Flash floods surged through in the middle of the night, but many local officials appeared unaware of the unfolding catastrophe, initially leaving people near the river on their own.

Several hundred people gathered at Tivy High School’s stadium in Kerr County, Texas, to mourn the victims of the catastrophic flash floods that hit the region and to honor the many still missing.

Henry DeHart, interim president and CEO of the American Camp Association, said counselors typically receive training in areas such as risk management, emergency procedures, child development, abuse pr...