Floods in the same part of Texas swept away 10 teenagers in July 1987.

Desperate parents posted photos of their children online, seeking any information, and others went to reunification centers to try to find missing loved ones.

Hundreds of emergency personnel were searching for stranded people, using 14 helicopters and ground crews who were struggling to navigate flooded roads, officials said. They warned that the death toll was likely to rise.

“This is a tragic event. It’s going to be a mass casualty event,” Freeman F. Martin, the director of the Texas Department of Public Safety, told reporters Friday afternoon.

Mr. Patrick said Camp Mystic was contacting the parents of campers who remain unaccounted for. He said parents with children who had not heard from camp officials should assume their children were safe. The camp has some 750 campers, he said.

The flooding seemed to take many by surprise. Judge Rob Kelly, the chief elected official in Kerr County, said at a news conference that “we do not have a warning system” and that “we didn’t know this flood was coming.”

In Hunt, Texas, where the Guadalupe River forks, more than seven inches of rain has fallen since Thursday afternoon — the highest total that area since the early 1990s.

This led to a rapid rise of the Guadalupe River, which accelerated to over 29 feet before sunrise, the second-highest crest ever.

Here is what else to know:

• Searching for the missing: Downed power lines, flooded roads and spotty cellphone service were among the challenges rescue workers in Texas were facing on Friday as they searched for survivors.

• Federal help: Homeland Security Secretary Kristi Noem has activated the U.S. Coast Guard and the Federal Emergency Management Agency to help search for the missing, Tricia McLaughlin, a spokeswoman for the agency, said in an X post.

• Camp Mystic: The Christian summer camp for girls on the Guadalupe River is nearly a century old. Its facilities include a recreation hall that was constructed in the 1920s from local cypress trees.

• Past flooding: For those old enough to have lived through it, the flooding on Friday surfaced memories of a deadly swelling of the waters along the Guadalupe River on July 17, 1987.

• Kerrville and San Angelo: The police said on social media that they were working with the Kerrville Fire Department to evacuate residents, noting that “many roads and streets are flooding in town.” The Weather Service in San Angelo, Texas, said it had “received multiple reports of flooded roads and homes across Tom Green County.”

“We have plenty of resources on the ground,” Freeman F. Martin, the director of the Texas Department of Public Safety, told reporters on Friday afternoon.

Officials said they had received offers from residents who wanted to assist with the search. But they urged people to stay home and warned residents in the area to refrain from launching drones or using private helicopters, saying that doing so could endanger rescue workers.

“We don’t need any more first responders,” Mr. Martin said. “We don’t want anybody to self deploy.”

Mr. Martin said that emergency workers had managed to reach people who were stranded but safe in several locations in the flood zone. Officials were waiting for road conditions to improve before evacuating them.

“We’re able to bring food and water to them,” he said. “It’s just taking time to get them out of there.”

Mr. Martin said that as of 3:30 p.m. on Friday, about 25 roads in the area were impassable. He said that people should not try to drive or wade through flooded areas.

“If you can’t tell how deep it is, the best advice is to stay home,” he said.

Parents of campers who have not been accounted for have been notified, the email said. About 750 girls were at the camp this week, Lt. Gov. Dan Patrick of Texas said at a news conference on Friday.

The camp said that it was assisting with search-and-rescue operations, but that it did not have power, water or Wi-Fi and was struggling to get more help because a nearby highway had washed away.

On social media and in text messages, parents circulated photos of some of the missing girls, and exchanged hopeful stories that they were hearing about dramatic rescues: girls clinging to trees, or floating downriver to a boys camp five miles away.

Camp Mystic aims “to provide young girls with a wholesome Christian atmosphere in which they can develop outstanding personal qualities and self-esteem,”its website said. It draws from families around Texas and beyond, with the youngest campers entering third grade in the fall.

The camp was established in 1926, according to its website, and has been run by generations of the same family since the 1930s. The current owners, Dick and Tweety Eastland, have been with the camp since 1974. They worked alongside the previous owners for years after graduating from the University of Texas in Austin. Camp activities include archery, cooking, cheerleading, fishing and a variety of sports. Videos posted to the camp’s Instagram account show large groups of girls participating in group dance performances this summer.

