Thanks, Scott, for helping me identify this beautiful but very invasive wildflower.
Lythrum salicaria Common Names: Purple loosestrife, syn. purple lythrum, bouquet-violet
Side note: Lythrum salicaria is native to Eurasia and is very invasive, particularly in northern wetlands. It will crowd out other native aquatics that are valuable to waterfowl and other wildlife. (If I'm right) Too bad because it is beautiful in this shot.
If you get a more straight ton shot with the stem and leaves, I can be sure but I'm thinking: Scientific Name: Lythrum salicaria Common Names: Purple loosestrife, syn. purple lythrum, bouquet-violet