Godfather,
It is legal to run a motor in the Yakima anywhere BELOW the Roza boat-launch approx. one mile above the dam. It is above that point where most of the trout fishing is done, with above the dam selective fisheries rules (single barbless / no bait). Some water for about a mile below the dam can be good for trout at certain times/ water conditions. Access can be difficult unless you have a very shallow running sled. A reasonable launch is at Harlan's landing right alongside the entarnce to Selah on the Yakima side (I've not personally used this launch - yet). Hope this helps, good luck.