Where the next deadly flood wave could strike

CNN

4 days ago

The flash flood that struck the Guadalupe River the early morning hours of July 4 was swift and merciless. A wall of water tore through Texas Hill Country communities, killing more than 130 people. It only took two hours for the flood wave to sweep away campsites, cars and homes, and throw hundreds of families' lives into chaos and grief.
The ingredients that led to the Hill Country disaster — steep terrain, swollen rivers and unsuspecting people in harm's way — are not unique to Texas. Across the United States, there are pockets of vulnerability where geography, weather and human nature converge in ways that heighten the risk of flash flooding.
Some of these dangers are well-known: canyons and valleys that funnel water into narrow channels, rivers and streams with long histories of overtopping their banks. But others are less obvious — places where people flock for fun and time away from their urban lives, unaware that the same features that make these spots so inviting will also make them deadly in the right storm.
To better understand where such disasters could happen next, CNN partnered with First Street, a research organization that specializes in climate risk data, to identify some of the places where vulnerability to sudden, destructive flash floods remains high — and often overlooked.
Our analysis does not include all places that could experience something like what Kerr County, Texas, endured in early July. But you can use the interactive map below to explore the river locations most prone to flash flooding, according to a recent study.
Helen, Georgia, is an unexpected glimpse of Bavaria. Tucked into the folds of the North Georgia mountains, the Alpine-themed tourist spot cradles the Chattahoochee River.
On a hot and humid summer day, few places are more inviting than the cool river, where visitors can float past beer gardens and candy shops in brightly colored inner tubes.
While ''tubing the 'hooch,' which on most days trickles through the town at walking pace, it's hard to imagine how just a few inches of rain can turn the same waterway into a roaring torrent in a flash. But the geography that lends the area its charm also carries a quiet threat.
'Helen is located at the base of some of the higher mountains' in Georgia, National Weather Service senior hydrologist Laura Belanger told CNN while standing next to the flood gauge on Main Street. 'This is the same river that runs through Atlanta and all the way down into Columbus and along that Alabama border — it has a long journey, and it starts here.'
The Upper Chattahoochee winds through the rainiest counties in Georgia, and the head start comes with consequences. The region's topography and clay-based soil leave little opportunity for water to soak into the ground, she said. 'Instead, it runs off and can flow very quickly into our river systems.'
Helen has flooded before, most recently in the remnants of Hurricane Helene, which raised the river's water level by 3 to 6 feet. That pales in comparison to the record flood of 1967, triggered by more than 8 inches of rain falling in 24 hours, causing the Chattahoochee to rise 12 feet.
Since then, Helen has transformed from a deserted logging town into the 'Bavaria of the South,' with riverbanks filled with shops, restaurants and cabins — all vulnerable to the next major flood.
A flood as big as 1967's today would likely submerge homes and motels, cut off roads and inundate much of the town's infrastructure in 2 to 4 feet of water, according to guidance from the National Weather Service. The stylistic arched bridges built to match the European theme would act as dams in a flood, backing up water and worsening erosion.
'The safety of our citizens and the thousands of visitors who enjoy outdoor activities in and around Helen is our highest priority,' said a spokesperson for White County Public Safety, which is in 'early stages' of enhancing its severe weather warning systems.
'While flooding along the Chattahoochee River is certainly a concern, equally serious threats include lightning, high winds, and other rapidly changing weather conditions that can impact a wide range of outdoor activities — not just river recreation,' the spokesperson said.
Belanger is particularly worried about a community on the Hiwassee River, north of Helen, where permanent residential trailers sit near a river gauge that a recent study found was among the most-prone to flash flooding in the country.
Regardless of a community's readiness, that many people living that close to a river is a recipe for disaster, Belanger said.
'Those are the places that keep me up at night.'
The Nogales Wash originates in the hilly city of Nogales in Sonora, Mexico, then slices a path north through the desert landscape and across the border to its sister city of Nogales, Arizona.
The drainage channel, built in the 1930s, lies bone dry for most of the year. It is called into service when the summer monsoon hits.
'These storms are flashy; they drop a lot of water in a very short period of time,' said Allan Sanchez, a floodplain coordinator for Santa Cruz County. It's not uncommon to see the region pummeled by 2.5 inches of rain an hour, he added.
