The last time Oroville Dam was big news, people were rapelling down onto their boats

Thousands of people are being evacuated from towns downstream of the Oroville Dam in California as a section shows signs of giving way.

“It’s uncontrolled. It’s uncontrolled,” Department of Water Resources spokesman Chris Orrock said, when asked how much water could be released should the spillway break.

Just to be clear, it’s not the dam wall that is under threat. It’s the emergency spillway, and to a lesser extent, the main spillway.

Here’s a clear explanation:

There’s some good news in that a hole which had appeared in the spillway was not eroding as quickly as authorities originally thought. There’s talk of using a fire helicopter to fly in a large rock and drop it into the hole to ease the pressure.

And the dam has been opened up enough to release 100,000 cubic feet per second. At the other end, floodwaters are entering the dam at 40,000 feet per second.

But traffic is reportedly at a standstill as thousands of residents attempt to flee. If the spillway does collapse, an estimated 10-metre wall of water could roar down the valley.

It’s a stunning way to break a drought. Here’s a few pictures from just three years ago when it looked like water would never touch the dam wall again after a four-year drought.

Pastures grew around ‘floaters’ used to stop boaters entering the spillway:

Picture: Getty Images

Here’s the main spillway in 2014, compared to today:

Picture: Getty Images

On the left, you can see just how far water had retreated:

Picture: Getty Images

There’s a stunning set of photos of houseboats dropping with the water level here. At its worst, owners were forced to rapel onto their boats.

But in 1997, there was a recommended evacuation during flash flooding because of potential levee breaks.

Once that danger had passed, and the big dry set in, a new development was built in Marysville below the dam. CBS Sacramento reported that development was one of the first which could be hit if the spillway collapses.

Business Insider Emails & Alerts

Site highlights each day to your inbox.

Follow Business Insider Australia on Facebook, Twitter, LinkedIn, and Instagram.