The 2015 World Hydropower Congress has just concluded in Beijing.Hydropower has so far proven to be the most reliable and cost-effective form of energy among all energy resources.With the global capacity for hydroelectricity passing the milestone of 1,000 GW, this congress examines the sustainable development in the next 35 years.Around 1,000 leaders and specialists from the hydropower sector discuss the challenges of climate change, energy and water security.It's the first time that the meeting is held in China, where half of the world's dams are built and where the Three Gorges Dam represents the largest hydro electric infrastructure in the world.However, problems like resettlement of residents and environmental issues do exist despite of the benefits brought by the large dams like the Three Gorges.So how do governments maintain the sustainability of hydropower?What is the optimal energy mix?