aka nodding beggarticks, sticktight; Bidens cernua; Asteraceae (aster family)
I found this annual plant along the sandy, exposed bed of the river when it was running very low, in mid-September. Although bur marigold likes really wet soils, it is not considered an emergent aquatic.
It can be found through most of the US and Canada, except for a few states in the south. Some authorities consider it weedy.
The more I see of composites, there more fascinated I am by them. Here’s a close up of the disk flowers.
By the way, it’s possible I’ve mis-identified this one. There’s another, almost identical species called smooth bur marigold (Bidens laevis); I spent a long time poring over details on the fantastic New England Wild Flower Society and John Hilty’s Illinois Wildflowers websites. These have become my go-tos whenever the books fail me.