I have never fished for walleye in the big river. From what I have read, you may want to start at Bonneville Dam and work your way up stream. The farther up stream you go, into eastern WA, the better. I have had guys tell my about monster walleye at Vantage, where I-90 crosses over the river.

Fishing & Hunting News is alway doing write ups about fishing for walleye in the big river. F&H News will often times have maps too. Pick up a copy at your local news stand.

Hope this helps!
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Carl C.