Small post and beam project sanity check
Can I use your canoe rack to drive my quad up and work on it? 
That is a bit over built but will be rock solid. Make your cuts tight when using pegs everything is a friction fit but you have it well laid out. 
It appears you will be just setting it on the ground so you can move it that would be my justification for the lower horizontal braces. This should hold up fine for whatever size canoe you can hoist up there.
My grandfather always used hard wood pegs, we used fur for the timbers but I only did a few small additions with him to his barn and the hard wood was important but a pain in the back side to cut off on his tree limb ladders.