PowerPoint Show by Andrew

The death toll hit 21 and was expected to inch up as recovery operations continued in the face of still more rains in the soaked region.

In Wharton, outside Houston, the mayor asked residents to voluntarily evacuate about 300 homes on the west side of the city due to the predicted rise of the Colorado River.

In the Parker County town of Horseshoe Bend in North Texas, officials asked about 250 residents to leave their homes as the Brazos River was expected to rise. 

Some parts of Oklahoma, which was also hard hit by the deluge that swamped Texas, expected inches more rain on Thursday.

Aerial footage reveals the dramatic extent of flood damage in hard-hit Houston, where experts say flood control has been offset by the population boom around Houston, one of the fastest-growing areas in the nation.

Mailboxes stand along a road covered in flood water from the San Jacinto River Thursday at Cooper Park in Kingwood, Texas.

The Harris County Flood Control District, the agency working in recent years with the U.S. Army Corps of Engineers on hundreds of millions of dollars in projects to ease the flooding impact, has been around since 1937, itself a product of catastrophic flooding two years earlier.

Flood control efforts on Buffalo Bayou alone, one of several that meander throughout the nation's fourth-largest city, have cost a half-billion dollars over the past decade.

Improvements to Buffalo Bayou have included bridge replacements, and the addition of detention ponds for rain runoff and green spaces that serve as parks in normal times.

Even though Houston is 50 miles from the Gulf of Mexico, any flood control efforts are under the specter of possible rising sea levels that will bring 'ongoing consequences' for rivers and canals.

Flood debris removed from homes sits following days of heavy rains and flash flooding in Houston.

A car sits submerged in a bayou near an overpass following days of heavy rains and flash flooding in Houston, Texas on Wednesday.

Relatives of a flood victim react with shock after learning a loved one was found drowned.

Porsha Peck, 12, left, and Dave Resendez, both from Corpus Christi, Texas, helps organize donations of supplies in San Marcos, Texas, Wednesday.

A photo recovered through debris is held by Kayla Massie as she helps search for people who are missing after heavy flooding Wednesday around Umphery Ranch located between Wimberley and San Marcos.

Ashley Hryszko, left, and Haley Hryszko clears the storage room of their friend's home on the bank of the Blanco River in Wimberly, Texas.

Lance Norman and his 7-year-old son, Peyton, observe rising water near the West Shore restaurant in Garland City on Wednesday.

Phil Archer walks through his bedroom at his home on the bank of the Blanco River in Wimberly, Texas, on Wednesday.

The Padera Dam near Dallas was dangerously close to overflowing, and sending a cascade of floodwaters into a nearby busy highway, as workers scrambled to pump off the excess on Wednesday.

A tornado and torrential rains hit Texas and Oklahoma, killing over 20 people and damaging hundreds of homes and buildings. At least 11 remained missing Thursday.

Volunteers search for missing bodies along the Blanco River May 26, 2015 in San Marcos, Texas. The area was one of the hardest hit from recent storms and over a dozen remained unaccounted for Wednesday.

A home in Wimberley, Texas is left mud-filled and uninhabitable as floodwaters slowly recede and reveal the devastation wrought by relentless storms.

An overturned vehicle close to Blanco River which had flooded 2 days earlier.

Over 11 inches of rain had fallen in parts of Houston, America's fourth largest city, where flooded highways have forced hundreds to abandon their cars.

A destroyed car is submerged in the Blanco River after the flood.