Here's a closer shot.

Lythrum salicaria, Purple Loosestrife?

L. Salicaria is under quarantine in South Dakota as an invasive, noxious, invasive plant - which is why you see it in ditches, choking ponds, streams, and lakes and not for sale (illegal) in garden centers.

It is beautiful in a clump like yours, but if you keep it, make sure you deadhead it and dispose of the deadIheadings in a bag buried in the garbage (although burning is the preferred method). One plant can drop 300,000 to a million seeds, literally. Also, the mat forming roots can produce 30-50 plant stems annually.

Some years ago, our DNR issued a voluntary removal of this from home gardens. Now it's not voluntary.

Thank you! What do you mean it's under quarantine? Just that it can't be sold? Is the problem just that it seeds too easily?

Not sure of the exact application of "quarantine", but there are Federal and state measures to keep the seed out of native/wildflower mixtures, etc.

It is illegal to import, sell, offer for sale, or distribute the seeds or the plants of purple loosestrife in any form in, I think, 32 states - possibly more now.

If you've never seen acres of loosestrife infestation - look at the first couple of pictures in the attached Google images link.

Here is a link that might be useful: See what purple loosestrife can do