Texas Flooding Map: See How the Floodwaters Rose Along the Guadalupe River

In the early hours of July 4, floods swept through Hill Country, a region of Central Texas also known as “Flash Flood Alley.” Its propensity for high levels of rainfall combined with thin soil, exposed bedrock and steep terrain make it especially vulnerable. Rainfall gets funneled through the hilly terrain and canyons into the valleys of the region.

By 1 a.m. Friday, the Weather Service said a very dangerous flash flood event was unfolding in Kerr County, and rainfall rates were reaching up to three to four inches an hour with no indication of easing. The Guadalupe River rose 20 feet in three hours, according to data from a river gauge near Hunt. By 10 a.m., it swelled in the town of Comfort, surging to 34 feet from three feet in about 90 minutes.

In under 10 hours, from the late evening of July 3 to the pre-dawn hours of July 4, the flow rate of the Guadalupe River went from that of a small stream you could wade across (about 10 cubic feet per second), to a raging and destructive torrent of 120,000 cubic feet per second, according to a New York Times analysis. That’s greater than the average flow rate across Niagara Falls.

The floods washed away cabins, R.V.s and cars and toppled down large trees. In its rush downstream, the river ravaged Camp Mystic, a girls’ summer camp in Kerr County, where at least 27 campers and counselors were killed in the floods.

At least 75 of those killed in the floods were in Kerr County, northwest of San Antonio, authorities said. Other people were killed in Travis County, Burnet County, Kendall County, Williamson County and Tom Green County.