changing the flow characteristics of a river is a massively disruptive event, not just for the species in the water but also the animals that rely on tertiary characteristics and effects of that river. It changes the erosion patterns and sediment carrying capacity of the stream, it changes the seasonal flows that ecosystems have come to rely on, it changes nutrient transport, changes water temperature, changes oxygen content, and a million other things that make the stream less compatible with certain species and more compatible for others. Best case this results in loss of biodiversity, worst case it results in spread of invasive species that have even further impacts on an even wider swatch of the ecosystem. It doesn't make the ecosystem go away, it just changes it to a less settled one - generally when that happens the complexity of the ecosystem goes down until natural selection can catch back up and everything adapts to fill the same niches again, which is not a fast process.

It's hugely disruptive to ecosystems.