This is a great idea. I hope it works well.
My suggestions for placement:
River banks, any ledges on the outside of a river bend, a place that is most likely to erode naturally. Where they would fall away from the edge, top down, roots up.
Can you make "Log Jams", this would be places in rivers where obstructions create natural barriers for fallen and floating logs, rapids and the tops of waterfalls. Obviously more of this in heavily wooded areas, less in the open tundra area. And some along the edges of mashes and swamps, they would tumble over time.
Skyrim is great for wandering. I've always loved the streams, rivers and forests, but it's always bothered me how un-naturally clean the riverbeds are. These fallen trees and debris that should be there are what make real life natural locals so great.
That's it. Thanks. If it's too much to ask, don't worry it's still a good mod idea.

I've currently got this mod "faved", awaiting offcial completeion then I'm certain I'll add it right away!