The largest hydro project in Iceland, diverted one glacial river into another. Both rivers were previously turbid and carried substantial amount of silt and other glacial sediments. The receiving river is now even more turbid than before, resulting in less primary production while the other has lost its turbidity and primary production is increasing. No environmental flow was designed for the old riverbed for the project, but groundwater, due to the permeability of the bedrock, as well as small tributaries, fill it relatively quickly. CPUE data shows how fish stocks, mainly Arctic charr, have decreased in the more turbid river, while in the other river, juvenile production and catches are increasing. Data showing how the ecosystem has changed will be presented and the possible effects, if environmental flow would have been incorporated into design, are discussed.