William Martin LaFever of Colorado Springs, Colo., told rescuers that in addition to the bits of food he scavenged, he drank water from the Escalante River while attempting to walk from Boulder, Utah, to Page, Ariz., a distance of approximately 90 miles or more by the route he appeared to be taking.
The Garfield County Sheriff's Department estimated he had traveled about 40 miles before he was found.
"It is some of the most rugged, unforgiving terrain you will find anywhere on Earth, jagged cliffs, stone ledges, sandstone, sagebrush, juniper," sheriff's spokeswoman Becki Bronson said in a telephone interview.
"Where William was hiking, there just isn't anyone out there," she said. "There are no people. There are no towns."
The sheriff's department said it was remarkable that searchers aboard a helicopter were able to find LaFever at all, much less alive.
Deputy Ray Gardner, who had recently completed training in search and rescue operations for people with autism and was aboard the helicopter, said LaFever would not have survived another 24 hours.
The helicopter took LaFever to Garfield Memorial Hospital in Panguitch. The hospital said it could not release any information on his condition.
LaFever was trying to get to Page because his father, John LaFever of Colorado Springs, told him he would wire money to him in there, the sheriff's department said in a written release.
William LaFever had called his father on June 6 or 7 to say he was hiking in the Boulder area with his dog, and that someone had stolen some of his hiking gear and he had run out of money. John LaFever told his son to catch a ride to Page to collect the money.
Unbeknownst to his father, William LaFever apparently decided to hike down the Escalante River and then hitch a boat ride along Lake Powell to Page, rather than try to catch a ride, the sheriff's department said.
LaFever set out along the river but ran out of food. His dog left him, and LaFever began abandoning his gear until all he had was the clothing and shoes he was wearing when he was found, the sheriff's department said.
The dog hasn't been seen since. Authorities do not know why the dog ran off, Bronson said.
The early June phone call was the last time the family heard from LaFever, and his sister reported him missing on Monday, the sheriff's department said.
A telephone message left at the LaFevers' home in Colorado Springs wasn't immediately returned.
Gardner's training in searching for people with autism taught him they are naturally drawn to water, so the helicopter search focused on the Escalante River, the department said.
The helicopter team spotted LaFever Thursday afternoon, sitting in the Escalante River about five miles from Lake Powell, weakly waving at the aircraft.
Gardner was dumbfounded when LaFever identified himself because of the long odds of finding anyone in that country, the sheriff's department said.
"In all my career I have never seen someone so emaciated," Gardner was quoted as saying in the sheriff's department release. "I could not believe that he was alive, and feel certain that in another 24 hours he would not have been alive."
Gardner didn't immediately return a phone message late Thursday.
LaFever was so weak that he couldn't stand, but he was so eager for human contact that at first he would not stop talking long enough to eat or drink anything, the sheriff's department said. He eventually took a drink and ate a granola bar.