All people living near the Guajataca River in northwest Puerto Rico have been told to evacuate immediately because of an "imminent dam break".
About 70,000 people in the area of the Guajataca Dam have been told to evacuate.
Buses have been brought in to help people leave.
Speaking on CNN's "Out Front", Puerto Rican Secretary of State said the evacuations are difficult because the island was heavily damaged by Hurricane Maria, which struck earlier this week.
Many parts of Puerto Rico are flooded and more than 3 million people don't have electricity.
The dam was constructed by the Puerto Rico Electric Power Authority to create a lake that provides drinking water to residents of the area.
