@The Hood
Holy moly. I think you need an environmental consultant to sort this one out. They should be able to find a quick solution for you.
If the riparian zone is that big the creek must be fairly large?
Yep any soil brought in would need to be local unpolluted, and a bush regen team could do stabilisation works (geomatting, terracing, gabion baskets, planting native tubestock, fairly cheaply).