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

The dams are very useful for the people and their needs, but they also very often cause problems in the ecosystems in the rivers on which they are built on. For starters, the dams limit the movement of the species, as well as the movement of the nutrients, and considering the fact that this effects the lifestyle of certain species, as well as changing the nutritional quality of the water, it heavily affects the ecosystem. The water upward from the dam is accumulated, and because its movement is largely stopped, the water is more stagnant. The more stagnant water is getting warmer much more easily, so the temperature change of it results in change in the habitat of the species, often resulting in destabilizing the ecosystem, with some species dying out, while others exploding in numbers.

the answer is cell division : )