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

Midchannel islands were conventionally considered undesirable additional hydraulic resistance to flow for many years and tended to be removed from the channels as a measure against flood. 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, slope, form factor, and relief) and fluvial (i.e., sediment yield) variables. Eleven 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 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, the morphometric analysis of MCIs in the individual scale revealed that when MCIs develop from the preliminary stage to the advanced stage, the contraction effect becomes more prominent which increases the streamlining effect.