Sedimentary characteristics and formation of riverine source bordering dunes in a humid region: an example from the lower reaches of Kiso River, central Japan

Riverine source bordering dunes are formed by the interaction of fluvial and aeolian processes. Even in humid regions such as central Japan, small-scale, elongated aeolian dunes parallel to river channels occur intermittently; a typical example is identified where sand bars had formed along the left bank of the lower reaches of Kiso River. This paper presents the sedimentary characteristics and luminescence chronology of the aeolian dunes along the Kiso River. The sand constituting dunes is well-sorted, medium-grained and the grain size shows little variation with depth. The sand would be supplied from the riverbed via the bars by the strong west-northwesterly to westerly winds promoted by the monsoon in the dry winter when the water level of the Kiso River drops significantly. Fading corrected infrared-stimulated luminescence (IR₅₀) dating suggests that the dunes occurred and grew rapidly upward after about 600–700 yr ago. Increased sediment yield caused by human activities such as deforestation in the drainage basin, and the resulting increase in sediment supply to the lower reaches may have promoted the formation of dunes. However, with the construction of continuous levees, the reduced sediment supply related to dam construction, and the lowering of the riverbed, the connectivity between sand bars and dunes declined, and most of the dunes appear to have been stable for the past century. Fluvial sand supply influenced by human activities as well as strong winds during the dry season plays a crucial role in the formation of riverine source bordering dunes in humid regions.