Landlocked Laos doesn’t have the famous beaches of its Southeast Asian neighbors, so it instead attracts visitors with the pristine beauty of its mountains and rivers and historical sites. The crown jewel is Luang Prabang, a UNESCO World Heritage site where all those elements merge. But a multibillion-dollar dam project underway 15 miles upstream on the Mekong River has prompted concerns that the city may lose its UNESCO status. The project has also raised broader questions about what the government’s ambitious plans to build multiple dams will do to the river, the lifeblood of Southeast Asia.