America watches Oroville dam as 188,000 are evacuated in the middle of the night

America's tallest dam has been damaged by erosion and record rainfall.

Around 188,000 Californians have been evacuated from before the tallest dam in America as official fear a catastrophic flood. Erosion at the Oroville dam has endangered nearly a quarter million lives.

LOS ANGELES, CA (California Network) -- The Oroville dam is America's tallest dam, at 770 feet. It is 44 feet taller than the Hoover Dam. Opened in 1968, the dam's reservoir holds 3.5 million acre-feet of water.

A long drought caused the reservoir to drop to just 33 percent capacity, a historic low for the facility. The reservoir feeds much of the California Central Valley, and Southern California.

The previous year has seen abundant rains steadily refill the reservoir, and recently the lake has reached record high levels.

It was last week, as the lake was reaching its record high level that erosion under the dam's main spillway forced operators to close the channel after chunks of concrete were sent flying.  The water carved a massive hole in the earth.

As waters rose behind the dam, operators were forced to reopen the spillway, in spite of the hole. However, this was to no avail. Warm rains and melting snow continued to fill the reservoir. Last the water finally topped an emergency spillway that has never been used in the history of the dam.

Oroville dam's emergency spillway is just a low section of the dam that allows water to spill over land and into the river below. It's a simple but effective protection that ensures the water does not run over the top of the dam. It's similar to the drain on a bathtub which prevents a tub from overflowing. If water were to crest the top of the dam, it would erode the structure entirely.

Unfortunately, water released by the emergency spillway cannot be controlled and may result in flooding downstream.

As the water ran down the hillside, officials noticed erosion in front of the spillway. This filled the river below with mud and debris making it much more dangerous. Their concerns also grew that the dam could fail as erosion developed.

On Sunday, as water topped the emergency spillway, officials declared an emergency and ordered an evacuation of the town of Oroville, just miles downstream of the dam. Evacuation orders were later sent to other areas in the path of the potential floodwaters.

Gov. Brown ordered the state's entire contingent of National Guard to be prepared for mobilization.

People had little time to leave. Some left pets behind. Motorists packed into gas stations, and stripped store shelves bare. The 40-minute drive to Sacramento took most drivers six hours because the roads were jammed.

As of this morning, water levels in the reservoir have dropped enough to stop going over the emergency spillway. Officials believe the dam will hold and people are safe at the moment.

However, a new round of storms will strike the area on Thursday, and there is no estimate of how much water will be added to the reservoir.

For now, evacuation orders remain in place but should be lifted soon, at least temporarily. But residents will be on notice that sometime after Thursday, they might have to evacuate all over again.

Should Oroville dam collapse, a wall of water over a hundred feet high could rush downstream, wiping out everything in its path.  The situation is being closely monitored.