Ok here's some info on Dollies and Bull Trout.

The only Dolly Varden in Washington exists between the Cascades and the Sound in the Snohomish System and northward. Their range then continues north all the way to the Mackenzie River in the Yukon. The Dolly Varden we have is actually one of two sub species, the Southern Dolly Varden, which has a range from the Snohomish System to the Kenai Peninsula. The Northern variant has a range from the Kenai around the coast of Alaska to the Mackenzie River. So the only true Dollies found in the Lower 48 exist in a small area here in Washington from the Snohomish north.

Bull trout on the other hand are found in a much larger range. Bull Trout (actually char) are found in all of Washington, most of Montana west of the Continental Divide, most of Idaho except a portion in the south eastern part of the state. All rivers in Oregon that drain into the Columbia but they are also found in the northern Klamath and its tributaries also in Oregon. The McCloud River in California oddly used to have Bull Trout but they have been extinct since 1970 because of the Shasta Dam. In Canada the Bull Trout has a range of all of B.C, the southern tip of Alaska and the western half of Alberta. Oddly no native Bull Trout are found on any off shore islands in B.C or Alaska while Dollies are native on some of these islands.

Bull Trout and Dollies do occasionally hybridize but do not make a "hybrid swarm" like rainbows do to cutthroat populations. Instead they maintain their identities, much in the same way as pinks and chums also can accidentally form a hybrid, so if a Bull/Dolly hybrid spawns with a Bull Trout the offspring would be pure Bull Trout. The Skagit river does contain the highest number of hybrid fish.

I hope this shines some insight into the mysterious world of our beautiful native char.