Abstract: Water ecological health of upstream watershed of Miyun Reservoir plays an important role in ensuring water safety of Miyun Reservoir. In order to systematically evaluate the health status of river ecosystems in the upstream watershed of Miyun Reservoir, a survey was conducted on macrobenthos in 2019. A total of 39 sampling points were set up, and 128 species of benthos were collected, including 106 species of Arthropods, 16 species of Molluscs, 5 species of Annelida, and 1 species of Platyhelminthes. From the perspective of pollution tolerance, the proportion of sensitive groups was 23.1%, and the proportion of pollution-tolerant groups was 0.9%. In terms of the occurrence frequency of groups, the top three benthic animals were Ephemera, Cheumatopsyche and Stenopsyche McLachlan, with the occurrence frequencies of 82%, 79% and 74%, respectively. The three benthic animals were all indicative organisms of the clean class. With macrobenthos as indicator organisms, the distribution range analysis, discriminant ability analysis and correlation analysis of 44 candidate biological indicators were carried out. The B-IBI index system of the upstream watershed of Miyun Reservoir was constructed by five indicators, namely, total taxa, the percentage of individuals of sensitive taxa, the percentage of Ephemeroptera individuals, the percentage of aquatic insect classification units and the percentage of sensitive groups. The quartering method was used to unify the parameter dimensions, and the 25% quantile method of the reference point B-IBI value distribution was used as the evaluation standard for river ecosystem integrity. The results show that 23% of the samples were at the "health" level, 33% at the "sub-health" level, 33% at the "normal" level, 8% at the "poor" level, and 3% at the "extremely poor" level.

Key words: upstream watershed of Miyun Reservoir, B-IBI index, macrobenthos, ecological health assessment