Climate-driven channel change of Orkhon River, Mongolia

Rivers in the Mongolian Plateau play essential roles in surface water resources in Eurasia. Orkhon River is a major tributary of Selenge River, which is a headwater of Lake Baikal in the North Arctic Ocean drainage basin. It is the second longest river in Mongolia (1,124 km) draining Mt. Suvarga Khairkhan in the Khangai Mountain Range in central Mongolia. This study considers the long- and short-term valley geomorphic evolution and channel morphology of Orkhon River. Geochemical imprints in distribution of major and trace elements in sediments in the Orkhon River valley imply that the sediments are primarily transported by fluvial process at the midstream and fluvial and aeolian processes at the downstream. The channel planform shows that the channel configuration of the river responds to modern climate changes in warming and drying during the last 50 years. The river is anastomosing and braiding at high degree in the middle and lower reaches. At the midstream its paleochannel called Khugshin Orkhon resulted in formation of an oxbow Lake Ugii in the northeast of the Khangai Mountain Range. At the downstream the river valley includes wide floodplains including natural levees and back marshes. The lower reaches have undergone significantly widening. Recent braiding of the lower Orkhon River is consistent with more frequent floods, i.e., the present channel width corresponds to the flooding driven by extreme precipitation events.