A storm like that can overwhelm the wash. Water in the mostly concrete-lined section that runs through Nogales, Arizona, quickly turns from a benign trickle to a dangerous, gushing torrent as rain sweeps down the hills.
Flood risk is exacerbated by the area's geography and aging infrastructure. Like many other towns at risk, most of Nogales was built on a floodplain because that was the flattest spot, Sanchez said. The city's population has also swollen over the decades, meaning more roads, houses, roofs and driveways — hard surfaces unable to absorb the rainfall.
The problem is complicated by the fact that beneath the Nogales Wash lies a sewage pipeline carrying millions of gallons of untreated waste from Sonora to a treatment plant in Arizona.
'When that thing overtops, then you also have the risk of contamination,' Sanchez said. During monsoon thunderstorms in 2017, floodwaters damaged the pipe, causing raw sewage to pour into the Nogales Wash, prompting the city to declare a state of emergency.
High levels of development and the growth of informal communities in the floodplain on the Sonora side puts people in harm's way.
The Nogales area is not only a hub for cross-border trade but is one of the busiest spots for illegal border crossings. 'Right where they're crossing is where some of the water is,' Sanchez said.
'Everybody on both sides (of the border) understands the severity of the rainstorms that we get here,' Sanchez said. 'We're all always kind of half an eye to the sky.'
He has noticed the storms have become more intense in recent years. Instead of one or two in monsoon season, last year brought multiple powerful storms unleashing large amounts of rainfall in very localized areas, Sanchez said.
It's 'something that I wasn't used to seeing, and I've been doing this for 30 years.'
It's easy to forget the danger in Santa Barbara County. The sun-drenched coastline, Spanish style architecture and warm, Mediterranean climate offer the illusion of calm. But just behind the serene facade is a landscape built for disaster.
Steep terrain, torrential wintertime storms and wildfires combine to put the Santa Barbara coastal watershed at high risk of flash flooding and mudslides, as rainfall screams down canyons carved into the Santa Ynez Mountains. The threat is growing as atmospheric rivers —intense plumes of moisture — coming in off the Pacific Ocean get stronger and wetter.
'We have a long history of flash flooding and debris flows,' and the terrain makes everything worse, said Kelly Hubbard, director of the county's Office of Emergency Management.
The mountain slopes tend to erode when saturated, especially after wildfires, resulting in potentially deadly debris flows.
Just weeks after the Thomas Fire tore through the hills above Montecito, an intense winter storm dumped several inches of rain in January 2018. The fire-scarred hillsides couldn't withstand the deluge. A wall of mud, boulders and debris thundered through the town as people slept. Twenty-three people were killed and more than 100 homes were destroyed.
As the city expands further into the foothills, the risk to residents grows. 'It tends to increase the vulnerability of people in homes when you start building right up against the hills,' said Jayme Laber, a hydrologist with the National Weather Service in Los Angeles.
'There's only so much flat land you can build on,' Laber said.
The area where neighborhoods press up against forests, brush and open terrain — known as the wildland-urban interface — is becoming more prominent in Santa Barbara. It was also a major factor in the destruction wrought by the Los Angeles fires in January.
'You get that increased flood risk and events that come out of the hills directed right towards those homes, buildings, and all that infrastructure that's built right there,' he said.
Santa Barbara County uses a patchwork of overlapping tools to warn people of danger: helicopter flyovers; emergency texts and calls; police cars that broadcast special tones, different from a typical siren, that tells residents to sit up and pay attention. Mobile sirens are best here, Hubbard said, since stationary sirens on poles can burn down in a wildfire.
There are 6,675 properties at risk of flooding in the next 30 years in Santa Barbara, according to research by First Street; 1,227 in Montecito; 1,970 in Carpinteria. But what keeps Hubbard up at night are the risks she doesn't know yet, like in 2023 when the county 'saw flooding in places we've never seen flooding before, and it took us a little bit by surprise.'
'What scares me the most is how it continues to change,' Hubbard said. 'It's really for something that for hundreds of years has been rather predictable. It's becoming unpredictable.'
The picturesque Catskill Mountains, north of New York City, are lush with rolling hills and winding streams. But the scenery belies significant flood vulnerabilities.
