On July 4, the broken remnants of a powerful tropical storm spun off the warm waters of the Gulf of Mexico so heavy with moisture that it seemed to stagger under its load. Then, colliding with another soggy system sliding north off the Pacific, the storm wobbled and its clouds tipped, waterboarding south central Texas with an extraordinary 20 inches of rain. In the predawn blackness, the Guadalupe River, which drains from the Hill Country, rose by more than 26 vertical feet in just 45 minutes, jumping its banks and hurtling downstream, killing 109 people, including at least 27 children at a summer camp located inside a federally designated floodway.