First, let's go over a few shrubs that are ideal for creek banks... You might try Elderberry (Sambucus canadensis) or Ural Falsespirea (Sorbaria sorbifolia). Both of these are suckering shrubs that will rapidly anchor the bank if you're concerned about erosion. Other shrubs that might not spread as aggressively include Summersweet Clethra (C. alnifolia), Spicebush (Lindera benzion), and Chokeberry (Aronia arbutifolia). You've got a lot of opportunities with perennials, as well, including Meadowsweet (Filipendula purpurea), Astilbe, Ligularia, Lobelia, Euphorbia, Primula, and the beardless Irises, which include Japanese, Siberian, and Flag Iris. Any perennials listed for rain gardens will thrive on creek banks, too.