Our river bank willows grew from the low-water edge all the way up to our back "lawn"which is about 60' higher - until the drought of 2016. All the higher trees died, but the ones within about 30 feet of the river did fine even though they were on a fairly steep slope. When the river floods, most of them are under water, but they hang onto their piece of terrain and thrive. I hope that helps you with your plan.
