Rescue operations were underway to find anyone lost in the debris after the catastrophic weekend storm, which caused the sprawling Guadalupe River to swell rapidly to near-unprecedented levels.

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...