Water levels at America's tallest dam have dropped, easing fears that an emergency spillway at the dam could collapse and devastate the communities below.

Tens of thousands of people living near Northern California's Lake Oroville Dam were evacuated from their homes earlier amid concerns the crumbling spillway could give way.

The dam itself was not damaged. But because its water levels were so high following heavy rain, it was feared the emergency slipway could collapse.

Engineers have been carefully releasing water from the dam some 65 miles (105km) north of Sacramento after it was discovered that large chunks of concrete were missing from the spillway.

Early on Monday emergency crews prepared to use helicopters to drop bags of rocks to shore-up the spillway, according to Sacramento television station KCRA.

Meanwhile, about 188,000 people were ordered out of their homes in the area below the dam on Sunday, leaving Oroville itself largely deserted overnight.

By Monday, state and local officials said the immediate danger had passed with water no longer flowing over the eroded spillway but they cautioned that the situation remained unpredictable.

"Once you have damage to a structure like that it's catastrophic," acting Water Resources director Bill Croyle told reporters.

But he stressed "the integrity of the dam is not impacted" by the damaged spillway.

The earthfill dam is just upstream and east of Oroville, a city of more than 16,000 people north of the state capital of Sacramento.

At 770 feet (230 meters) high, the structure, built between 1962 and 1968, is the tallest U.S. dam, exceeding the Hoover Dam by more than 40 feet (12 meters).