San Marcos emergency management coordinator, Kenneth Bell, said the body of one man had been recovered but had no more details. Three more people are missing.
Kristi Wyatt, communications director for the town, said: "We have people on car tops and rooftops awaiting rescue. People in homes are going to higher levels."
She said hundreds of people were now in evacuation centres and that floodwaters had washed away five police cars.
Several hundred houses were destroyed in the town of Wimberley.
"It looks pretty bad out there," said Hays County emergency management coordinator Kharley Smith.
"We do have whole streets with maybe one or two houses left on them and the rest are just slabs," she said.
A tornado hit Houston briefly on Sunday, damaging buildings and injuring at least two people.
Warnings of more tornados have been issued for parts of Iowa, Kansas, Missouri and Illinois.
The storm sewer system in parts of San Antonio saw water exploding from sewer covers.
A mandatory evacuation was ordered at Lake Lewis, 50 miles (80km) north of Houston, which itself saw high winds bringing down trees and blowing out windows.
Wichita Falls, also in Texas, was warned its river could burst its banks.
A fire-fighter died in Oklahoma after being swept away as he took part in a rescue effort in the town of Claremore, in the north-east of the state. He was named as Capt Jason Farley.
A woman was also killed in a weather-related traffic accident in Tulsa.
Residents were rescued from roofs in the state's Cleveland and Comanche counties.
The National Weather Service says Oklahoma City already has a new monthly rainfall record for May - at 18.19 inches.
In Colorado, El Paso and Pueblo counties and the city of Sterling were badly affected.
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