FEMA administrator Brock Long is urging volunteers to get involved in assisting those in Houston as southeastern Texas continues to be hit with torrential rain and rising floodwaters.

"We need citizens to be involved," Long told reporters during a briefing Monday morning. "This is a landmark event. We have not seen an event like this."

The National Weather Service said more than 30 inches of rain has fallen in parts of Harris County, which includes Houston, in the last 72 hours, with peak flooding expected on Wednesday and Thursday.

The U.S. Coast Guard, local, state and federal law enforcement have been working to rescue people from the rising floodwaters.

Long said Monday he expects more than 30,000 people to be placed into temporary shelters, and 450,000 to file for assistance as a result of Harvey, which downgraded to a tropical storm.

"We are not out of the woods yet," acting Homeland Security Secretary Elaine Duke said Monday morning.

As water levels rose and rain continued to fall, the U.S. Army Corps of Engineers began releasing water from two reservoirs in the Houston area to relieve the stress of the rain and prevent flooding in the downtown area.

Law enforcement continued to rescue stranded people by air and boat, with videos and photos making the rounds on social media of good Samaritans using their own vessels to reach those in need of help.

But as federal authorities focused on rescue operations, questions arose as to why the city didn't evacuate.

Houston Mayor Sylvester Turner said it would've been a "nightmare" to empty out the city and county at one time.

"You literally cannot put 6.5 million people on the road," he said in a press conference Sunday. "If you think the situation right now is bad, you give an order to evacuate, you are creating a nightmare."