Abstract: Qinhe Basin, a small watershed in North China, was chosen, and spatial and temporal variations of chemical composition in river water were described to illustrate the controlling roles of natural factors and anthropogenic activities on the base of two sampling campaigns during flood and normal seasons for mainstream and tributaries combined with onsite determination and indoor analysis. The results indicated that: (1) Cations were dominated by Ca²⁺ and Mg²⁺ in river water samples of Qinhe Basin during the two seasons accounting for 45% and 41% of total cations in terms of equivalent values during flood season and 43% and 41% during normal water season, respectively. Anions were dominated by HCO₃^- and SO₄^2- in river water samples and their ratios of equivalent values were 46% and 39% for flood season and 55% and 27% for normal season, respectively. The water type was mainly HCO₃^--SO₄^2--Ca²⁺-Mg²⁺. (2) The average values of δD and δ¹⁸O were -65.4‰ and -8.76‰ in flood season and -64.6‰ and -8.68‰ in normal season respectively. Atmospheric deposition was the main recharge sources for river waters in Qinhe and some of them with positive hydrogen and oxygen isotope values experienced evaporation. (3) The ratios of SO₄^2- and HCO₃^- in terms of equivalent values were 0.96 and 0.54 during flood and normal season respectively, and the ratios between Ca²⁺+Mg²⁺ and HCO₃^- equivalent values were 1.88 and 1.78 in flood and normal season respectively. The sulfuric acid derived from acid rain and oxidation of sulfide minerals had participated in chemical weathering process of carbonate minerals. (4) Nitrate and total mercury concentrations of river waters gradually increased downstream, which indicated the influences from anthropogenic activities mainly including sewage water, industrial waste water and agricultural activities.

Key words: cold-waterlogged paddy field, paddy rice-upland crop rotation, photosynthetic characteristics, soil aggregate, soil profile configuration, economic benefit.