Thompson.Man, Chris would be able to answer your jack spring questions, as he does the occasional trips up there in late summer. If you do a search on the forum by using the search engine, you will find some of the past fishing reports on Thompson chinook jacks.
The rainbow trout fishing is good from July until September, depending on the water condition. You can flyfish for them, either by nymphing or dry flyfishing. Before you head up, stop by Reaction Fly and Tackle in Abbotsford, talk to Daryl and pike up some of the stonefly nymphs, dry flies that are available at the store.
The fish are between 14 and 20 inches in length. A 4 - 6wt is ideal, depending on the type of fishing you are doing. They are pretty fish, take a look at these photos, which are mostly rainbow trout from last year around the Thompson.
http://www.fishingwithrod.com/fishing_photos/index.htmlYou can also spincast for the rainbow trout if you want, which is even easier. Here is a video:
http://www.fishingwithrod.com/video/0704.htmlPlease do check the regulations before you go up. The salmon regulations may change during the season once DFO has a better estimate on the escapement, so please be aware of that. Wear a pair of wading boots even if you are wet wading. Watch out for snakes on the rocks.
Good luck.