Nearly 200,000 people have been told to leave the area around the Oroville Dam in California, in the US, after heavy rain and snow.
The dam is around 230 metres high and holds back a large lake.
The unusual amount of rain and snow has caused a lot of damage to two parts of the dam that let water escape. Those parts are in danger of collapsing and therefore flooding the areas below.
Officials said that they will use bags of rocks to try to plug the hole at the head of the emergency spillway, and also will reduce the water level at Lake Oroville to stop the stress on the spillway.
Oroville dam is the tallest dam in the United States.