HOUSTON — Torrential rains have killed at least 16 people in Texas and Oklahoma, including four in Houston, where floods turned streets into rivers and led to about 1,000 calls for help in the fourth-most populous U.S. city, officials said Tuesday.

The death toll is set to rise with numerous people still missing in Texas after the storms slammed the states during the Memorial Day weekend, causing record floods that destroyed hundreds of homes, swept away bridges, and even unearthed a coffin from a Houston cemetery. It washed ashore on the banks of a bayou.

“A lot of folks drove their car into high water and had to abandon those vehicles,” Mayor Annise Parker said at a news conference.

Two of the dead in Houston were found in their cars and another two were found in a bayou. One was likely to be a male victim lost when an evacuation boat capsized during a water rescue by emergency crews in the morning.

The Oklahoma medical examiner’s office said six people have died in weather-related incidents over the holiday weekend in the state.

Though Parker said parts of the city were unscathed, more than 1,000 vehicles were submerged in the Houston floods and people took instead to bicycles, kayaks and surfboards to navigate water-covered streets.

The Houston Fire Department brought about 500 people to safety in boats, local media reports said.

President Barack Obama said Tuesday he had assured Texas Gov. Greg Abbott that he could count on help from the federal government as the state recovers from the floods. Abbott has declared a state of disaster in at least 40 Texas counties, including Harris County, home to Houston.

Abbott said he has deployed the state’s National Guard and was worried the death toll could rise.

“It’s devastating to see what I saw on the Blanco River when this tidal wave of water just swept away neighborhoods,” he said, recalling a disaster area in central Texas.

Twelve people are confirmed missing and about another 30 are unaccounted for due to flooding that hit along the Blanco River, county officials said. The missing were from two families whose vacation home was swept off its foundation in Wimberley, a town about 30 miles southwest of Austin.

Dogs and boats were being used to search for the missing. The river rose so quickly and with such force, it caused a flood gauge to break, Hays County officials said.

There was no damage estimate available for the state, which has a $1.4 trillion-a-year economy and is the country’s main domestic source of energy as well as an agricultural and manufacturing power.

Houston resident Dutch Small, 40, climbed onto the roof of his car when the water came up to his knees inside his vehicle and was eventually rescued by a passing tow truck driver.

“It happened so fast. Every person that died in the flooding, I know what was going through their minds. They didn’t measure the threat accurately. They were like me,” Small told Reuters.

The National Weather Service issued tornado and thunderstorm watches for later Tuesday and said more rain is expected this week in Texas and Oklahoma.

More than 200 flights had been canceled by early Tuesday evening at airports in Houston and Dallas, some of the nation’s busiest, as blocked roads made it difficult for workers to get to their jobs. A sinkhole also closed a runway at the Dallas/Fort Worth International airport.

Roughly 100,000 customers lost power throughout the state after the storm due to high winds and rising waters that caused power poles to snap.

In Houston, about 11 inches of rainwater fell Monday while parts of Austin were hit by as much as 7 inches. Helicopter crews in both cities plucked people to safety who had been stranded in cars and on top of buildings.