We’re introducing a new blog series called “When Rivers Rise Fast” to support our community in preparing for and responding to flash floods.
This series will provide valuable information, including a detailed look at the new flood warning system we’re implementing, and a clear explanation of what flash flooding is and why it’s so dangerous. We’ll also highlight essential tools and resources available to the public, empowering you to plan safer routes and avoid flood risks when you’re on the road. In this second blog of the series, we’ll define flash flooding: what it is, how it happens, and why it’s so dangerous.
What is a Flash Flood?
A flash flood is a fast, powerful flood caused by a large amount of rain falling over a short period of time, usually six hours. It can fill dry creeks and streets almost instantly. The Texas Hill Country—called “Flash Flood Alley”—is especially vulnerable because of steep rocky hills and soil that can’t soak up water well.
In urban areas, concrete and asphalt prevent water from soaking into the ground, so it quickly accumulates on streets and in low-lying areas. Because of these conditions, a thunderstorm doesn’t just bring rain—it can trigger flash floods that rise with alarming speed and turn dry creek beds into torrents of rushing water.
Flash flooding in action — the San Antonio River at Mission Road before (Left) and after (Right) a large rainstorm in April 2023.
During a flash flood, a “wall of water” can form—a fast-moving surge filled with debris, mud, and tree limbs that can carry away cars, homes, and lives.
Flood Terms You Should Know
It is important for the public to know common terms and phrases used by the River Authority and the media when talking about flooding. These terms are:
- A flood watch means heavy rain may happen soon, and flooding might occur, so be advised.
- A flash flood warning means that dangerous flooding is happening right now—take action and move to high ground.
- Flood stage: when a river reaches a danger level that may cause flooding in towns.
- Gauge: a sensor that measures water in creeks or rivers.
- Floodplain: land around a river that can Floodplains are mapped so that developers know where to build and people and homeowners can be made aware of their flood risk.
- Turn Around, Don’t Drown: A safety slogan—don’t drive into flood waters, even if they seem shallow. Just a foot of moving water can carry your car away.
Flash floods are a serious and fast-moving threat. They can turn a peaceful day into a life-or-death situation in minutes. By understanding what they are and respecting the power of water, you can make informed decisions that will keep you and your loved ones safe.
Stay tuned for part three of our “When Rivers Rise Fast” series, where we’ll look at the River Authority’s role in protecting our watershed from flooding, publishing on September 19, 2025.
