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I've been up Knight Inlet a few times but have not explored the rivers. I have explored the rivers at the head of some of the other inlets up here over the years. By the way, exploring the Klinaklini River has been on my radar for quite some time and I may do it this summer as well. Knight Inlet is about a 50 nm trip so you need to leave early, it's better to have daylight to anchor back there.
Not sure how much you know about the inlets up here but here goes. Generally you can get some pretty nasty winds funnelling through the inlets but managable if you have lots of chain and don't mind heavy air sailing. They usually pick up in the afternoon. From my experience, sounders don't work very well way back in the inlets because of the runoff, so I usually use a lead line near the head. I try to arrive near low tide so I can spot most of the stumps/trees/old pilings etc. If I get there on a high tide I slow way down and usually shoot for a bit deeper water to set the hook. Looking at the chart it looks that you would be pretty exposed to the outflow winds pretty much anywhere but the holding should be pretty good.
Exploring the rivers... In July, the rivers are usually still running pretty strong. you won't be able to see the bottom even at 6 inches due to the milky colour of the water. Kayaking would be quite a workout and if your using a skiff/motor, generally swing wide at the turns but you can get a good indication of the depth by the look of the water surface. You could expect to hit bottom a few times, I've always drifted back down (that's the best part). I have been up rivers with no indication of any winds only to look up at the tree tops, to see them laid right out. This makes it difficult on your way back at the mouth of the river and it will generally be quite rough getting back to your sailboat and into your sailboat so be prepared for that. You will see grizzlies up there for sure. You won't see anyone back there so your basically on your own. As I say, I haven't been up that river but it should be spectacular and definately a must do. Kingcome Inlet would be another good one to try. Your up for a great adventure for sure and have fun.
Hope this helps,