Basically a specific extreme flood risk as the ground is mostly rock with little drainage. I heard it was one of the two most at risk river valleys in the USA
https://www.texasmonthly.com/news-politics/beauty-and-danger-of-guadalupe-river/
David Wolters/Getty
“When the river takes you, don’t fight it,” my parents told me when I was about nine years old. “If it takes you under, focus your effort on holding your breath—it will push you back up.” We would practice floating on our backs, feet downstream, like tiny giggling canoes moving diagonally across the Comal, Medina, San Marcos, and other lesser currents that made up the landscape of our gorgeous, dangerous childhoods, until we were ready for something bigger.
One of those bigger rivers was the Guadalupe, which runs from the Texas Hill County to the Gulf of Mexico, crossing aquifers and collecting tributaries as it plows southeast with a depth millions of years in the making. Before there were roads or fences, the Guadalupe was already cutting its way through limestone in a slow choreography of water and stone. It’s that clever pathway that makes the river so beautiful. It’s that perseverance through rugged terrain that makes it so deadly.
“This whole landscape has basically been carved out by running water,” said Kimberly Meitzen, a fluvial geomorphologist at Texas State University. Fluvial geomorphology is the study of how rivers and the earth shape one another—a cool name for a poetic phenomenon that’s been contouring the Hill Country for millions of years. Massive flooding events like the one over the July 4 holiday weekend played a large role in creating the stunning canyons and cliffs, which in turn contribute to more powerful flash floods.
Most of the time, Meitzen said, those cloistered landscapes—rocky riverbeds beneath the canyons lined with rows of cypress trees—are complemented by the tranquil, cool spring-fed flows of the three tributaries that make up the headwaters of the Guadalupe. The North Fork, the South Fork, and Johnson Creek stay reliably strong even in seasons of drought. Meitzen spent her 2018–2021 summer field seasons with a research team walking along, paddling in, and wading in the upper Guadalupe, where the recent floods wreaked havoc. She was looking for barriers to the movement of aquatic wildlife, such as Guadalupe bass, but as she gazed up at the steep canyon walls around her, she often wondered, “How would you get out of here in the event of a flood?”
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