Thank you for all the comments and sage advise.
A couple answers to a couple comments...
-The only easement type requirement that I'm aware of is that my culvert not back up water to my neighbors property, which is not really possible since it is about a half mile up steam.
And fwiw, my culvert is about 1/4 mile from any county road. Not sure many people or my neighbors even know/care it exists.
-I hear you about the pro/con about the lawyer talk. That is a big reason I posted the question before talking to my neighbor. Nice to know ramifications (socially and legally) before broaching the subject.
-As for "less than 54 inch lengths"... good thought, and I'll do that, but in the next deluge there will be more stuff than what I cut and clear headed downstream so I'd doubt that it would clear their culvert. The stream is a foot deep and gentle normally so stuff will lay in my culvert till next storm. It drains 3-4 square miles of farm land that is tiled. So when it gets up it gets up it gets up fast and angry enough for my eight foot culvert not to be able to handle. Ha. And I thought I overbuilt for a 50-100 year storm. Not. It overtopped twice in past 2 years.
-The material came from upstream. The stream runs through standing timber. We had quite a rain, so a couple years worth of fallen/dead trees floated downstream.
-I could try hauling the trees up and out of the culvert ditch with my... oh yeah I forgot to have sons a couple decades ago.
-Yes culvert is across my driveway. Not a road commission thing... and the less officials know of it's existence the better.
I took a couple pictures today... I'll try to post.