A 28-year-old autistic man from Colorado who had been living on frogs and roots for weeks while wandering the remote Escalante Desert of southern Utah was found emaciated but alive, authorities said.

William Martin LaFever of Colorado Springs, Colo., told the people who rescued him that besides eating the bits of food that he found in the wild, he drank water from the Escalante River, Associated Press reported.

He had attempted to walk approximately 90 miles or more from Boulder, Utah, to Page, Arizona.

Authorities from the Garfield County Sheriff's Department said that the man was likely to have traveled about 40 miles over at least three weeks before he was found on Thursday.

"It is some of the most rugged, unforgiving terrain you will find anywhere on Earth, jagged cliffs, stone ledges, sandstone, sagebrush, juniper," said sheriff's spokeswoman Becki Bronson, according to AP.

"Where William was hiking, there just isn't anyone out there," she said. "There are no people. There are no towns."

Garfield County authorities said that it was incredible that searchers aboard a helicopter were able to find the 28-year-old at all, much less alive.

Deputy Ray Gardner, one of the rescuers who was aboard the helicopter and who recently completed training in search and rescue for people with autism, said that LaFever would have survived another 24 hours.

Rescuers had taken LaFever was to Garfield Memorial Hospital in Panguitch, and the hospital has not released any information about his condition.

LaFever had been trying to get to Page because his father, John LaFever of Colorado Springs, told him he would wire money to him in there, according to a statement released by the sheriff department.

The 28-year-old 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 that he had run out of money.

His father told him to catch a ride to Page to collect the money.

However, instead of following his father's orders, LaFever decided to hike down the Escalante River and then hitch a boat ride along Lake Powell to Page instead of catching a ride.

While hiking along the river, he had run out of food and his dog left him.  He had abandoned all 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 has not been seen since, and authorities do not know why it vanished.

The June phone call LaFever made to his father was the last time the family heard from him and his sister reported him missing on Monday, the sheriff department said.

Authorities said that autistic people are naturally drawn to water, so the helicopter search focused on the Escalante River, the department said.

Rescuers from above spotted LaFever Thursday afternoon as he was sitting in the Escalante River about five miles from Lake Powell, desperately waving at the aircraft.

Gardner had been stunned when LaFever identified himself because of the unlikelihood of finding anyone in that country, the sheriff's department said.

"In all my career I have never seen someone so emaciated," Gardner said 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."

The department said that LaFever was so weak that he could not stand, but he was so happy for human contact that initially he would not stop talking long enough to eat or drink; he eventually took a drink and ate a granola bar.