(Courtesy LaFever family)

An autistic man survived by eating frogs and roots as he wandered for weeks through the desolate Escalante Desert in southern Utah until he was rescued yesterday.

William LaFever, 28, of Colorado Springs, Colo., told rescuers that in addition to food he scavenged, he drank water from the Escalante River he was traveling along.

Becki Bronson, spokeswoman for the Garfield County Sheriff's Office, said that people come from all over the world to hike in the Escalante Desert because it is so challenging and dangerous.

"People hiking out there want a challenge and there are people that have been killed," said Bronson. "They underestimate the conditions. There are jagged rocks, sheer cliffs, sliding sand, sagebrush, juniper trees and blistering hot temperatures."

Bronson also said that the area is extremely remote, difficult and very unforgiving.

"It's the most incredible set of circumstances to come together for him to be found alive," said Bronson.

Bronson said LaFever's journey began after he managed to hitch a ride from his Colorado Springs home to Boulder, Utah to go hiking with his dog in early June. By June 6 or 7, he had to call his father, John LaFever, from Boulder to ask for money after some of his hiking gear was stolen. He had run out of money.

LaFever's father told him to travel to Page, Ariz., where he would be able to wire him some money. But Bronson says the details of how LaFever would travel to Page were never discussed, and his father had no idea his son planned to hike along the river to Lake Powell. According to Bronson, once LaFever arrived at Lake Powell, he planned on catching a boat ride to the northern Arizona city.

After his family didn't hear from for weeks, his sister, Lisa, called Garfield County authorities on Monday.

Officials then launched a search and rescue effort.

"We got all the information and decided that we'll fly the river and by some hope that by some long shot that maybe he is still out there," said Bronson.

Then, miraculously, the two-man helicopter crew, Utah Highway Patrol pilot Shane Oldfield and Garfield County Deputy Ray Gardner, spotted LaFever sitting in the Escalante River late Thursday afternoon, waving weakly at the aircraft.

Officials say LaFever had been lost in the desert for at least 3 weeks and they suspect it could have been much longer. They also believe he traveled at least 50 miles over that time period.

Oldfield said that once they saw a person in the middle of the river, it was obviously the man they were looking for.

"We were just flying the river and it just twists and turns and we were just flying on the river looking for anybody," said Oldfield. "Once we found him, we gave him a moment to calm down. He was horrifyingly emaciated, but surprisingly hydrated."

Bronson said Gardner, who recently completed his training for search and rescue operations for people with autism, remembered that autistic people are drawn to bodies of water. They say this information helped determine their search planning.

LaFever told Oldfield that he had been in that location for many days and that he would roll and sit in the water and roll back to shore at night, which helped him stay hydrated.

Oldfield also said that the rescue effort could have been much more difficult.

"We're out on these searches all the time and if people are where they say are going to be we can find them easily," said Oldfield. "But if they change their plans it becomes much more difficult. He was smart enough to stay on his planned route."

"My brother is a survivor," said Lisa LaFever. "There were a bunch of different scenarios that we feared that could have happened. We worried so much."

Lisa said that initially authorities weren't very helpful because the region was so massive, but she was grateful once officials launched the rescue effort.

"I told him [Gardner] that my brother had been missing for weeks and that he was autistic," said Lisa. "I was working with Ray all week and I just gave him as much information as I could."

After weeks of worrying and fearing the worst for William, LaFever's family was elated after they learned of his rescue.

"We were overwhelmed and overjoyed," said Lisa. "It's been so hard since he left and we're so grateful that my brother is alive. We're in shock from all the support we received and we couldn't be happier."

"It's almost too good to be true that they found him," John LaFever told ABCNews. "I thank God my daughter made the phone call, and I thank God that Ray Gardner was the person she talked to."

Bronson said that LaFever was able to stay alive because he staying in the river to cool down and stay hydrated.

The Sheriff department said that LaFever was so desperate for human contact that at first he couldn't stop talking long enough to take a drink of water or eat something.

LaFever was flown to Garfield Memorial Hospital where he remained until Thursday night. According to his father, William is now being transported to a hospital in St. George, Utah.

According to Bronson, efforts to find the dog have not been successful.

Katie Bosland contributed to his story.