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Characterization of stable isotope compositions (delta H-2 and delta O-18) of surface water and groundwater in a catchment is critical for refining moisture sources and establishing modern isotope-elevation relationships for paleoelevation reconstructions. There is no consensus on the moisture sources of precipitation in the Yellow River source region during summer season. This study presents delta H-2 and delta O-18 data from 111 water samples collected from tributaries, mainstream, lakes, and groundwater across the Yellow River source region during summertime. Measured delta O-18 values of the tributaries range from -13.5 parts per thousand to -5.8 parts per thousand with an average of -11.0 parts per thousand. Measured delta O-18 values of the groundwater samples range from -12.7 parts per thousand to -10.5 parts per thousand with an average of -11.9 parts per thousand. The delta O-18 data of tributary waters display a northward increase of 1.66 parts per thousand per degree latitude. The delta O-18 data and d-excess values imply that moisture sources of the Yellow River source region during summertime are mainly from the mixing of the Indian Summer Monsoon and the Westerlies, local water recycling, and subcloud evaporation. Analysis of tributary delta O-18 data from the Yellow River source region and streamwater and precipitation delta O-18 data from its surrounding areas leads to a best-fit second-order polynomial relationship between delta O-18 and elevation over a 4,600 m elevation range. A delta O-18 elevation gradient of -1.6 parts per thousand/km is also established using these data, and the gradient is in consistence with the delta O-18 elevation gradient of north and eastern plateau. Such relationships can be used for paleoelevation reconstructions in the Yellow River source region.