Rescuers worked through mud and debris in areas devastated by a flash flood along the Himalayan border between Nepal and China. They searched on Friday for hundreds of people still missing as the death toll rose and Nepali and Chinese authorities warned of further flooding from a lake that formed upstream after Wednesday’s disaster and was already overflowing. They urged people in potentially affected areas to move to safer ground. Police in Nepal said Friday morning that the country’s death toll had risen to 469 from 390 a day earlier. China had not updated its report of three deaths in Tibet. Together, the two countries have said more than 1,500 people are missing.