Identification by: Don Knoke who comments (via email), "Delphiniums are tough to key out. I led a field trip up Stafford Creek in May this year, and I see have Delphinium nuttalianum on the plant list we made. In Hitchcock, Delphinium lineapetalum is considered a variety of D. nuttalianum, so we were probably looking at the same species. D. lineapetalum is essentially a pale version of D. nuttallianum, with the petals nearly white and veined with purplish-black. I saw this pale version near Leavenworth while with a couple of Forest Service botanists, and it seemed much lighter than the one in your picture. I believe it is more common on the Chelan County side. With Delphiniums, I'm happy to stay with species and not try to identify the varieties."