Accurately determining true ice microstructure and material parameters is a basis for ice disaster theoretical research on the Yellow River. In this work, natural Yellow River ice was collected, and ice crystals parallel and perpendicular to the ice surface were photographed using an orthogonal polarizing mirror. Morphologies of ice microstructure were extracted, and equivalent ice grain sizes were calculated. The results show that Yellow River ice mainly consists of granular ice and columnar ice and vary greatly in different time and space ranges. The ice crystal shape is irregular, and the ice crystal size is larger span, and mainly between 1 mm and 10 mm. Ice crystal initial defects come from bubbles, sediment particles, impurities, and microcracks; among them, bubbles are the most common and have a relatively large impact. In addition, a calculation model of the Yellow River ice microstructure was constructed according to the ice crystal test results. Based on the experimental data and numerical model, the obtained Yellow River ice parameters provide help for analyzing ice disaster mechanisms along the Yellow River.

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The eco‐flow metrics (ecosurplus [ES] and ecodeficit [ED]) based on flow duration curves (FDCs) are currently less popular than other metrics used to assess the ecohydrological conditions of rivers. To fully utilize their application potential, the seasonal ES and ED are redefined based on discharge hydrographs (DHs) rather than their FDCs. The annual ES and ED are redefined as the sum of the four seasonal ES and ED values, respectively. The impact of reservoirs can be measured by the rate of increase of the cumulative seasonal eco‐flows. Taking the seasonal EDs calculated by the 10% DH and 20% DH as the two threshold values, a risk evaluation method is proposed that classifies the seasonal ecohydrological conditions into four risk levels: no risk, low risk, moderate risk, and high risk. A daily discharge time series ranging from 1956 to 2015 in the Yellow River is used to perform a case study. The results showed that the Shannon diversity index (H) had a strong relevance to the annual ED. Riverine biodiversity was more sensitive to summer and autumn droughts than to floods. The Xiaolangdi Reservoir affected the seasonal eco‐flows much more than the Sanmenxia Reservoir. The winter season is currently classified as having a high risk and should be given more attention by river managers in the future. After the ecological regulation measures are implemented in the future to improve the winter conditions, their effects can be re‐evaluated by the above framework.

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