The unknown wildflower

I have no idea what this is, so hopefully one of you will! I thought at first it was loosestrife, but the leaves didn't look right, nor did the color of the blooms. These flowers are growing all over in the prairie restoration area near the Lake.   At any rate, it's my entry for the Wildflower challenge--thanks for a great idea & for hosting Miranda1008. It's been hotter than blazes here, hitting the high 80's today, and the weather people are saying it could be 100 on Friday! Yikes! That's expected in the South, but not here in the Midwest! We're got that A/C on around the clock!   :))