Basin-scale factors in emergence of midchannel islands: a process-based morphometric analysis

Abstract

Midchannel islands were conventionally considered undesirable additional hydraulic resistance to ow for many years and tended to be removed from the main channel as a measure against the ood. Today, it is known that these geomorphic units provide key eco-geomorphological services in the active corridor of natural waterways. This study examined the dynamics behind the occurrence mechanism of midchannel islands considering basin-scale (i.e., land use, elevation, main channel slope, form factor, relief) and uvial (i.e., sediment yield) variables. Nine drainage basins were selected as samples from the different ecoregions on Earth. Morphometric and hydrological characteristics were determined using hydrological datasets and satellite-based images by means of ArcGIS. Sediment delivery magnitudes for each examined basin were calculated based on an empirical expression. In addition, the development process of isolated midchannel islands was investigated by quantifying their planform morphometric features. The basin-scale analysis showed that the main channel slope, i.e., gravity-induced factors, controls the emergence location of MCIs to a great extent, and the basin sediment yield impacts the density (i.e., number of islands) of MCIs in a unit length of the river. Furthermore, at the individual scale, the morphometric analysis of MCIs revealed that during the MCI growth, the streamlining process diminishes the skin friction. This effect becomes more prominent, particularly at the mature stage of MCIs, and the form drag prevails over the skin friction.