Abstract:

The elementary compositions of river water  in   the Yangtze River headwater  area are derived from evaporite, carbonate and silicate dissolved in rain（snow,ice）. Major cations in main tributaries of Tongtian River named Qumar River and Tuotuo River, Na⁺, Ca²⁺ and Mg²⁺, account for 97% or more  in total cations. Ratios of Ca²⁺/Na⁺, Mg²⁺/Na⁺ and K⁺/Na⁺ are relatively low, and the values of 87Sr/86Sr range from （0.708,954±0.000，020）and to（0.711,860±0.000，011）, indicating  that elements of rivers flowing on  the northern slope of  the  Tanggula Mountains mainly come from evaporite rocks. Major cations of Nujiang River headwater area originating from southern  slope of Tanggula Mountains, Brahmaputra River and its tributaries， Ca²⁺, Mg²⁺ and Na⁺, account for 97% or more in total cations. But the ratios of Ca²⁺/Na⁺, Mg²⁺/Na⁺ and K⁺/Na⁺ are high and ⁸⁷Sr/⁸⁶Sr ranges from（0.705,534±0.000,016） to （0.722,856±0.000,014）. Elementary compositions of these rivers are mostly derived from carbonate and silicate. Na⁺ and Cl^- rooted in evaporite rocks are the most  abundant chemical compositions in the Yangtze River headwater  area.Calculations show that the physical erosion rate is 77 kg/(km²·y), and the chemical erosion rates of evaporite rocks, carbonate rocks and silicate rocks are 49-72.7, 15-20 and 2-5 t/(km²·y), respectively. However, on the southern slope of the Tanggula Mountains, the physical erosion rate is 34 kg/(km²·y), and the chemical erosion rates of carbonate rocks and silicate rocks are 25-30 and 7-10 t/(km²·y), respectively. These features are explained by the facts that in the high elevation and dry environment, the evaporite has a higher chemical erosion on the northern slope; on the other hand, carbonate and silicate have higher chemical erosion on the southern of the mountains because the precipitation on the southern slope is higher than that on the northern slope.

Key words: Yangtze River headwater area, river water, erosion, major element, Sr isotope, Tanggula Mountains