I can recommend some people who do research on the intersection between fire and water; they would know the answer.
In my limited opinion, I think it has to do with one's philosophy on "natural." Dumping strained ashes into a current likely has little biological affect on remote rivers with little traffic. More popular rivers with more people doing it... it probably depends. Moreover, it all washes out and starts over eventually.
However, people need to absolutely have the mindset that rivers (and lands) are public and for everyone; that is to say that while we all have the right to use and enjoy our rivers and mountains, we should not have the right to ruin it for the next person. That is why the leave no trace ethic is vital in areas where that is the expectation (MFSR, for instance). Other more popular daily runs, maybe the expectation is not as stringent.
It bothers me that there are people out there who do take the lazy, cheap, or easy way out. I guess they figure they're more important than the rest of us; and granted, isolated events really don't matter. But if you do it, you can and should expect everyone else to do it, and then what do you have?
Originally Posted by SummitSurfer
On a more serious note, I wonder if there are any scientist or biologist out there that can provide some facts on river health, its life cycle and our over all impact. Because I have to admit there is some pretty compelling evidence on both sides of this debate.
I myself believe my enviormental impact is big enough, therefore I tend to subscribe to the LNT phylosophy.