Riding a river and debris

Ok so I know there are a lot of people on here that go to rivers for the awesome water conditions, myself being one of them so this is my question:

How big does debris (stick and limbs) need to be to do damage to your boat?

I try to avoid anything I see while riding and as a result I always ride with seat bolster up however there is a SAN that rides my river and I have seen them just plow right through some stuff that I would never take my boat through. This is especially an issue after a big storm up north, it seems all the loose stick find their way into the river.