The desperate messages and horrifying photos washed across social media like so much deadly floodwater this weekend.

There was the mother trapped on her roof with five children who were “running out of time” and the pregnant woman who was in labor and unable to reach police. Miles away, on Houston's northeast side, a photo emerged showing four children asleep on a counter in a flooded kitchen, including a little girl reportedly on a ventilator.

“Still trying to get a water rescue,” the message said. “Please send some help quick.”

Each heartbreaking story — shared widely on Facebook, Instagram and Twitter as floodwaters swelled — offered some variation of the same message as Tropical Storm Harvey unleashed chaos across Houston:

"Here's my address – Please send help!"

When social media was in its infancy, Americans watched in horror as Hurricane Katrina inundated New Orleans, marooning residents on rooftops, where they helplessly waved white sheets and held up signs for passing helicopters, often to no avail. Twelve years, several smartphone releases and billions of tweets later — as a powerful storm hovered over America's fourth-largest metropolis — social media allowed many Houstonians to take their fate into their own hands. Using social media, flood victims who still had power were able to communicate with public officials directly or to bypass them entirely and coordinate their own rescues with private citizens.

With floodwaters trapping people in homes and emergency phone lines overwhelmed, many had no other option but to post a plea online and hope it was received by a heroic stranger. In the process, thousands of Texans offered the world a window into their gripping struggle for survival and the latest example of the evolving role that social media can play in large-scale disasters.

Between Saturday night and Sunday morning, Twitter accounts normally reserved for photos of kids, dinner recipes and celebrity gossip were suddenly transformed into lifelines capturing people's efforts to stay alive.

“I have 2 children with me and the water is swallowing us up,” Maritza Willis tweeted in the early morning hours Sunday, along with her address. "911 is not responding!!!!!!”

A little over an hour later, she had been saved.

Many of the city's local celebrities with hundreds of thousands of followers — such as rappers Paul Wall and Kirko Bangz — tried reposting pleas for help on their social media feeds in hopes of drawing rescuers' attention, a tactic known as a “signal boost.” The most wrenching posts spread across Houstonian's social media pages, where people offered prayers and asked for updates. Social media users typically keep their addresses, phone numbers and intimate family details hidden from public view, but the brutality of the storm suspended those rules.

John Nova Lomax — a Houston resident and senior editor at Texas Monthly who has written extensively about the city's flood problems — said he noticed a Facebook post Sunday asking for someone to rescue a woman trapped on her roof with two dogs. The victim's address was less than a half a mile away, he said, leading him to embark on a hapless rescue effort that ended when he reached an impassable bridge several blocks from his house in northwest Houston. Despite his failed attempt, he said, social media spawned numerous successful rescue efforts over the weekend, often through Facebook groups that formed to share information and facilitate rescues.

The Facebook group "Hurricane Harvey Helping Hands” has multiple examples of homegrown rescuers posting photos of their boats and trucks along with their phone numbers. Police have also asked boat owners to help authorities with rescues using social media.

“The government can only do so much for a city of 7 million people,” Lomax said. “We have a lot of outdoorsy people down here and a lot of people with boats and lifted trucks and there's a Texas machismo here that says, 'I'm not going to wait, I'm going to get out there and help people myself!' "

“The difference with social media this time around is that it allows someone to pinpoint where the victims are,” Lomax added. “It's actually helping.”

In so many cases, those seeking help were the city's most vulnerable — children, the elderly and the infirm. Houston Police Chief Art Acevedo instructed residents to continue using official channels to ask for help, but many social media users complained they couldn't get through as floodwaters swept across Southeast Texas. In some cases, however, Web-savvy officials responded to victims almost immediately.

There's no official tally of how many people were saved this weekend because of social media.

Harris County Judge Ed Emmett told the Houston Chronicle that the county had carried out 1,500 to 2,000 high-water rescues since Saturday night, a number he called “unprecedented.”

Between 10 p.m. Saturday and 1 p.m. Sunday, 911 operators were slammed with more than 56,000 calls, according to the city's Office of Emergency Management. At a Sunday news conference, Houston Mayor Sylvester Turner asked residents to limit their calls to the most urgent emergencies.

“If you’re stranded in your vehicle, but you are in a safe place, or a dry place, let’s give preference to those who are in a situation in their home where water is rising very quickly,” he said. “I ask that you continue to call. We are manning 911. But a lot of calls have come in.”