Officials in northern California ordered tens of thousands of people to evacuate Sunday, warning that a spillway at the tallest dam in the United States could fail. 

The focus of their concern is the Oroville Dam, where damage to the main spillway last week has been followed by weakness in an emergency spillway. 

The dam is 235 meters high and holds back Lake Oroville, playing a role in both California’s water and power systems.  The spillways allow water to drain from behind the dam in a controlled manner, but the damage at the Oroville Dam has come as snow and rainfall rapidly pushed up the water level. 

Authorities said there was no threat of the dam itself failing, but warned that the substantial flow of water that would come if the spillways gave way would be extremely dangerous to those living in valley towns below. 

Efforts were under way to release water from the lake and lessen the pressure on the spillways, while also dropping containers of rocks into the damaged areas in order to try to stabilize them.

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