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Major ion concentrations were measured in lake water, river water, ground water, and rainfall water around the Lake Qinghai catchment. The dissolution of carbonate particles in dust has a strong impact on the chemical composition of rainfall as inferred from the Gibbs model and the ternary plots of the major ions. The chemical composition of the ground water can be divided into two groups, one influenced by river water and the other dominated by deeper aquifers. The chemical composition of lake water can be mainly ascribed to evaporation and crystallization. Total dissolved solid (TDS) of river waters during the wet periods is higher than during the dry periods. Calcium and bicarbonate are the dominant cations and anions of the river waters, and they have higher fractions during the wet periods than during the dry periods. Comparisons between (Mg(2+) + Ca(2+))/TZ(+), (Mg(2+) + Ca(2+))/(Na(+) + K(+)), HCO(3)(-)/Na(+), and Cl(-)/Na(+) suggest that the chemical composition of the river waters is mainly controlled by carbonate weathering and that the carbonate weathering is much stronger during the wet periods than during the dry periods. Seasonal melting may be one of the important factors that influence the seasonal variations of water chemistry of the rivers. (C) 2008 Elsevier Ltd and INQUA. All rights reserved.