Also I think you are right - common law seemed to have done the job here - initially at least. But the problem for me is that without an EPA like institute (maybe a state level one would suffice), the crime needs to be committed before the retroactive action is taken and it is fixed. Anything with that sort of feedback loop isn't very efficient [often times this is the worst cost effective approach if a long term analysis is done]. It would be better to monitor on an on-going basis these types of activities before the river was destroyed. The EPA like organization can note the environmental issue resolved by common law in this case, and make it part of its knowledge base for monitoring in the future to avoid having such a long feedback loop.
On the whole I agree though - it is very hard to compare standards now vs. a previous generation. What is acceptable changes over time. The only way a previous generation of bad decisions should be punished by law is if there is evidence supporting that the decision makers had information at the time indicating they were knowingly making a bad decision that would harm people and break the law (either a specific law or common law).