The Ada County Sheriff's Office says an adult male in his 40s was swimming with a group when they got separated and he hasn't been seen since.
Officials at the scene tell us the man swam from the Ada County to the Owyhee County side of the Snake River and then back again.
Witnesses told them that the man appeared to struggle and lose his footing on the rocks when he reached knee-deep water, and that was the last time he was seen. Deputies say alcohol could be a factor.
The sheriff's office says that no one saw him go underwater, so they are searching for the river and the riverbank to see if he was able to get out at some point.
Swimmers often take a dip in the reservoir upstream from the dam in the summer, but the river is more unruly downstream where the swimmer disappeared. There are strong currents, slippery rocks, and in this very steep canyon, very few places where it is easy to climb onto the bank.
Crews from Ada, Canyon and Owyhee counties, as well as the Boise Fire Dive Team and the Kuna Fire Department are assisting in the search.
Ada County Dispatch says the call came in at 8:22 p.m. Thursday evening.