We didn't actually go into the creek bed--just rode along it. Because we have a bunch of new shrubs in there for erosion control we have a fence blocking it from the pasture. We stayed on the pasture side. The Mrs. Thinks the problem might have been the tall grass blowing in the wind spooking him. I think he was afraid of Horse Eating Monsters. Who knows?
We went down there a few days later and he was fine. Go figure.