Longday,
There is a report someone posted on the home page of Washington Lakes from 6/10/2005. Sounds like they had some pretty good fishing.
You will need a tribal permit to fish on the reservation. $7.50 per day. I also believe you can buy a three permit. When you get your license, ask for the regulation handbook put out by the Colville tribe. It will give you limit, regulation info and what type of fish are available in the different lakes.
Most of the small store in Grand Coulee and Coulee Dam sell them. I'm sure the resorts at the lake also do.
There is also McGinnis & Buffalo lakes you can fish on the reservation. Don't forget Roosevelt also.
Have fun. If this is your first trip there, you'll really enjoy the scenery. One of the nicest places in this state. It reminds me of the Rocky Mountains. Keep an eye out for black bears, I've seen alot up there in that neck of the woods. Always fun to see them out in the wild.
Brian