I found this in a river in southeast Michigan. I can't make out any obvious signs on it being worked, but it fits nicely in my hand so I thought I would get a second opinion.
Hello from Michigan! (Adrian area here). When I was a kid I was walking a local baseball field and found a worked and large arrowhead....I also have friends that find them all the time....so good question as there was a lot of Indian activity in these areas. I'm curious as well.
Welcom to Treasurenet, you have certainly come to the right place. Unfortunately, I think its just natural. But if you keep looking, you're bound to come across something.