Here are some of the most powerful photos of the devastation and a before-and-after look at the affected areas.

If you can do so safely, please share your own photos and videos here, or leave us a voice mail message. Listen to Thursday’s episode of “The Daily” to learn about how Houston was built to flood.

The human toll

At least 39 people have died, including a Houston police officer, a family trapped in a van beneath surging floodwater, and a mother whose shivering 3-year-old was found clinging to her unresponsive body.

Survivors face hurdles of their own, with help likely to be slow in coming. Managing the shelters that are housing tens of thousands of them remains “the biggest battle that we have right now,” Brock Long, the administrator of the Federal Emergency Management Agency, said on Thursday. Rich and poor alike have had their homes and lives upended.

There were many positive stories, too. A huge volunteer effort operated with little official guidance. A businessman turned his mattress store into a safe haven. And the women and cat in a viral photo of a flooded nursing home are safe. The Houston Independent School District also said that all students will receive three free meals a day for the school year.

Here’s a look at how victims sought out help online.

What made Harvey so powerful?

What set Harvey apart was its rain. Once it made landfall, the storm essentially stalled, turning roads into rivers. Scientists say it was fueled by a deadly combination of environmental factors.

While it has been called a “500-year flood,” that term may be misleading: A similar storm may not be as far off as you might think.