The affected stretch of the Guadalupe River is home to several summer camps for children, including Camp La Junta for boys, about five miles from Camp Mystic. The boys camp informed families on Friday that all campers there were safe and fed, but evacuations would not begin until at least 7 p.m. local time on Friday.

The Facebook page for another nearby girls camp, Heart o’ the Hills, reported that it was not in session when the flooding took place.

Ten of the teenagers were swept away and killed; 33 others, and four adults, were rescued. Some of the survivors held on to the upper branches of cypress and pecan trees, praying until helicopters arrived to carry them to safety, The New York Times reported. At the time it was the worst flooding of the Guadalupe River in 55 years, The Times reported.

Rob Kelly, the Kerr County judge, said at a news conference on Friday morning that the flooding this time might exceed what took place in 1987, based on the waterline at his property along the river.

As in Friday’s flooding, in 1987, five to 10 inches of rain fell in the upper headwaters of the Guadalupe River basin. In Hunt, Texas, where the Guadalupe River forks, over seven inches had fallen since Thursday afternoon, which was the highest total at the site since the early 1990s.

The rapid rainfall led to a fast rise of the river that accelerated to over 29 feet before sunrise on Friday, the second highest crest ever recorded. Still, it might have even gone higher. As the waters rose 20 feet in two hours, they flooded the instrument box that the U.S. Geological Survey uses to send river gauge data.

The box stopped transmitting at 4:35 a.m. local time on Friday.

In 1987, though, warnings were issued early in the morning to summer camps along the river, and it was unclear why the church vehicles crossed the swollen river. All but one body was recovered.

More than a year later, a sheriff’s deputy in Kendall County, Texas, said of the missing camper that a cresting river would create sandbars, and a body could end up buried, possibly until another big flood comes to “turn things upside down again.”

In Kerrville, some people huddled inside a church’s activity center, and others looked distraught, shivering under blankets. Brian Eads, 52, was hoping for information about his wife, Katherine, after aggressive floodwaters ravaged their trailer at around 3:30 a.m.

“I have no idea if she’s made it,” Mr. Eads said. “We both got swept away, and then I lost her.”

The couple were awakened by rushing waters, and managed to escape with a man driving a recreational vehicle. But the water caught up with them about 20 feet away, Mr. Eads said, and the vehicle’s engine died. Both he and his wife were swept underwater. He tried to swim toward her voice, he said, but lost her when he was struck in the head by debris. He survived by holding onto a tree and making his way to dry land.

Outside Ingram Elementary School, about seven miles west of Kerrville, people hoped to find missing loved ones, including girls who were staying at Camp Mystic nearby. Some hugged each other outside the main entrance, while others stood waiting to hear from their relative.

Randy Bush, 59, said he had not heard from his fiancée Charlotte Huff, 55, since last night. He had already been at a local Walmart, where others were searching for relatives.

“I have no idea what happened to her,” he said.

His fiancée lives at an R.V. park in the Kerrville area. As soon as he heard about the floods, he rushed to the park, but was stopped by road closures and emergency vehicles.

“When I was there this morning, they were doing water rescues with helicopters,” Mr. Bush said. “From what I saw that park was gone,” he added as he made his way to the school. “It was just all water. It didn’t look like there was anything there. That whole park was done.”

Parents of campers were hoping for the best Friday afternoon. Betty Gerlach, whose 14-year-old grandson is a camper at Camp La Junta, a boys camp about five miles along the river from Camp Mystic, said the boys camp had informed families that all campers were safe and fed. But an evacuation plan was still in development, and would not begin until at least 7 p.m. local time on Friday.

The camp asked families from Houston, the Dallas-Fort Worth area and out of state to begin traveling to the area. But families in nearby Austin and San Antonio were told to “stay put for now,” to avoid overcrowding.

With several camp building washed away in the flooding, the campers had taken shelter in two small cabins while they waited for evacuation, Ms. Gerlach said.

By midafternoon, emergency crews had started to bring some of the stranded girls to Ingram Elementary. One man saw his daughter sitting in the passenger seat of an emergency vehicle and ran after it with a smile.

