Discovery of Riverine Dunes along the Nagara and Ibi Rivers and Identification of Their Sediment Provenance

Abstract

 On the Nobi Plain, all currently recognized riverine dunes are limited to the Kiso River and its former distributary, the Saya River; no riverine dunes have been reported along the Nagara and Ibi Rivers. Grain-size analyses of sediments from alluvial fans, riverine dunes, and natural levees in the plain are conducted and compared with results for riverine dune sediments from the Nakagawa lowland and coastal dune sand from Otake beach, Kanto Plain. Sediment mineral and chemical composition are also analyzed using stereomicroscope and X-ray fluorescence to estimate the provenance of small elevations at Nakasu, Suka, and Suwaki along the Nagara and Ibi Rivers. The small elevations are considered to be riverine dunes because their sediments are distinguishable from those of coastal dunes and natural levees in terms of mean grain size and skewness. Combined analyses of grain size, mineral composition, and chemical composition suggest that Suwaki dune sediments are a mixture of sediments from the Kiso, Ibi, and Nagara Rivers, and that the Suka dunes are mainly composed of sediments from the Nagara and Ibi Rivers. Nakasu dune sediments located near the Ibi River are similar to Neo River sediments, suggesting a change in flow path after dune formation. Combined analyses of grain size, mineral composition, and chemical composition of riverine dune sediments are, in short, useful for estimating their provenance and river channel avulsion histories. Although typical riverine dunes rarely develop in Japan, finding small riverine dunes and applying the above-mentioned analyses can be used to evaluate channel stability and sediment transport systems on alluvial plains.