Whether this is the three gorges or not, the dam in this video has water cresting the top even with the spill ways fully open. Dams have a safe water level limit that is usually a percentage of the total height of the structure down from the top. if water starts pouring over the top, it can lead to a major structure damage to the dam and can cause it's collapse.

As a side note, it also means the spill ways cannot handle the amount of water flowing into the area behind the dam. If you can't clear the water coming into the dam, and the current water is starting to crest the top of it, you're in a death spiral leading to failure. It's like a plane that lost all it's engines, it can coast for a while but eventually it'll be flying into the ground.