Abstract: The alternation of gaining and losing reaches is a common manifestation of stream-groundwater exchange, and studies on the stream water source composition where source means the location of gaining reaches have become one of the frontiers in stream water quality research. However, the characteristics of stream water source compositions in arid and semi-arid areas or at large watershed scales remain unclear. By taking the Dosit River in the Ordos Plateau as an example, this study built a coupled stream-groundwater numerical model and analyzed the characteristics of stream-groundwater exchange in the Dosit River under three annual precipitation scenarios as well as their impact on the stream water source composition along the river. The results show that the Dosit River predominantly receives groundwater discharge, but the stream-groundwater exchange patterns on reach scale are heterogeneous, with a percentage of losing reaches of more than 40%. There is a concentration effect on the stream water source composition, e.g., in the KuShuigou section, 80% of the stream discharge under the three precipitation scenarios originates from only 12.3%, 9.1%, and 17.5% of the total river length, respectively. The concentration effect of stream water source composition correlates well with stream discharge, with the location of stream water sources becoming more concentrated as discharge decreases. This study is the first to investigate the characteristics of stream water source compositions in arid and semi-arid regions, highlighting the crucial role of key reaches in maintaining stream discharge. These findings can provide guidance for the rational management of water resources and effective prevention of river pollution in arid and semi-arid regions.