Ecological flow determination is very important for all hydropower projects to be more sustainable and less impacting for the environment.Ecological flow needs to be determined both for reservoir impoundment period as well as for the normal operation of the plant, especially in cases where the water is diverted from the main river course, to be released in a different river or a significant length downstream the dam and intake structures.This paper will show the studies that resulted in the determination of the ecological flow of the belo monte HPP, located in Xingu River in Brazil (11.3 GW of installed capacity) and the S(a)o luiz do tapajós HPP, located in tapajós River in Brazil(8 GW of installed capacity).This large scale projects located in the amazon region are in high inflow rivers with large seasonality, since flows vary a lot from dry season to wet season, both projects the powerhouse is located some kilometers downstream of the dam, and to avoid large environmental and social impact of keeping a stretch of the river, downstream of the dam with insufficient flow, an adequate assessment and determination of the ecological flow was carried out.The paper will present this assessment, including the environmental and social constrains, limitations and demands that were raised for the reduced flow stretch of river, the hydrological studies performed to consider different solutions for minimizing this impact and the final solution achieved.the paper will also present a discussion and considerations for a broader application of such methodology in other situations/projects.