Favoritos de mertensia

I saw two plants of magenta color at separate locations in a 20 mile float:

see also: https://www.inaturalist.org/observations/94468815

After @mertensia set me on the right track, I found the following from Flora of Missouri, Volume 2 (at the end of the description for L. siphilitica):
"Lobelia siphilitica and L. cardinalis are closely related. Ordinarily, the two species are isolated by differences in floral morphology and pollinator preferences, but occasional natural hybrids are produced. In Missouri, hybrids have been collected most often along the Current, Jacks Fork, and Eleven Point Rivers. The hybrids are intermediate between the parents for most characters, and they usually have distinctive reddish purple or less commonly somewhat blue-, purple-, and white-variegated corollas."

This individual was growing in a patch of blue lobelia, on a dolostone bluff (Eminence formation) just above the river (photo take from the canoe), as was typical of the blue lobelia we saw on the float trip. Most of the cardinal flower I saw was growing on gravel bars, not in immediate proximity to blue lobelia. I'd guess that blue lobelia was the mother plant and pollen came from a cardinal flower via an unknown pollinator.