As we move into the twenty-first century, global economic prosperity is driving the consumption of energy to grow faster and faster. Hydropower, which is presently the only large scale renewable alternative to fossil fuel generation, is increasing its share in the world Total Primary Energy Supply. Hydropower projects are popular and increasing in number and scale, mainly because they are regarded as renewable, clean, with less impact than other options and generate less greenhouse gas. But this point of view is under challenge. Hydropower projects can lead to changes in the riparian ecosystems and may affect the biodiversity, making the project not sustainable. The impact of hydropower on the riparian ecosystems has been studied intensively by many scientific communities but the mechanisms are not fully understood especially in the special field of groundwater. This study was made in order to give future researches on this subject a systematic and integrated view of the impact of hydro projects. The study is a comprehensive literature review of physical, physical-chemical changes of the river water, and the groundwater as well as of accompanying changes in the ecosystems. Hydropower projects can change the flow pattern and flow regime of the river water, the hyporheic exchange, the thermal properties of the river water, and the transport of nutrients and metals. The properties of the river-near groundwater can also be changed. The comprehensive result of these changes is the change in the riparian ecosystems, such as disturbing the fish migration paths, reducing the activity of the floodplain and riverbed, destruction of habitats, changes in the riparian communities and so on. Today most studies regarding to this topic are focusing on the changes in the ecosystems and not the physical and chemical changes in the river water which are very important and are the causes of the changes in the ecosystems. It is worth noticing that impact on the groundwater has been underestimated. Now the groundwater dependent ecosystems are attracting more attention and the impact of river regulations on the groundwater calls for more research.