America's fourth-largest city is underwater.

Residents use boats to evacuate flood waters from Tropical Storm Harvey along Tidwell Road in East Houston. | (REUTERS/Adrees Latif)

In the days since Hurricane Harvey was downgraded to a tropical storm, Houston and other parts of southeast Texas have suffered cataclysmic flooding. The storm has dumped more rain, and at a faster rate, than any other storm in U.S. history. By the time it's over, the worst-hit areas will see between 40 and 50 inches of rain — more than some regions of the U.S. will get in an entire year.

Those rising waters have put the city in an extended search-and-rescue mode, with first responders stretched thin. But citizens have been stepping up to help, saving neighbors from their flooded homes by any means possible — boats, kayaks, inflatable pools.

FEMA estimates some 30,000 people will need shelter for the next several days. The city has already been forced to use dump trucks to transport large groups of displaced residents to area convention centers. Soaked refugees arrive with just a few belongings in plastic garbage bags, worried of what they've left behind, fearful of what they'll return to.

When the rain finally stops, and the water recedes, Houston residents and officials will have a new disaster on their hands.

Below, see how Houston is wading through one of the worst floods in U.S. history.

Beaumont Place, Houston. | (REUTERS/Jonathan Bachman)

A truck driver walks past an abandoned pickup while checking the depth of an underpass. | (BRENDAN SMIALOWSKI/AFP/Getty Images)

A child makes his way through floodwaters while checking on neighbors at his apartment complex in Houston. | (AP Photo/LM Otero)

People wait to be rescued from their flooded home. | (Joe Raedle/Getty Images)

An elderly woman and her poodle use an air mattress to float above flood waters while waiting to be rescued from Scarsdale Boulevard in Houston. | (REUTERS/Adrees Latif)

A man standing in the doorway of his flooded home responds to an evacuation offer in a Houston neighborhood. | (AP Photo/Charlie Riedel)

Alexendre Jorge evacuates Ethan Colman, 4, from a Houston neighborhood. | (AP Photo/Charlie Riedel)

Isiah Courtney carries his dog Bruce through flood waters in Beaumont Place, Houston. | (REUTERS/Jonathan Bachman)

Shardea Harrison looks at her 3-week-old baby, held by Dean Mize, after being rescued. Mize and Jason Legnon used an airboat to help flood victims. | (Joe Raedle/Getty Images)

A Texas National Guardsman carries a resident from her flooded home. | (Lt. Zachary West/Army National Guard via Getty Images)

A man helps children across a flooded street as they evacuate their home. | (Joe Raedle/Getty Images)

Evacuees are airlifted in a Coast Guard helicopter. | (U.S. Coast Guard/Petty Officer 3rd Class Johanna Strickland/Handout via REUTERS)

Residents embrace after being rescued in east Houston. | (REUTERS/Jonathan Bachman)

Flood victims arrive at a shelter in the George R. Brown Convention Center in Houston. | (BRENDAN SMIALOWSKI/AFP/Getty Images)

**Click here to find out how you can help Houston's flood victims.**