This paper assess the controversies and tensions being created by the target of ambitious hydropower development plans in the name of climate mitigating energy in the Upper Tista River Basin of Sikkim (India) which is a part of Eastern Himalayan region. However, in the face of climate change, hydropower projects have become a contested developmental paradigm in the Easter Himalayan region of Sikkim. The paper highlights potential issues and conflicts between climate mitigation approaches through hydropower projects and anti hydropower forces of local people. The paper critically examines pre and post hydropower development conflicts and cooperation along Upper Tista river basin. In fact, the region has witnessed that the Hydro Power Project have been undermining environmental and social security over the past years. The study also analyse the different roles of dam affected people, local politicians, NGOs and dam developers in advocating and protesting of the hydropower dam projects in the region. Using an environmental security framework the study critically analyse the environmental risks and impacts being aggravated by combined forces of climate change and hydropower development in upper Tista catchment of Sikkim Himalaya (India).