this would be more of a brief passing. there is no way enough food grows around a river for a large herd of sauropods long term. They likely migrated across the continent stopping only to breed and drink for a short time. if they did not migrate then they would likely either run out of food or do severe damage to the ecosystem given how much they must eat. I agree the tail makes an effective weapon and a predator would not want to approach from behind but I fail to see how an animal so heavy it cannot lift more then a single foot at a time aside from a stable bipedal stance supported by their tail could possibly maneuver fast enough to counter a fast, lightly built predator, or far more likely considering they are carnosaurs, a pack. All they'd have to do is get a good strike on the neck to sever the long windpipe or blood vessels supplying the brain. this is likely why they lived in herds, to cover the fronts of other individuals. Plus a sauropod would provide much more food for a predator then another predator of similar size. Similar risk for a much greater reward in terms of hunting predators.