I don't know exactly where you are, but to start: ferns typically do well beside creeks. I'm sure someone will have better suggestions, but it's a start :)

Oops! Forgot that I don't live on Vancouver Island anymore, goodbye zone 7ish and hello zone 5ish. I am in Eastern Ontario, home of the ostrich fern and trilliums.
There is a huge jack in the pulpit across the creek that I stumbled upon, 3 feet high and full of jacks. Could I capture the seeds and try them? I am hoping for a cardinal lobelia and a yellow bog iris.

Gomanson, ferns are a great idea, there are a few there but I am not sure of the type. Wish there was a photo upload capability here.

Cheers, Joyce

One thing I do recommend is that you research before you plant anything. Some water/creekside plants are invasive and will crowd out native plants. When this happens the wild life that is dependant on that particular food looses its source of food.
I live by a creek and wetlands so I try to plant natives along with plants I know wont be invasive. I'm also zone 5.
Ligulara loves wet areas and likes morning sun.My Astilbe is doing great on the creek bed. So are hostas. We have native skunk cabbage which to me is just as attractive as Hostas, so I leave them. Lilies will do good if they get sun. Blue flag Irish does good on stream banks if they are getting sun, The Yellow flag iris is more invasive.
I love living by the creek, ours is small and sometimes dries up. We also have a couple ponds. ITs fun to watch the wildlife and waterlife.

that was iris not irish. LOL

Your research should include the high water flood level. I have seen some very extensive gardens washed out when the person did not take into account water level and force of flow.