Sounds like you do not have an area where water puddles and sets awhile before it drains out (that would be a rain garden). What you have is a creek. It flows fast and hard very suddenly like for a short time and then is gone, correct? You say it has eroded areas of the yard. This means that the movement is powerful enough to move soil. By placing rock in and actually creating a creek, instead of a muddy bed, you can help to prevent some of the washing out. I would suggest plants that root deeply and have strong root systems. They will need this to prevent further washing out of the soil and to keep themselves from washing out. Iris I think would root too shallowly for this. There are some iris that prefer water and those may do ok, but not the bearded kind.
