With a poke from his pitchfork, Joe Espinoza found the red door of an old Chrysler and wondered if the entire car was buried beneath it, in the mud. He called to his wife and two friends, fellow volunteers from Kerrville, Texas, who were searching the Guadalupe River floodplain for neighbors who were swept away by the July 4 deluge.

The four volunteers bent low and pried the door free. Underneath, they found only more mud.

It has gone that way often for the army of searchers hunting for more than 100 people who are still missing along the Guadalupe. More than 2,100 search workers from 12 states have descended on Kerr County, Texas, said Sergeant Jonathan Lamb of the Kerrville Police Department, including public workers from Nebraska, Louisiana, Virginia and even Mexico. Indiana alone sent people from 15 different fire and police departments.

Then there are volunteer groups, dozens of them from across the country, some who received donations for private flights into Kerrville-Kerr County Airport to begin searching as quickly as possible.

It is painstaking work, full of disappointments like the Chrysler door.

“You think you find something that might help someone, a body, or just a drivers license,” said Evan Cervantes, 34, who joined Mr. Espinoza in the search on Thursday after their shifts as psychiatric nursing assistants at Kerrville State Hospital. “But then you find nothing.”

But there is also solidarity in the struggle.

“It’s overwhelming to see so many people come and help in the search,” said Amy Vanlandingham, 38, a Kerrville resident who spent hours on Thursday searching along the river. “This is our town. I do it so I can sleep.”

The search for human remains is focused on an area of Texas that is unlike many of the places where recovery professionals are accustomed to looking, several experts said. Most major search operations in recent years have happened in large urban areas hit by hurricanes, said Mr. Koester and Scott Hammond, a professor in the Department of Management at Utah State University who studies search-and-rescue teams.

In the flood plain of Central Texas, by comparison, searchers are dealing with a relatively high number of people who are missing and presumed dead, spread across an expanse of mostly narrow, rural territory, spanning more than a hundred miles of shallow valleys along the river.

The destructive power arrived with little notice, in a relatively constrained river valley where there are few homes or other buildings to serve as likely search targets. The recovery efforts are therefore focused on the massive piles of debris.

That will continue to make the search especially slow, dangerous, painstaking and long.

“This effort in Texas will go on for some time,” said Mr. Hammond, a canine search handler who is part of the Utah County Sheriff Search and Rescue team.

Since the disaster, many have asked whether local officials responded quickly enough to warn people that floods were imminent. In the aftermath, rescuers and experts have said that the search effort had been well organized and efficient. In Kerrville, most teams gather every morning at the Kerr County Emergency Operations Center or at a satellite location to receive orders for the day. Some volunteers said they also coordinated with the Salvation Army.

Leaders meet with searchers when they return, Mr. Hammond said, who has used social media to track searchers’ accounts of the process. The organizers debrief about what they found, use that information to plan for the next day and then explain why those next steps are important, he said.

Along the Guadalupe River this week, the searchers were as varied as their tools. An official order grounded privately operated drones for much of the week to prevent them from crashing into helicopters. Then a no-fly zone was declared on Friday as President Trump arrived to survey the damage.

That left Jordy Marks, the owner of LA Drone in Lafayette, La., only one day to fly. He got an early start on Thursday and flew until sunset, inspecting 20 miles of the Guadalupe River with a drone capable of spotting individual stones in a gravel pile from 1,000 feet away.

“We can go under fallen trees, right along the top of the river,” Mr. Marks said. “I’m just trying to cover as much ground as possible.”

Meredith Pool spent Wednesday morning scouring the east bank of the river in Kerrville with Mini, her black Labrador retriever, and Kodak, her golden retriever. They walked into the wind, “letting the smells come to us,” said Ms. Pool, 49, who lives in Ardmore, Okla., and volunteers with a nonprofit called Gideon Rescue Co.

Mini paused, marking the scent of a cadaver. Ms. Pool notified the Kerrville Fire Department. Hours later, she returned with her dogs to search the opposite bank. Behind her, eight Kerrville firefighters paddled across the river in a blue inflatable raft, carrying the remains that Mini had found.

Mini did not pause to watch. Neither did Ms. Pool. Their search continued.

“Mini, check-check!” Ms. Pool said, using her command to focus the dog’s attention. “We’ve got a pocket of debris here to sniff.”

Down the river, near the town of Comfort, two other cadaver-sniffing dogs marked a low-lying spot behind a gravel quarry that had been inundated by water. On Thursday afternoon, two large pumps were forcing water out of the temporary pond so that volunteers could search the depression for bodies. The operation was run by the United Cajun Navy, a search and rescue group created in Louisiana after Hurricane Katrina.

As the water level dropped, it revealed a truck half buried in mud.

“We didn’t even know that truck was there when we started” pumping, said Alex Harkrider, 38, from Carthage, Texas. “That shows you how deep the water got. I’ve never seen destruction like this.”

Mr. Harkrider had worked at the site for three days. By Wednesday he was feeling lightheaded, he said, overwhelmed by the afternoon heat, which reached 92 degrees. So he rested in a folding chair beneath a tent, rolled up his sleeve, and asked Heather Orum, a nurse, for an IV of saline solution.

“This reminds me of a war zone,” said Ms. Orum, 52, who works for a company called Lone Star IV Medics.

Two miles away, Deb Gonzalez and her friend Teal Harris noticed a team of sweaty dog handlers along the road. The women parked their truck and hurried toward the men, carrying a bucket of ice and wet towels.

“We saw that people were being overwhelmed with the heat,” said Ms. Gonzalez, 60, as the men pressed the towels to their burning skin. “Everybody’s like, ‘That’s all we needed, just one moment.’”