The damage to the main spillway was "first reported" on 6 February [1] (although it could have existed for longer and gone unnoticed). They had to leave the spi

The damage to the main spillway was "first reported" on 6 February [1] (although it could have existed for longer and gone unnoticed). They had to leave the spillway in use because of the amount of inflow, but on Friday they reduced the spillway outflow to avoid damaging electric pylons [2]. They also had to halt outflow via the main hydroelectric discharge. On Saturday, the emergency spillway started being used for the first time in the dam's history. The operators thought this might happen, but didn't see it as a risk. The evacuation order was given when they saw the amount of erosion caused by the outflow from the emergency spillway, fearing that if it carried on, the emergency spillway could collapse. As a result, they've since fully-opened the main spillway (100,000 cfs, rather than their previously reduced limit of 55,000 cfs), accelerating the damage to the main spillway. So the damage to the main spillway existed at least a week ago. The emergency spillway was seemingly undamaged, but once put it into use, caused damage showing it wasn't really fit for purpose. And in trying to save the emergency spillway, the main spillway is now sustaining further damage. [1] https://twitter.com/CANGJ3OPS/status/829769462077284352 https://twitter.com/CANGJ3OPS/status/829769462077284352 [2] http://www.water.ca.gov/news/newsreleases/2017/021017oroville.pdf http://www.water.ca.gov/news/newsreleases/2017/021017orovill... (PDF)