SAN FRANCISCO, Feb. 14 (Xinhua) -- A mandatory evacuation order issued Sunday for people living downstream of the tallest dam in the United States was reduced to a warning Tuesday afternoon, local authorities said.

"An evacuation warning means the immediate threat has ended but the potential for an emergency remains and therefore residents must remain prepared for the possibility of an evacuation order," said Kory Honea, sheriff of Butte County, where Oroville Dam is located.

The evacuation order, effective Sunday afternoon for some 180,000 people, was prompted by fear of possible failure of the emergency spillway at Oroville Dam, the tallest in the United States with an earthfill embankment of 770 feet (230 meters) high on the Feather River east of the city of Oroville.

At the time, a hole developed in the emergency spillway structure as water cascaded down the dirt ravine. Overflow in the spillway began Saturday morning and the erosion appeared to be spreading upward toward the structure.

The California Department of Water Resources (DWR) worried that further erosion at the head of the spillway would unleash an uncontrolled torrent from Lake Oroville, the second largest man-made lake in the state of California.

Oroville Dam, which has been said to be sound by the DWR, is a separate structure from the emergency spillway.

To avert more erosion at the top of the emergency spillway, the DWR had doubled the flow down the dam's main spillway, so as to lower the water level in the reservoir more rapidly and stop water from flowing over the emergency spillway, and tried to reinforce the emergency spillway.

While the area was being continually monitored from the ground and in the air with the use of drones, the DWR said in a news release Tuesday afternoon that the level of the reservoir continued to decrease, and was projected to possess the capacity to absorb inflows from incoming rain.

Earlier forecasts said rain was expected late Wednesday through Thursday in the area that encompasses Oroville and communities south of the city.

Oroville is nearly 70 miles (113 km) north of California's state capital of Sacramento, and about 150 miles (240 km) northeast of San Francisco.

Oroville Dam was built by the DWR from 1961 to 1968 as one of the key features of the California State Water Project (SWP) mainly for water supply, hydroelectricity generation and flood control.