As rescue teams continue searching for hundreds of people missing along the Nepal-China border following catastrophic floods, humanitarian organizations are mobilizing to aid survivors.

More than 300 people died in the region when a deadly torrent of water and mud several stories high swept through communities Wednesday following a glacier collapse. More than 1,400 were still missing Thursday.

The Nepalese army has been using helicopters to rescue people because so many roads and bridges have been washed out.

Dan Hovey, vice president of emergency response at Direct Relief, which provides medicines and healthcare to disaster-struck areas, said his organization had already pre-positioned emergency supplies in Nepal for the upcoming monsoon season. That preparation has enabled Direct Relief to help quickly during these unexpected floods.

“Nepal is a very disaster-prone country,” he said. “On...