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Evacuation order lifted near Oroville Dam

Officials in Northern California have lifted an evacuation order for residents living in communities below the Oroville Dam, but warn that they must remain "vigilant and prepared" as "conditions can rapidly change."

The Butte County Sheriff's Office reduced the evacuation order to an evacuation warning at 1 p.m. Tuesday afternoon, allowing the 188,000 people who left on Sunday to return to their homes. The Oroville Dam is the tallest dam in the United States, and last week, workers discovered that the primary spillway, which allows a controlled release of water from the dam, had developed a giant hole. After switching to the emergency spillway, it was determined that spillway is eroding, which raises the risk of the dam collapsing, and authorities quickly ordered that residents nearby evacuate.

While the dam itself isn't damaged, if the emergency spillway should collapse, it could unleash a torrent of water onto the communities below, state officials told NBC News. There is more rain in the forecast, but for now, the water level in Lake Oroville, which lies behind the dam, has dropped.