I agree with Chelle about the weedblock (which I believe is misnamed as it doesn't seem to do that!) I can't think of anything that isn't going to require more labor, either. Dig deeper, lay in organic material as per Dave's comment, use more rock, bring in larger rock....
I also agree with whomever commented that it isn't really a rain garden situation. More of a flash flood creek situation. Water irises were one thing I thought of but I don't know how they would do under the pressure of flooding. The ones I've seen grow in marshy pond edges. But I bet there are reeds and maybe sedges that can root deep (taking advantage of any water stored deeper in the soil) and thus grip and avoid being washed away during flood events.
Thanks for the pictures!
