沖積平野の微地形

The landforms on recent alluvial plains of large rivers in Japan can be classified into three "landform areas": the area of alluvial fan, natural levee and delta, which are arranged from the upper stream downward. Such arrangement is called "a standard of alluvial plain morphology in Japan". Micro-landform units on each area have close relation to their sediments. Main composing sediments of the area of alluvial fan are gravels, but the surface layers on it consist of sandy materials (0.5-2 m thick) with scanty silt component. Many abandoned channels exist on the surface of alluvial fan and are filled with thin muddy fine materials. The river bed on this area is characterized by gravelly braided channels and many channel bars. The micro-landform units on the area of natural levee are classified into meandering abandoned channells, natural levees along meandering abandoned channels and back swamps. In general, sandy deposits are on such relatively higher part of the plain as natural levees, and muddy deposits on such relatively lower, poorer drained land as back swamps. River dunes develope on leeward location of the river beds or abandoned channels, in which large quantities of sands have been or are being surplied. The area of delta is very flat swampy lowlands 0-1 m high. The lowland whose coast has not been closed by sand bars is composed of sandy layers on the surface. Small valley bottom lowlands which dissect surrounding uplands and hills are filled with high water content thick peaty deposits, the main valley alluvial deposits have shut up their mouths of small valleys. Standard of alluvial plain morphology is modified by morphology of sedimentary basins, geology of drainage basins and crustal movement. The deltaic lowland on the alluvial plain of the Nobi Plain extend west, even northwest of the plain, and peaty layers accumlate on the surface of the plain. As the cental part of the plain of the Sagami Rive r have been closed by coastal sand bars and sand dunes, this area becomes stagnant, and the transported muddy materials accumlates in the closed water. Thus accumulated cohesive muddy deposits as well as channel deepened by land uplift, fix the channel at the present course. The Yahagi River flows down the drainage basin of granific rocks. Therefore, the river bed sediments on the area of natural levee are generally, graveliferous coarse sands with braided channel morphology, and cause frequent shift of the channel, so that island-like or butte-like upland fields are scattered over this alluvial plain.