The culturally and ecologically diverse region of the Eastern Himalayas is the target of ambitious hydropower development plans. Policy discourses at national and international levels position this development as synergistically positive: it combines the production of clean energy to fuel economic growth at regional and national levels with initiatives to lift poor mountain communities out of poverty. Different from hydropower development in the 20th century in which development agencies and banks were important players, contemporary initiatives importantly rely on the involvement of private actors, with a prominent role of the private finance sector. This implies that hydropower development is not only financially viable but also understood as highly profitable. This paper examines the new development of hydropower in the Eastern Himalayas of Nepal and India. It questions its framing as green energy, interrogates its links with climate change, and examines its potential for investment and capital accumulation. To do this, we also review the evidence on the extent to which its construction and operation may modify existing hydrogeological processes and ecosystems, as well as its impacts on the livelihoods of diverse groups of people that depend on these. The paper concludes that hydropower development in the region is characterized by inherent contentions and uncertainties, refuting the idea that dams constitute development projects whose impacts can be simply predicted, controlled and mitigated. Indeed, in a highly complex geological, ecological, cultural and political context that is widely regarded to be especially vulnerable to the effects of climate change, hydropower as a development strategy makes for a toxic cocktail.