|ABSTRACTEnvironmental assessments and environmental flows are important components of modern hydropower development. In the developed countries, comprehensive and detailed assessments including research works are being carried out for defining and setting environmental flows. However, current practices in Nepal are variable; from a little focus on minimum flows to the application of simple hydrological methods. This trend is changing in the recent developments where detailed studies and comprehensive procedures are being employed in environmental flow assessments. In this study, current minimum flow practices for a number of hydropower projects in the planning, development and operation phase are considered to observe the trend in setting minimum flows and environmental flow consideration over time. The Upper Trishuli- I Hydroelectric Project (UT-I HEP) has been taken as the case study where field studies and detailed assessments are being carried out. This study presents the analysis of environmental flows in the Upper Trishuli-I Hydroelectric Project that is currently in the planning phase using an eco-hydraulics based approach where modelling is integrated into the environmental assessment to study the effects of proposed minimum flows and possible changes to improve the effect of compensatory releases. The current practices of setting minimum flows in the Nepalese hydropower system was studied using a number of Environmental Impact Assessment (EIA) reports of the developed projects and using existing literature. Based on the reports and information collected during the field study in summer 2013, a trend in setting minimum flows in Nepal was plotted. Out of 130 different projects considered between the years 1995 to 2014, 114 projects have considered or planned minimum flow setting during project development with all projects licensed after 2001 following a minimum flow criterion. Thus, a positive trend in setting minimum flows was observed. It was found that only the hydrological index method has been adopted for environmental flow setting in these projects in contrast to different methods used in other developed countries.The environmental flow regime for the UT-I HEP was determined based on the existing environmental conditions of the project area as mentioned in the EIA report and the river habitat data collected during the complementary baseline survey by Nepal Environmental and Scientific Services (NESS) Pvt. Ltd. Environmental flow requirement for key ecological targets, mainly for the Snow Trout, was determined using the Modified Tenant Method based on the habitat suitability criteria. The evaluation of bypass release scenario proposed in the EIA report revealed the minimum ecological condition for January, February and December. Whereas, the evaluation of the bypass release based on the Hydropower Development Policy, 2001 resulted in the severe ecological condition of the dewatered river section for January and December and minimum condition for February and April indicating that the effect of regulation was pronounced. In order to account for the best spawning period for the Snow Trout, additional flow to the one proposed in the EIA report was made for these months and other months of the year to achieve good ecological condition based on the Tenant rating scale. In order to see the effects of the river regulation in the flow regime alteration, Indicators of Hydrological Alteration (IHA) statistics were plotted for the cases with and without regulation. The new flow regime determined using the Modified Tenant Method was used as an input to the regulated case. The plots for 25th percentile, Median and 75th percentile of flow showed that the newly defined flow regime follows the same trend as the natural flow regime indicating that the ecological functioning is maintained. Using the hydrological and the production data, a model of the UT-I HEP was developed in nMAG. The bypass release based on the new flow regime defined for UT-I HEP, the one proposed in the EIA report and the bypass release based on the existing practices were evaluated using the nMAG simulation. The results of the simulation showed a loss in the production value of about 7.3 million US $ for the proposed release compared to no bypass release case. But, the loss compared to the bypass release based on the HDP, 2001 is about 4.5 million US $. The new flow regime was then compared with the existing flow regime to observe their resemblance to the natural variability of the flow in the river. It was observed from the comparison that the new flow regime has addressed ecological objectives compared to the existing practices.Based on the results of the study, it can be concluded that there has been positive trend in the minimum flow setting and environmental flow consideration in the Nepalese hydropower system which is going to be augmented by more detailed assessments in the future. However, some approximation in the flow regime determination exists due to the limited river habitat and eco-hydraulics data. Improvements in the flow regime determination can be made with more detailed assessments and field studies supplemented by input from multidimensional expertise.