In Brazil, given its privileged hydrology, the unexplored economic use of water resources has many dimensions, such as hydroelectric power. This energy will face increasingly rigorous social and environmental impact assessments (40% of potential is located in the Amazon region). Hydropower inventory studies conducted over decades, with solutions such as ecological river flows, that flood smaller areas and reduce natural river flows modifications, are being reviewed. The river extension from dam to the point where the waters are returned after the powerhouse is known as the Reduced Flow Stretch (RFS). Even mega-projects, such as the 11.3 GW Belo Monte dam, are designed with deviating flows reaching an astounding 13,000 m 3/s (excavated material higher than Panama Canal). RFS requires to be carefully studied to achieve appropriate ecological flows, since RFS flows increased reduces the plant's electricity production to the same installed capacity. Balancing RFS requirements and hydroelectric power remains a challenge and, clearly, there is no consensus. Here, we performed an analysis of the main environmental impacts caused by RFS requirements, considering the multiple water use specific for each dam site. The natural variability of river flows provides diversity of habitats and maintains the richness and complexity of biological communities. Therefore, the present study has great ecological, social and economic relevance, since proper evaluation of the RFS requirements avoids potential destabilization of biological communities and even loss of biodiversity. This type of arrangement was more common in dams located in headwaters of rivers, as in the slopes of the Andes mountain range, and in regions like the Alps. There are many hydroelectric plants in South America and Europe that have this type of arrangement of engineering works. But the times are different and the environmental impacts have to be better evaluated. A final aspect also involves the maintenance of ecological flows downstream of dams. Regularization reservoirs need to keep downstream, even if they do not have a TVR, adequate flows that represent minimally the seasonality of the river, with floods and droughts, that propitiate the maintenance of the ecosystems downstream. There are cases such as the Sobradinho Plant in the São Francisco River that has been much questioned in this regard, especially when the climate is changing in the basin, with long periods of drought, and with increasing water use. So this is a very important and increasingly current issue.