Abstract
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.