SAN JUAN, P.R. — Two days after Hurricane Maria flattened this island of 3.5 million people, knocking out all its power and much of its water, the rebuilding of the services and structures needed for people to resume some semblance of ordinary life was looking more complicated by the day.

All or part of three towns in the northwestern part of the island — Isabela, San Sebastián and Quebradillas — were being evacuated Friday because of fears about structural damage to the nearby Guajataca Dam. Close to 70,000 people could be affected if the 90-year-old dam, which is 120-feet high and can hold about 11 billion gallons of water, collapsed, said Puerto Rico’s governor, Ricardo Rosselló.

And with everyone from the governor of Puerto Rico to the mayor of San Juan predicting that it could take four to six months to resume electrical service, people were contemplating empty refrigerators, campfire cooking, bathing in their own sweat and perhaps wrangling for fresh water on an island accustomed to hard times but nothing like what the future may bring.

“It’s been hard to see infrastructure deteriorate in Puerto Rico, but it has been harder to meet citizens who have lost it all,” Governor Rosselló said.