Puerto Rican authorities have said they will evacuate areas near the Guajataca dam after it showed signs of failing.
The dam has experienced increased water pressure after Hurricane Maria and recent heavy rainfall.
Authorities earlier ordered people leaving in the nearby areas to evacuate and today security forces check to ensure the areas had been vacated.
Hurricane Maria, the most powerful storm to strike Puerto Rico in nearly 90 years, has killed at least 16 people and wiped out the power and communications systems.
It has also made it difficult to get food, water and fuel to the island.