Most tide books have a base time for a specific spot in the Columbia, usually Astoria. There is another little table that has a correction time amount to get the tide at, say, Kalama. You add this time to the AStoria time, and that gives you the Kalama tide time. I think Kalama is about 2:45 for high tide. So if the Astoria high tide is 6:00am, Kalama high tide will be about 8:45am.
The easier answer is to use a tide website that gives you the exact tide. This one is pretty good.