I would suggest you contact a local university or your local extension service for help with this.* The seed could be from just about anywhere up-river, but a local source would probably give you the best chance for an id.
Oh, and you shouldn't have to keep checking back if you checked the box for e-mail notification when you registered.* If you aren't certain or don't receive a notice that I've replied, you can click on 'My Account at the top of this page, then click on 'Preferences'.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.