Though flash floods are scary, experts say you can increase your odds of survival by staying informed and having a plan. Here’s what to do in advance — and in the moment — to get through a flash flood safely.

Understand the difference between the various alerts.

The National Weather Service currently issues severe weather alerts in English and Spanish.

If there’s a “flash flood watch,” according to the service, flooding is not guaranteed, but conditions are favorable enough for it to be possible, so be prepared to change your plans.

A “flash flood warning” means a flash flood is imminent or already occurring, and you should immediately move to higher ground if you’re outside or in a basement apartment.

The most dire alert is a “flash flood emergency,” which indicates that not only is flooding occurring, but it’s posing a severe threat to human life. In 2021, New York City received its first notification of this type during heavy rainfall caused by Hurricane Ida.

Before a flood

Make a plan

Long before rain is on the radar, the first step is to figure out how your family will communicate, meet and evacuate if there’s a flash flood emergency. How will you escape from your home if needed? Who will be responsible for your children? Where will you meet if your family becomes separated? The American Red Cross has printable templates to help guide your conversation.

You’ll also want to evaluate the flood risks to your home, work and school, as well as the routes between them. The flood maps developed by the Federal Emergency Management Agency are a good place to start. (If you live in a flood zone, you might also want to consider purchasing flood insurance.)

Prepare a ‘go bag’ now

If you need to leave your home in a hurry, it’s crucial to have an easily accessible emergency kit packed with supplies. Consider adding shelf-stable food; water or a portable filtration system; a change of clothing; a headlamp or flashlight with batteries; a phone charger; cash; and a first-aid kit. If you have pets, don’t forget food, leashes and portable bowls for them, too. Ready.gov also advises creating “password-protected digital copies” of important paperwork, like birth certificates, identification cards, insurance policies, wills, deeds and titles.

If this seems like overkill, it’s not, said Dr. David Markenson, chief medical officer at American Red Cross Training Services. “The human nature side is obviously not to worry,” he said. And many people think, “‘It’s not going to happen to me.’”

But having a plan can help you make better choices in an emergency, said Sabine Marx, a senior trainer at Columbia University’s National Center for Disaster Preparedness. As she described it, she wouldn’t want to “have to come up with this decision on the spot when I’m also possibly fearing for my life.”

During a flood

Stay alert

If a storm is in the forecast or underway, pay attention to local weather alerts via your phone, radio or TV. In the case of a power outage, a battery-operated radio can come in handy.

Be prepared to evacuate

If there’s a chance you may have to leave your home, gather essential items that are not already in your “go bag” — driver’s licenses, credit cards, medications and key documents — and seal them in a waterproof pouch. (A plastic freezer bag works just fine.) Make sure your phone is charged and, if you have time, unplug small appliances so they won’t get fried by electrical surges. Move valuables to a higher floor (if you have one).

If you live in a basement apartment, be extra vigilant when it comes to monitoring rainstorms, said Julie Munger, founder of Sierra Rescue International, an organization that has trained swift-water rescuers for 35 years. If you believe you may be in danger, she recommended immediately moving to a higher floor or evacuating to another location. (To find an emergency shelter, text SHELTER and your ZIP code to 43362.) FEMA warns against climbing into a closed attic, as you could become trapped by rising floodwaters. If necessary, climb onto the roof.

If you find yourself in the worst-case scenario, with water rushing into your apartment, you need to act fast, Ms. Munger said. “Don’t wait, don’t grab anything, just exit,” because if you can’t get out, she added, your only option is to “hope that the water doesn’t fill up the apartment completely.”

According to Dr. Markenson, it’s essential to follow updates closely, since conditions can change rapidly. If you’re told to evacuate, do it. Check road closures on your state’s department of transportation website before heading out if there’s time, and take an alternate route if you encounter a flooded road.

The biggest issue with flash floods, Dr. Markenson said, is that people don’t always evacuate when they’re told to do so. But by trying to ride it out, he warned, you’ll endanger yourself and rescuers.

