These Texas 'flash flood alley' towns have suffered most in horrific flooding

At least 90 people have died and rescue workers are still searching for survivors as heavy rain threatens additional flooding in Texas this week.

Twenty-seven children and counselors from Camp Mystic, an all-girls summer camp near the Guadalupe River in central Texas, have died in flash floods, said Kerr County Sheriff Larry Leitha at a news conference. Crews are still searching for people missing from the camp.

Learn more: Where the most rain fell in Texas

Six Texas counties have reported flood-related deaths since Friday. Kerr County, 65 miles northwest of San Antonio, has recorded at least 68 deaths.

Nearby Travis, Burnet, Kendall, Williamson and Tom Green counties also have reported fatalities.

What areas in Texas have flooded?

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Floods took place in ‘flash flood alley’

Local officials said they were caught off guard by the fast-rising floods, giving them little time to act, USA TODAY reported.

The rapid increase in river levels happened in what’s called flash flood alley, an area in the Texas Hill Country. It's known as a dangerous region for flash floods, according to the Lower Colorado River Authority.

Flash flood alley is a curved-shaped region about 300 miles long and 50 miles wide in central/south-central Texas, extending from north of Dallas to southwest of San Antonio. Its location, weather patterns and geology often combine to create devastating floods.

Those factors include:

• Heavy rain: The region lies between cold, dry air from the north and wet, warm air from the Gulf. The air masses converge and create intense rainstorms.
• Rocky soil: Limestone in the area prevents rainwater from soaking into the ground. Instead, the water rushes into valleys, Texas Public Radio says.
• Steep terrain: The Balcones Escarpment is a series of fault lines in central Texas. It creates a series of cliffs and hilly terrain.

The Guadalupe River has flooded more than a dozen times since 1978, according to the U.S. Geological Survey, but the Independence Day flood is among the worst in its history, USA TODAY reported.

CONTRIBUTING Dinah Voyles Pulver, Rick Jervis and Christopher Cann

SOURCE USA TODAY Network reporting and research; Reuters; National Weather Service; Lower Colorado River Authority