How can a change that occurs downstream in a river ecosystem (that is, a dam) affect the rest of that ecosystem?

Well, dam is used as the main source of water that irrigate the whole ecosystem.
dam will hold the water to be used by the bottom half of the ecosystem, but this could result in negative effect such as it will flood will unclean water since the water is not flowing as normal.

Cr(oh)3 -> cr(3+) + 3oh-