Abstract: We seasonally collected river water samples at four stations along the Yellow River basin, namely Tangnaihai and Qingtongxia stations (upstream), Huayuankou station (middle stream), and Jinan station (downstream) for a period of two years. Major ion concentrations in the river water samples were determined to illustrate possible controlling roles of natural factors and anthropogenic activities. The results showed that: (1) The pH value of river water in different stations in the Yellow River basin ranged from 6.90 to 9.00 with a mean value of 7.94, and TDS values varied from 178 to 685 mg·L^-1, with an average value of 453 mg·L^-1. The average pH values decreased and TDS values increased along the flow path; (2) The water hydrochemical types varied at different stations. The water hydrochemical type was mainly HCO₃Ca at Tangnaihai station, while HCO₃SO₄Ca-Na at both Qingtongxia and Huayuankou stations with SO₄ Ca-Mg and Cl-Na at Huayuankou station in some months, and HCO₃SO₄Na-Ca at Jinan station with Cl-Na in some months; (3) The dissolved ion concentrations varied in different seasons. The values were often low in flood seasons, mainly in July and August, due to the dilution of rainwater, and high in winter (December) and ice melting (March) seasons; (4) The results of this study suggested that variations in water chemical compositions in Yellow River basin are mainly controlled by natural factors. Carbonate weathering dominates at Tangnaihai station with sulfur acid acting. As the river flows into Loess Plateau, the silicate weathering and evaporite dissolution become dominant. Nitrate concentrations increased slightly in May and June at Huayuankou and Jinan stations, possibly related to agricultural irrigation.

Key words: AZTI marine biotic index (AMBI), ecological assessment., benthic biotic index, benthic polychaetes amphipods (BPA), benthic opportunistic polychaetes amphipods (BOPA)