The Geology of Texas’ ‘Flash Flood Alley’ Defined
A hydrologist explains why Texas Hill Nation is called Flash Flood Alley and the way its geography and geology can result in heavy downpours and sudden, harmful floods
A person conducts a search close to the Guadalupe River in Kerrville, Texas on Sunday, July 6, 2025. Devastating floods swept by way of Central Texas on Thursday night and Friday morning, elevating the dying toll to 51 and numerous extra who stay lacking, together with 27 women from Camp Mystic.
Desiree Rios for The Washington Publish by way of Getty Pictures
The next essay is reprinted with permission from 
The Dialog, an internet publication masking the newest analysis.
Texas Hill Nation is understood for its landscapes, the place shallow rivers wind amongst hills and thru rugged valleys. That geography additionally makes it one of many deadliest locations within the U.S. for flash flooding.
Within the early hours of July 4, 2025, a flash flood swept by way of an space of Hill Nation dotted with summer time camps and small cities about 70 miles northwest of San Antonio. At the least 75 folks died in Kerr County, and 10 women from one camp had been nonetheless unaccounted for, officers stated on July 7. Greater than a dozen extra deaths had been reported in close by counties.
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The flooding started with a heavy downpour that despatched water sheeting off the hillsides and into creeks. The creeks poured into the Guadalupe River.
A river gauge at Hunt, Texas, close to the camp, confirmed how rapidly the river flooded: Round 3 a.m. on July 4, the Guadalupe River was rising about 1 foot each 5 minutes on the gauge, Nationwide Climate Service information reveals. By 4:30 a.m., it had risen greater than 20 toes. Because the water moved downstream, it reached Kerrville, the place the river rose quicker.
Flood skilled Hatim Sharif, a hydrologist and civil engineer on the College of Texas at San Antonio, explains what makes this a part of the nation, referred to as Flash Flood Alley, so harmful.
What makes Hill Nation so susceptible to flooding?
Texas as an entire leads the nation in flood deaths, and by a large margin. A colleague and I analyzed information from 1959 to 2019 and located 1,069 folks had died in flooding in Texas over these six a long time. The following highest complete was in Louisiana, with 693.
A lot of these flood deaths have been in Hill County. It’s a part of an space referred to as Flash Flood Alley, a crescent of land that curves from close to Dallas all the way down to San Antonio after which westward.
The hills are steep, and the water strikes rapidly when it floods. This can be a semi-arid space with soils that don’t take in a lot water, so the water sheets off rapidly and the shallow creeks can rise quick.
When these creeks converge on a river, they will create a surge of water that wipes out houses and washes away automobiles and, sadly, anybody in its path.
Hill Nation has seen some devastating flash floods. In 1987, heavy rain in western Kerr County rapidly flooded the Guadalupe River, triggering a flash flood just like the one in 2025. Ten youngsters being evacuated from a camp died within the speeding water.
San Antonio, on the jap fringe of Hill Nation, was hit with a flash flood on June 12, 2025, that killed 13 folks whose automobiles had been swept away by excessive water from a fast-flooding creek close to an interstate ramp within the early morning.
Why does the area get such sturdy downpours?
One purpose Hill Nation will get highly effective downpours is the Balcones Escarpment.
The escarpment is a line of cliffs and steep hills created by a geologic fault. When heat air from the Gulf rushes up the escarpment, it condenses and can dump lots of moisture. That water flows down the hills rapidly, from many alternative instructions, filling streams and rivers under.
As temperature rise, the hotter environment can maintain extra moisture, growing the downpour and flood danger.
The identical impact can contribute to flash flooding in San Antonio, the place the big quantity of paved land and lack of up to date drainage to manage runoff provides to the danger.
What may be finished to enhance flash flood security?
First, it’s essential for folks to grasp why flash flooding occurs and simply how briskly the water can rise and movement. In lots of arid areas, dry or shallow creeks can rapidly replenish with fast-moving water and turn out to be lethal. So folks ought to pay attention to the dangers and take note of the climate.
Bettering flood forecasting, with extra detailed fashions of the physics and water velocity at totally different places, can even assist.
Probabilistic forecasting, for instance, can present a spread of rainfall eventualities, enabling authorities to arrange for worst-case eventualities. A scientific framework linking rainfall forecasts to the native impacts, equivalent to streamflow, flood depth and water velocity, might additionally assist decision-makers implement well timed evacuations or highway closures.
Schooling is especially important for drivers. One to 2 toes of shifting water can wash away a automotive. Individuals might imagine their vans and SUVs can undergo something, however fast-moving water can flip a truck and carry it away.
Officers can even do extra to barricade roads when the flood danger is excessive to stop folks from driving into hurt’s method. We discovered that 58% of the flood deaths in Texas over the previous six a long time concerned autos. The storm on June 12 in San Antonio was an instance. It was early morning, and drivers had poor visibility. The automobiles had been hit by fast-rising floodwater from an adjoining creek.
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