Distribution, diversity, and geomorphic evolution of floodplain wetlands and wetland complexes in the Ganga plains of north Bihar, India

Floodplain wetlands are important features in alluvial settings across the world because of their ecological and hydrological significance. Understanding the geomorphic evolution of such wetlands is therefore not only interesting from an academic viewpoint but also necessary for their conservation and protection. Using a hierarchical system of geomorphological ‘elements’ and ‘units’, detailed geomorphic investigations have been carried out in the Kosi-Ganga interfluve, Eastern Ganga Plain (EGP) in the Himalayan foreland to understand the distribution and diversity of the floodplain wetlands in this region. The major geomorphic elements in this region consisting of lentic (wetlands, lakes) and lotic (streams) water bodies were mapped using multispectral satellite images. The distribution of the wetlands analysed through chord diagrams suggests that these are highly localised and associated with the regional geomorphic setting and the fluvial dynamicity. The morphometric characterisation of these wetlands using their count, % area, average area, and total area shows significant variability among the geomorphic units. The equal-area circle (EAC) such as the shoreline development index (DL) was found to be inadequate to assess the geomorphic maturity of the lentic water bodies and hence a new maturity index MCH, has been designed in this work using the convexity and area of the water bodies. Further, the space-for-time substitution approach has been used to develop the evolutionary pathway for the wetlands and the wetland-complexes in this interfluve over time scale of 10–10² years. We suggest that the evolution of the wetlands in a dynamic fluvial system such as the interfluve region of the EGP is governed by two different processes. Firstly, fluvial scouring and channel abandonment provide negative relief at a regional scale, and secondly, erosional processes (cut and fill) acting on the local scale transform the narrow and deep fluvial scours into large and shallow wetland systems.