In this paper, the relationship between hydroelectric projects and the river environment is analyzed. Recently, the large-scale regulation of runoff by large hydroelectric projects in the Ningxia Reach of the Yellow River has altered natural runoff processes, causing an increase in the probability of low discharge and an overall adjustment of riverbed evolution and river characteristics. During low-flow years, the combined effects of these two changes can weaken the self-purification capacity and reduce the water environmental capacity of the river. This is one of the main reasons for the recent decrease of water quality in the Ningxia Reach. This research shows that it is necessary to implement river training projects to maintain stable flow paths, not only for adjusting river regimes and for flood control, but also for increasing the self-purification capacity and the water environmental capacity of the river. Methods and proposals for coordinating the operation of hydroelectric projects with the protection of the river environment are presented in the interest of promoting sustainable development.