The National Hurricane Center is calling Harvey the biggest rainstorm on record.
Rainfall potentially reached 50 inches by midweek in some places like Houston, which has a city population of 2.3 million people.
President Trump declared a state of emergency in Louisiana and Texas.
“We're having a white house being very responsive, very concerned about people of Texas, and they have been a tremendous help to us,” said Texas Gov. Greg Abbott.
Governor Abbott activated the Texas National Guard in response to Harvey, bringing the total number of deployed guardsmen to 12,000.
The Houston Police Chief said 2,000 people have already been rescued from catastrophic flooding and 185 more critical rescues were pending Monday.
Making matters worse, Harvey is hovering over the Texas coast and threatening to turn eastward and make landfall again Wednesday before heading to Louisiana.
Environmental experts warn of bacterial infections brought on by sewage-laden floodwaters and potential for contaminated drinking water.