Chemical and physical weathering in the Min Jiang, a headwater tributary of the Yangtze River

The Min Jiang is a major headwater tributary of the Yangtze (Chang Jiang). Its source is in the undeveloped eastern Tibetan Plateau, but it flows through the heavily populated Sichuan (Four Rivers) Basin on its way to join the Yangtze main channel. The dissolved major element composition was determined in the Tibetan headwaters at the rising stage and at four hydrologic stations in a monthly time series. The Min Jiang is alkaline and has high dissolved load compared to other world rivers. Carbonate weathering dominates, especially in the source region of the Min Jiang main channel, and silicate weathering and evaporite dissolution gain some importance in the Dadu He tributary. The Si / (Na* + K) ratios of the dissolved load and the clay mineralogy of the bed material suggest that silicate weathering in the watershed is superficial, i.e. soluble cations are being preferentially leached. Many major elements show lower concentrations at high summer discharge but there is also additional input, such that the concentration–discharge relationship is not purely a result of dilution. The discharge-weighted total dissolved solid (TDS) flux, based on data from the three upstream monitoring stations, is 10 × 106 tons/year, ∼6% of the Yangtze at mouth, and the chemical denudation rate as measured by TDS yield is 115 tons/km2/year for the Min Jiang, higher than the average Yangtze basin (85 tons/km2/year). The common extrapolation using annual average discharge and spot sampling of dissolved and suspended material during rising or falling stage can reasonably estimate chemical fluxes to ± 20% but seriously underestimate the physical flux. Sampling the peak flash flow is critical for the suspended particulate material. Concentrations of SO4, Cl and Ca are significantly elevated as the river debouches onto the populated Sichuan Basin.