Geomorphic mapping of floodplain accessory channels with implications for floodplain morphodynamics

Floodplain morphology plays a key role in routing of floodwaters, sediment transport during floods, and the mechanics of the main channel. High-resolution topographic data (e.g., LiDAR) reveal a wealth of previously unappreciated geomorphic floodplain features. Using 1.5-m LiDAR data from Indiana, we found that the floodplains of Indiana rivers often contain a large number of secondary accessory channels. Accessory channels are subordinate to the main channel and are only active during flood stage, which is confirmed by Landsat images of inundated floodplains. Individual accessory channels are linear to sinuous, having widths ranging from 15 to 35% of the main channel, and lengths ranging from 500 to 3500 meters. On the White River, for example, the accessory channels account for ~25% of the floodplain area and these accessory channels are numerous and wide enough that during a ~10-year flood they conveyed all the flood discharge, leaving the inter-channel areas dry. To assess the pervasiveness of floodplain accessory channels, we inventoried all the floodplains of Indiana rivers having channel widths greater than 20 m. We constrained the mapping to the modern floodplains of each river, avoiding terraced floodplains related to the last glacial maximum. In total, we mapped 1100 km2 of floodplain; 55% of that area showed evidence for floodplain accessory channels. Moreover, there is a high concentration of floodplains exhibiting floodplain accessory channels in south-central Indiana occurring along the transition from glacial till into bedrock. Our preliminary hypothesis for the formation of floodplain accessory channels is that the channels form by connecting preexisting topographic lows, such as abandoned meander loops. Geomorphic mapping of a 7-km stretch of the floodplain along the White River reveals that the concave portions of accessory channels are associated with scroll-bar topography and abandoned meander loops from the main channel. In fact, 73% of the accessory channels annexed abandoned meander loops. Based on this, we conclude that accessory channels are associated with meandering rivers because frequent meander-loop cutoffs of the main channel create preexisting conduits that are then linked together during floods. These long coherent down-valley accessory channels have important implications for the morphodynamics of floodplains and flood-wave propagation.