Swollen Texas rivers prompt evacuations after floods kill 21

DALLAS (Reuters) - A new batch of heavy rain caused Texas rivers to overflow their banks and prompted widespread evacuations on Friday after a week of historic storms flooded Houston, killed at least 21 people and damaged more than 4,000 buildings.

Thousands of cars were trapped for about six hours on a suburban freeway blocked by floodwaters near Dallas, where rainfall reached about 7 inches (17.8 cm) between Thursday night and Friday. The Red Cross distributed Girl Scout cookies and water to stranded motorists.

"I am very frustrated. I feel like I am on an island and nobody cares," Vanessa Paterson, who was on the highway with her 6-month-old son, told TV station WFAA.

The National Weather Service issued a flash flood watch from Central Texas into Missouri, with the additional rain swelling Texas rivers already at dangerously high levels.

Dallas officials advised people to go home early and stay off streets that have seen more water than they can handle.

"This would be a great night to stay home, watch a movie and cook some popcorn," Dallas Police Deputy Chief Scott Walton told a news conference.

Governor Greg Abbott has declared 70 counties disaster areas and called on Texans to heed severe weather warnings.

The mayor of Wharton, about 60 miles (100 km) southwest of Houston, issued a mandatory evacuation order for about 900 people living near the Colorado River, which began flooding into the city on Thursday and has been rising steadily since.

Hundreds of Texans are set to spend the night in shelters after this week's floods turned streets into rivers, ripped homes off their foundations and swept over thousands of vehicles.

The Brazos River, which began overflowing its banks on Wednesday in Parker County about 30 miles (50 km) west of Fort Worth, is expected to see another surge due to recent rains.

"This situation will get worse before it gets better," said Parker County Emergency Management Coordinator George Teague.

The rushing water trapped people in cars and houses, prompting hundreds of calls for help in North Texas. One Dallas-area police officer had to be plucked to safety by a helicopter.

The rainfall for May across the state has already set a new record and more storms were forecast for Texas over the weekend.

The volume and intensity of rainfall has taxed water systems with the U.S. Army Corps of Engineers managing the difficult task of releasing water from dams reaching their limits and preventing serious flooding downstream.

"The system is operating as it was designed but there is a lot of water and it has to go somewhere," said Clay Church, spokesman for the Fort Worth District of the U.S. Army Corps of Engineers.

(Writing by Jon Herskovitz in Austin, Texas; Reporting by Lisa Maria Garza in Dallas and Jim Forsyth in San Antonio; Additional reporting by Marice Richter in Dallas and Heide Brandes in Oklahoma City; Editing by Lisa Lambert, Doina Chiacu and Sandra Maler)