But along the way to the unfortunate end you describe above, many good things came of the dam. Any calculus needs to evaluate the tradeoffs. "Were they worth it?" doesn't lend itself to objective-factual analysis, because "worth" is all about values.
Those tradeoffs are "hard to decide via objective truths" only because they haven't been decomposed enough. You have to dig deeper into them, split them apart, ask why do we care about something, and then ask again and again. It might get hard to keep track of the expanding tradeoff space, which is probably why people shy away from digging into things too deep. But if you do the work, you may find out that there is a way to solve the original problem in a way that satisfies everyone's low-level values.
Reminds me of essays of Bret Victor, who occasionally seems to be making a related point - a lot of policy debates quickly devolve into appeals to emotions, because we lack tools to effortlessly decompose and analyze actual problems. See e.g. http://worrydream.com/ClimateChange/#media for a good example.
Consider the towns and cities by bodies of water that have either dried up or been polluted. It's financially ruinous because it either/or kills tourism and/or fishing industry; the salty dust left behind by water causes horrible health problems; there's natural brain/brawn drain that leaves only the older and sickly behind to either fend for themselves or be on gov pay check. Just off the top of my head. I can easily think up a lot more effed up scenarios that have already happened in the world (see Aral Sea).