One such risky region is in Delaware County and stretches from around Margaretville to Pine Hill. The small community of Fleischmanns, population of about 230, has an 'extreme' risk of flooding, according to First Street's modeling.
Fleischmanns is vulnerable to river flooding and inundation from excessive precipitation, which is worsening as the climate continues to warm. Tropical Storm Irene brought severe flooding to the region in August 2011.
Delaware County ranks highest of any county in the state for federal disaster declarations since 1954, according to Steve Hood, director of emergency management for the county. The majority of those declarations were for flooding, and a recent assessment of the threats the county faces put flooding at the top of the list, he said.
Hood told CNN many flash floods there don't get into population centers as the region is sparsely populated, with river flooding constituting a somewhat bigger threat for the villages and towns.
The county lacks a siren system after a request to fully fund one hit a dead end in 2015, Hood said. That system would have warned areas downstream of two reservoirs; instead, officials rely on New York Alert, which is a state-run notification system, and reverse 911 to warn citizens of impending floodwaters, Hood said.
Tim Brewster, a hydrologist at the National Weather Service in Binghamton, New York, said Delaware County is a major flash flood hot spot within its forecast area — and terrain is the key factor.
'It's got really steep slopes and fast-draining, small river basins,' he said, which can lead to flash flooding from slow-moving thunderstorms, for example.
Forecasters are mindful of the significant influx of people during the summer who come to the county to escape the heat in the big cities along the East Coast, Brewster said, adding there are many summer camps located throughout the region. This gives it a similarity to Texas Hill Country.
'We definitely have to have a heightened sense of awareness of that population influx,' he said.
The Yadkin River and its tributaries in northwest North Carolina — surrounded by forests and dotted with vineyards — are no strangers to flash floods. But even the damage wrought by Hurricane Helene last year pales in comparison to the history of flooding in this scenic stretch of foothills of the Blue Ridge Mountains.
Elkin sits at the confluence of the Big Elkin Creek and the Yadkin River, which winds more than 200 miles through the state to the Atlantic Coast. The town is prone to intense storms, as warm, moist air from the Atlantic is drawn into the higher terrain, which can supercharge impacts. Steep-sided, narrow valleys funnel rain into the waterways below, causing them to rapidly swell and overflow.
When Hurricane Helene carved its 500-mile path of destruction from Florida to the Southern Appalachians in September 2024, Elkin and its neighboring towns — Wilkesboro, Ronda, Jonesville — were in its way.
Elkin was spared the deadly destruction wrought in other parts of the state, but the Yadkin River rose 22 feet, inundating homes and businesses, leaving roads underwater and swamping pickleball courts.
It took Jim Neese a week to clean up his campground, Riverwalk RV Park, nestled along the river, after Helene left it swimming in a muddy swirl of floodwater.
Floods are a part of life here, he told CNN. 'Anytime you get bad weather, you think about it.' They do tend to be foreseeable, he added: 'You see it coming and you know (the river is) rising… We keep an eye on everything. I watch three or four different apps.'
Vigilance is important; the town is vulnerable to much more catastrophic flooding.
Elkin was one of the many areas affected when a one-two punch of tropical cyclones led to a devastating flood in 1916. Then in 1940, the Yadkin River reached 37.5 feet, its highest crest on record, when the remnants of a hurricane pummeled parts of the river basin with more than 8 inches of rain, causing extensive damage to the town.
The Yadkin River is better protected today, in part due to the W. Kerr Scott dam, built in 1962 just upstream from Wilkesboro by the US Army Corps of Engineers, said Nick Fillo, a hydrologist with the National Weather Service in Blacksburg, Virginia.
It absorbs a lot of the runoff and holds back water, he told CNN. 'In order to see another flood like we saw in 1940 or 1916 we would need much, much more rain.'
For now, people in this tight-knit community feel prepared for the flooding that punctuates their lives. But Helene has shown they cannot be complacent.
'When we see something coming, it is a concern,' said Brent Cornelison, Elkin's town manager. 'And after Helene, it will be a much larger concern.'
Freedman reported from Washington, DC. O'Kruk reported from New York. Ory reported from Atlanta. Paddison reported from London. Miller and Weir reported from Helen, Georgia. Angela Fritz contributed reporting from Washington, DC.