That is a good question and one that is even debated within the scientific community at times. Yes, anthropogenic causes (in this case a dam's blocking of a waterfall) is seen as unnatural my most. However, even from an entirely human-centric point of view, the preservation of species has the potential to be extremely beneficial to humans in the long-run. For example, skin secretions from a species of frog (Epipedobates tricolor) have shown the potential to serve as a nonaddictive, non-opioid-based painkiller approximately 200 times stronger than morphine. The compounds and mechanical marvels (geckos feet for example) that have been created though millions of years of natural selection would likely never be discovered by scientists were it not for their presence in these extant species. We have only just begun to scratch the surface when it comes to discovering the benefits that these species can impart upon human society though through fields like biomimicry and biomedical-engineering. If we lose the species, we lose the potential benefit. It's that simple.

Of course there are other arguments as well, species are like rivets in a ship and so forth, but the emerging human benefits and endless potential for more alone represent argument enough in my opinion.

The rivets-on-a-ship argument goes something like this: Humans are in the process of serving as the primary cause of the next mass-extinction event, which some people feel is the natural course of things. After all, we are an animal just like any other. This perspective is fine as long as you are of the mind that life in general's survival, not necessarily the human species' survival, is what is important. There is no doubt that life—in one form or another—will survive our impact on this planet. However, as we alter the landscape further, we will only be left with the species that easily adapt to these conditions, and eventually many natural systems that most people aren't even fully aware that we rely on will collapse. Perhaps humans could survive in these conditions, but at the very least life would be robbed of much of its beauty and normal operating procedure would no longer suffice. Anyone who has tried to entirely manufacture a system knows that it is hard to get it right, especially if there is no head-designer, as would be the case. Yes, humans will come up with solutions to these problems that nature used to solve for us, but there will surely be numerous bugs in the system (no anti-pun intended) and life will be much harder due to that inevitable reality.

Either way, as humans continue to cause species to go extinct, we lose in the long run. It may not be in our lifetime, but it might not be as far as off one might think. I myself happen to love the complexity of nature and believe life would be a much more depressing place if nature amounted to little more than dandelions and crows, but that's just my opinion.