I am a huge fan of phlox, whether creeping or otherwise. This purple plant has just exploded along the banks and resembled a purple phlox, but only has 4 petals. I found it in a light violet as well as a deeper purple. It appears to be Dame’s Rocket ( Hesperis matronalis ). As you might expect, it has a wealth of other names, including Gilliflower, Mother-of-the-Evening, and Damask Violet.