Houston - Bryan Curtis normally rides his jet ski for fun. On Sunday, he turned it into an emergency rescue vehicle.Curtis - who lives in Conroe, north of Houston - was one of the many civilians who pitched in to help evacuate those stranded in rapidly rising floodwaters in Texas's biggest city unleashed by monster storm Harvey."I'm not even thinking about myself right now to tell you the truth. It's just people need help, I'm here to help, I want to do my part," he said.With the official emergency services overwhelmed by the scale of the disaster in Houston, the fourth biggest city in the US, it was sometimes left to an army of volunteers to save the day.Some formed human chains while others fired up their motorboats to pluck their fellow Texans to safety from the floodwaters swamping the Lone Star state.READ: Rescuers pluck hundreds from rising floodwaters in Houston'Inundated'After more than 60cm of rain fell in Houston in a 24-hour period, some streets resembled canals, making it impossible for cars to navigate the city.Some of the few vehicles that could be seen on the streets were giant trucks, loaded with evacuees who had gratefully clambered on board after hearing offers of help."We continually go through these neighbourhoods and are actually calling out on our loudspeakers trying to get people's attention and saying: 'Hey, are you ready to evacuate?'" said local law enforcement officer Alan Rosen."We're just inundated. There's not enough resources - there never is enough resources - to actually go around and save everybody," he told the local KTRK TV channel after loading up one of the rescue vehicles with dozens of evacuees."We're doing absolutely the best we can."One man was spotted clinging to a tree on Saturday night by police patrol officers who then formed a human chain across a bayou to save him. He later posed grinning broadly without his rescuers on the local police department's Twitter feed.With so many roads under water, a boat ride was the only option for some of those stranded in outlying areas.James Lofton, a resident of the suburb of Spring Valley, made multiple journeys on his boat to ferry residents to safety from a nearby hotel to safety."We've just been carrying people back and forth from the Omni most of the afternoon," Lofton said.Dramatic rescueOne of the last to leave the hotel was a woman who recently had back surgery who was delicately brought on board."Obviously she's had surgery and is in a lot of pain. This was a very painful ride for her," said Lofton.Curtis said he and a friend had contacted the emergency management office to offer their jet ski services."We're just waiting on a phone call from them to see where they need us at," he said.Some of the most dramatic rescues came as helicopters winched those stranded on the upper floors of houses to safety.The Houston-based ABC 13 channel captured footage of a father and his 6-year-old son, both called Jeremiah, climbing out of a second floor window and into a makeshift basket dangling from a chopper, with one bag on each of their backs."This is all that we got," said the father. "We thank God. We thank God."