Residents have been stocking up on water since early Thursday when Beaumont first announced it lost access to clean water, part of a broader concern that the state called “the biggest threat to public health at this time.” Dennis Johnson had collected 26 gallons, which he was loading into the back of a taxi. “I don’t want to take any chances,” he said.

The authorities set up locations to distribute bottled water — the heat index in Beaumont exceeded 90 degrees on Friday, according to the National Weather Service — and reported heavy demand. National Guard officials said they were moving resources east of Houston as conditions deteriorated in places like Beaumont.

Some in the city said they are concerned about what might happen in a place in such dire and immediate need.

“I just worry when you got people that are desperate — no gas, there’s lines for blocks to get in grocery stores, they’re out of water, they’re out of food, desperate people do strange things,” said Prof. James Love, a retired professor of criminal justice at Lamar University in Beaumont. So far, Prof. Love said, he has heard few incidents on the police scanner.

With severely limited access to water service in Beaumont, one of the city’s hospitals, Baptist Hospitals of Southeast Texas, said that it had transferred 110 patients since Thursday and that it hoped to move another 83, including 14 newborns, from its main wards on Friday.

“Our E.R. is cleared out,” said Mary Poole, a spokeswoman for the hospital. “Our dialysis is gone, and almost all of our I.C.U. is gone.”

At the same time of the water shortage, the area around Beaumont is facing the opposite problem: a 150-foot breach in a levee holding back a 5,500-acre retention pond that has so far resisted attempts at repair.

“We lost a piece of equipment up there last night,” said Richard LeBlanc, general manager of Jefferson County Drainage District No. 6, the area flood-control agency, who called the breach the biggest concern in his district. “We almost lost a man.”

In the scrambled and bizarre geography of disaster, many homes even in Beaumont have come out relatively unscathed, while whole neighborhoods in other places had been submerged.

The population of Mauriceville, a heavily flooded bedroom community, seemed to have largely reconstituted in the gym at Buna High School about 20 miles up the road.

This was the big shelter in this little town. It, too, had been without power on Friday morning, though a team of volunteers, in some cases having driven through the night or hitchhiked to get there, were barbecuing and tirelessly treating the sick and injured. The evacuees sat in an awful kind of idleness, just wondering what had become of their homes and when they would be able to see them again. All they knew of them now came from occasional phone calls with family members who had stayed behind with boats to take part in the huge rescue operation. Many of those in the shelter had come here after spending a terrifying Tuesday night when water started seeping through floor vents and creeping under doors, and sent butane tanks floating around backyards. By Wednesday morning, the only way out was by boat.

“What do we do when there ain’t no place to go home to?” asked a teary-eyed Rebecca Tally, 58, who left her house in a kayak that her son had just happened to see floating by his house nearby. “Where do we go?”

“We want to go home,” said her son, Dusty. “But we don’t have no home to go to.”

The talk in the gym was of dam safety and floodgates opening along the Sabine and Neches Rivers, and whether that would mean more damage. Of what had happened to their beloved pets. And of this other hurricane brewing in the Gulf, Irma, a bad one they say. What if that comes here? What would become of this shelter?

“If the Good Lord’s willing to take me I’m ready to go,” Ms. Tally said.

Within hours, she and the 300 others whose only current home was the gym, would need to go, again, to a shelter a hundred or so miles farther inland.