Water samples were collected twice per month over a two-year period from the Longchuanjiang River (Yunnan Province, China) to understand monthly variations in major elements and solute fluxes as related to rock weathering and associated CO2 consumption rates. Solute concentrations were 5 times the median of 65 mg/l for global average. Total cationic exchange capacity (Tz+) ranged from 2.4 to 6.1 meq/l; and the mean (4.4 meq/l) was significantly higher than that of the global river waters. Calcium and bicarbonate dominated the annual ionic composition, accounting for more than 70% of the solute flux that exceeded 71 × 106 kg/yr. Lower concentrations of most measured elements during the monsoon high flow period could be explained by dilution effects from precipitation. Three major reservoirs contributed to the dissolved load: carbonates, silicates and anthropogenic inputs, i.e., some 83% of the riverine cations from carbonates and 17% from silicates. The chemical weathering rate of 26.1 t/km2/yr, with respective carbonate and silicate weathering rates of 20.3 t/km2/yr (8.46 mm/kyr) and 5.75 t/km2/yr (2.13 mm/kyr), was comparable to the average for global rivers, but higher than that for the Changjiang River in China. The CO2 consumption rate was estimated to be 173.7 × 103 mol/km2/yr and 202.9 × 103 mol/km2/yr by silicate and carbonate weathering, respectively. The CO2 consumed by rock chemical weathering in the upper Changjiang River reduced the atmospheric CO2 level and constituted a significant part of the global carbon budget. Consequently the carbon sink potential of rock chemical weathering in the Qinghai-Plateau deserves extra attention. Population density and anthropogenic activities, particularly agricultural practices, contributed remarkably to dissolved solutes and associated CO2 consumption worldwide, and anthropogenic inputs probably contributed some 10.4% to the dissolved solutes in the Longchuanjiang River.