Chinook in Arrowrock

Hi F&G
I have been fishing arrowrock a bit this winter and happened upon a spot where I seem to have pulled out multiple chinook.
They are not kokanee, or trout, they are a salmon. People have told me that coho have been planted in anderson, as well as chinook. People seem to think they are coho because their jaw is not black.
The 3 I caught were roughly 16-18" spots on the back, silver body, spots on the tail, greenish hue to their back.
I have caught 3 in 2 days, are they washed out from anderson? Are they planted in Arrowrock? Are they spawning? I am a bit lost here any help would be appreciated.
Here is a picture of the fish
http://puu.sh/mZTrk/8cfe433aac.png Bottom is a trout, top fish is the one I am asking about.
How did this many salmon wash out into arrowrock?

Answer: 

There have been a few fall Chinook that have passed out of Anderson Ranch Reservoir and  caught downstream in Arrowrock Reservoir.  However, the fish in your picture are not fall Chinook or coho.  The fish in the bottom of the picture is clearly a rainbow trout while the upper fish appears to be a kokanee. 
IDFG used to stock coho in many reservoirs across the southern part of Idaho but after several years of evaluation of return-to-creel results, we found  few were being caught by anglers.  Coho were replaced in our hatchery systems with other strains of rainbow trout and more kokanee production - fish that are caught by anglers.  Our goal is to achieve a 40%+ harvest number for fish stocked at 9 - 12" in length.
Fall Chinook have been stocked numerous times over the years as a fish management tool to control excess numbers of kokanee.  They are an aggressive predator on kokanee and, as a benefit, provide an outstanding fishery for anglers interested in catching larger fish.  Deadwood Reservoir is another location where both kokanee and fall Chinook can be caught.

Answered on: 
Monday, February 8, 2016 - 9:11 AM MST