A helicopter crew will take to the sky today to look for Mike Marshall, 61, of Seaside.
Marshall was last seen on Saturday before becoming separated from his hiking partner on a difficult trail, Monterey County sheriff's officials said.
"That area is not for average hikers, the terrain is really rugged," said Monterey County sheriff's Sgt. Mike Burns.
Search and rescue teams have found some of Marshall's belongings, but he remains missing, deputies said.
Marshall reportedly became separated from his hiking partner on Saturday while the two were trekking down the Little Sur River to Old Coast Road, according to the sheriff's office.
The hiking partner reportedly spent four days searching for his friend, sustaining injuries from falling and becoming more and more exhausted as he traversed the difficult terrain, deputies said.
The partner finally reported Marshall missing on Tuesday when he met with family members who had come to pick him up on Old Coast Road, deputies said.
That day, the Monterey County Search and Rescue Team began a search of the area, including Little Sur River, according to the sheriff's office.
On Wednesday, search and rescue teams continued the search down the river, often on riverbanks without trails. They combed over the hiker's intended trail, including areas only accessible by water, where they found some of Marshall's belongings, deputies said.
On Thursday, search and rescue teams from Marin and Contra Costa counties also joined in the search. The search teams combed the river until the area became too dangerous to pass.
"The river is high there, which is dangerous," Burns said.