Dam-induced hydrological alterations and related ecological problems have created considerable concern for hydrologists, ecologists, and policy-makers. The East River basin in China is the primary provider of water resources for mega-cities within the Pearl River Delta and meets 80% of the annual water demand of Hong Kong. Daily streamflow data from 4 hydrological stations covering the period of 1954-2009 are analyzed with respect to eco-flow regimes of the East River basin using eco-flow metrics (ecosurplus and ecodeficit) and the Indicators of Hydrologic Alteration (IHA) technique. In this study, ecodeficit and ecosurplus were analyzed to determine the ecological impact of water impoundments. In addition, D_o and DHRAM (Dundee Hydrological Regime Assessment Method) were employed to evaluate the degree of alteration of hydrological regimes, and ERHIs (Ecologically Relevant Hydrologic Indicators) were used to analyze the influence of hydrological alterations on ecological diversity. Results indicate that the flow duration curve (FDC) is significantly affected by water reservoirs, which is largely mirrored by the running down of the upper curve tail and running up of the lower curve tail, especially in autumn and winter. In this way, the magnitude and frequency of high flows decrease and those of low flows increase because of reservoir regulation. Precipitation exerts greater influence on annual and summer ecosurplus, whereas seasonal ecosurplus and ecodeficit are more influenced by the hydrological regulations of the reservoirs. The ecodeficit in the autumn and winter is nearly zero, whereas the ecosurplus in these same seasons increases significantly. In addition, ecodeficit and ecosurplus in other seasons, particularly autumn and winter, are more influenced by reservoir regulation. Impacts of reservoirs on hydrological regimes and eco-flow regimes differ from one station to another owing to varying degrees of influence from the reservoirs on hydrological processes. The longer the distance between a reservoir and hydrological station, the weaker the influence the water reservoir has on hydrological processes. Because of changes in the streamflow mechanism, the fluvial ecosystems are under a high risk of degradation at the Longchuan and Heyuan stations and a moderate risk of degradation at the Lingxia and Boluo stations. The general alteration degrees of hydrology are 58.48%, 54.04%, 54.32%, and 52.47% at the Longchuan, Heyuan, Lingxia, and Boluo stations, respectively. The changes in the streamflow mechanism tend to increase seasonal ecosurplus and trigger risk of a high level of ecosurplus that causes a further decline in biodiversity. Comparing the eco-flow and IHA metrics demonstrated that combining these two groups has the potential to provide a sufficient measure of the change in the flow regime. Ecodeficit and ecosurplus can be accepted in the evaluation of alterations of hydrological processes at annual and seasonal time scales. This study provides a theoretical background for water resource management with consideration of eco-flow variations in response to reservoir regulation in other highly regulated river basins of the globe.