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Instream ecological flow is essential determinant of river health. In this paper, the monthly minimum flow calculation method, the (new) monthly frequency calculation method were applied to calculate and evaluate the minimum instream ecological flow and the optimal instream ecological flow for the Irtysh River, and the different criteria of instream ecological flow was calculated by the improved Tennant method. It is expected to provide a scientific basis for the reasonable allocation of water resource in Irtysh River basin by calculating the instream ecological flow.

Oujiang River was selected as the case study, and a dataset of daily flow series at Xuren Station was used to explore the hydrologic characteristics of rivers in humid areas, by using the ‘Indicators of Hydrologic Alteration’ approach and ‘Range of Variability Approach’. Results showed that the overall alteration of the hydrological regime for Oujiang River belonged to the low alteration category, and some key eco-hydrological characteristics should be protected in certain key periods to maintain the integrality and health status of river ecosystems.

Based on the daily data of temperature (T) and precipitation (P) of Heilongjiang River basin, Mann–Kendall test method, the linear least-squares regression model and hydrological statistical analysis methods were selected to analyze the variations of temperature, precipitation, and extreme events in Heilongjiang River. The results revealed that T had significant increasing trends, and annual P increased only in the lower reaches. The annual extreme low temperature (ELT) increased obviously. Both the annual extreme high temperature (EHT) had noticeable increasing trends. The outputs can provide a valuable theoretical basis for sustainable development and utilization of water resources in Heilongjiang River.