Filmmaker and drone pilot Manish Maharjan crossed into Nepal from China’s Tibet through Rasuwagadhi, a border crossing closest to where last week's catastrophic flooding began, two days before the disaster. Returning home, little did he know he would be using his professional skills to help with rescue efforts along a route he has known intimately from years of travel for work.

As authorities mobilized dozens of rescue and relief teams, Maharjan joined the Nepal Army’s rescue efforts a day after the flash floods, which have since killed more than 1,000 people and left thousands missing in Nepal and China.

Maharjan has been operating thermal drones to assist rescue teams, focused on locating and finding at least 900 workers who are still missing from 12 hydropower projects buried under mud and debris. About 500 of them are believed to be trapped inside power plant tunnels.

He said drones were effecti...