, we will pursue the president's vision for an overhaul of our nation's crumbling infrastructure," he added.

- No longer overflowing -

A giant hole opened in the dam's main spillway last week, forcing the authorities to activate the emergency overflow channel on Saturday for the first time.

But it began eroding, threatening a rupture that would have sent water surging toward the valley below.

The authorities are draining about 100,000 cubic feet (3,000 cubic meters) of water per second from the dam's main spillway. The goal is to lower the water level by at least 50 feet (15 meters) below the top of the reservoir.

The lake level has fallen 12 feet below the top of the auxiliary spillway, which is no longer overflowing, reducing the risk of erosion.

The California Department of Water Resources had indicated that the water level was dropping at three to four inches (eight to 10 centimeters) per hour.

California has placed its 23,000-strong National Guard on alert to help with the situation if needed.

Three environmental groups -- Friends of the River, Sierra Club and South Yuba Citizen League -- have accused federal and local authorities for more than a decade of failing to bring the dam up to modern safety standards.

The Oroville Dam, completed in 1968, is 770 feet high, the tallest in the country.