KATHMANDU, Nepal — Rescuers worked through mud and debris Friday in areas devastated by a catastrophic flash flood along the Himalayan border between Nepal and China, continuing the search for hundreds of people still missing as the death toll rose and authorities warned that a newly formed, overflowing lake could trigger further flooding. Nepal police said Friday morning that the country’s death toll had risen to 469 from 390 a day earlier, as rescuers recover more bodies amid the debris. China has not updated its report of just three deaths in Tibet. Together, the two countries have said that more than 1,500 people are missing. 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. China’s Ministry of Water Resources said Thursday that the lake held around 2 million cubic meters (70 million cubic feet) of water and was expected to receive another 3 million cubic meters (106 million cubic feet) over the next three