Avoid floodwaters when possible

The best thing to do is avoid all floodwaters if you can — or, as the National Weather Service’s grim catchphrase urges: “Turn Around Don’t Drown.” It takes just six inches of fast-moving water to knock you off your feet, so unless you’re ordered to evacuate, staying where you are is usually the safest choice. (Flash floods generally pass quickly.)

The most immediate risk of entering floodwaters is drowning, but you may also get exposed to various harms floating around the water, like human, animal and industrial waste; physical objects like cars, lumber and other debris; stray animals like rodents and snakes; and downed power lines.

If you’re caught in your car

Sometimes flash flooding happens when you’re out and about, and you may suddenly find yourself in a life-threatening situation. Nearly half of all flash flood deaths are vehicle-related, which is why you should never ignore barriers. “Don’t drive into a flooded street, period,” Ms. Munger said. “There really is no better advice.”

Not only is it difficult to gauge water depth and road conditions, but just 12 inches of water can float your car and 18 inches can carry off your SUV or pickup truck. “Everybody tends to underestimate the force of the water,” Ms. Munger said. “It takes very little current to wreak havoc.”

That said, if your car does get taken by floodwaters, first, roll down your windows, said Lynn Burttschell, an emergency medical worker, rescue swimmer and founder of Wimberley Rescue Training. If they won’t budge, he recommended breaking the glass with an escape tool (like the one in this Wirecutter guide, which you can store in your glove compartment) or using the metal pole of your headrest as a ram. It’s important to open the windows, Mr. Burttschell said, because “if the water continues to rise, then that car fills up and becomes more of a rock instead of a bobber floating downstream.”

Then, unbuckle your seatbelt and grip it as you climb onto the roof and call 911, Mr. Burttschell advised. Do your best to remain with the car until help arrives. Lie down on the roof to keep yourself stable, and don’t tie yourself to the car, in case it rolls.

During his 32-year career, Mr. Burttschell has found that people who stay with their cars survive at much higher rates than those who abandon them, simply because it’s easier for emergency services to spot a vehicle than a person. “I really don’t ever recommend leaving the vehicle,” he said. To make yourself more noticeable, you can also turn on your hazard lights, activate your car’s alarm with your key fob and, if possible, honk the horn.

If you’re walking, hiking or camping

If you happen to get caught in a flash flood while on foot, run perpendicular to the water and “get to the highest point possible,” Ms. Munger said — whether that means heading into the nearest building and racing up the stairs, climbing a tree or clambering onto a truck. The bigger and heavier the object, the better, she said, since it will be less likely to float away.

If you do get swept away, don’t attempt to stand up, as you risk trapping your foot in a drain, fence or other object. Instead, Ms. Munger advised swimming perpendicular to the current, as you would with a rip tide, until you get to safety. Since you’ll be fighting against drainages, debris and the current, she warned this is extremely difficult, even for strong swimmers. “People need to realize that most people who lose their footing in a flash flood don’t get out,” she said.

As for camping or hiking, Ms. Munger advised researching the region’s weather patterns and forecast before setting out. If there might be a rainstorm upstream of your destination, she suggested camping above any rivers, rather than beside them. If water starts to rise where you are, head immediately to higher ground.

If you’re on an underground train or subway

The subway is “the last place you want to be” during a flash flood, Ms. Munger said. “Because eventually, if the storm drains are overwhelmed, there’s no other place for the water to go.” Your best defense, in other words, is to avoid it altogether.

If you do find yourself underground during a flood, Ms. Munger urged exiting the station as swiftly as possible — even if that means forcing your way up flooded stairs. If you’re on a train that is stuck, don’t leave it until you’re instructed to do so, said Eugene Resnick, a spokesman for the Metropolitan Transportation Authority in New York.

Take flash floods seriously

While some of these steps may sound inconvenient, the reality is that following them could save your life. “You never want to be in a position where you’re looking back, or others are looking back, and saying: ‘Why didn’t you just heed simple advice?’” Dr. Markenson said.

Or, as Ms. Munger put it: “It’s going to be much more of a hassle and much more tragic when you don’t make it home.”

Susan Shain is a freelance journalist and future New York Times fellow based in Madison, Wis.