From drought to flood, nation's biggest dam topped for first time in 50 years

California has spent the last six years grappling with a brutal drought. Now parts of the Golden State are suffering from the opposite problem: too much water.

Extreme flooding this weekend washed away chunks of highway, derailed trains and inundated homes in Northern California. At Lake Oroville, a major man-made reservoir, water began flowing over a never-before-used emergency spillway at the Oroville Dam.

On Sunday evening, the situation turned hazardous for residents near the 770-foot-tall dam.

State officials warned around 4:40 p.m. local time that two of the dam's spillways could fail this afternoon due to severe erosion.

California's Department of Water Resources ordered residents to immediately evacuate from the low levels of Oroville and areas downstream. A flash flood warning went into place for parts of Butte County.

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Oroville Dam lies about 150 miles northeast of San Francisco. Opened in 1968, the colossal infrastructure supplies water for farms in the Central Valley and homes and buildings in Southern California.

Early Sunday morning, Lake Oroville crested at 902.59 feet above sea level -- a record high, according to the California Department of Water Resources.

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The reservoir is considered full at 901 feet, and it reached that level around 8 a.m. on Saturday. That's when water began pouring over the emergency outlet for the first time in the reservoir's nearly 50-year history.

Water had already started flowing through the dam's heavily damaged main spillway. Heavy rains late last week caused the concrete structure to unexpectedly cave in, leaving a 30-foot-deep hole that continues to grow.

"This is mother nature kind of kicking us a few times here," Bill Croyle, the California Department of Water Resources' acting director, told reporters Saturday.

The water level in Lake Oroville began falling by Sunday afternoon, but state officials said water could still keep washing over both spillways into Monday.

The deluge threatens to knock down trees and send debris cascading into the already-swollen Feather and Sacramento Rivers and into the San Francisco Bay.

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Despite the onslaught, the Oroville Dam is sound and doesn't pose an imminent threat to the public, authorities said.

In the unlikely event that it did fail, communities from Sacramento to San Francisco would face cataclysmic flooding.

Devastating drought to relentless rains

The torrential rainfall in California so far this year is a swift about-face for the Golden State, parts of which are still suffering from the worst drought on record.

In 2014 and 2015 alone, the state economy lost billions of dollars as crops withered and farm jobs vanished, researchers from the University of California, Davis, found.

Residents and businesses across the state were forced to sharply reduce their water usage as reservoirs turned into sunbaked patches of mud.

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Yet now, parts of Northern California are in the midst of their wettest winters in at least two decades.

A barrage of atmospheric "rivers" in recent weeks has continued to dump rain and snow across the region, helping to end the drought in northern areas.

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San Francisco, for instance, has received at least 21.86 inches of rain since Oct. 1, or 146 percent of its average for the season, the National Oceanic and Atmospheric Administration reported.

Southern California, however, is still a few more storms short of finally ending its drought.

This story was updated on Feb. 12, 2017, at 9 p.m. EST.