Nature Reviews Earth & Environment | 2021
Geomorphic change in the Ganges–Brahmaputra–Meghna delta
Abstract
More than 70% of large deltas are under threat from rising sea levels, subsidence and anthropogenic interferences, including the Ganges–Brahmaputra–Meghna (GBM) delta, the Earth’s largest and most populous delta system. The dynamic geomorphology of this delta is often overlooked in assessments of its vulnerability; consequently, development plans and previous management investments have been undermined by unanticipated geomorphic responses. In this Review, we describe GBM delta dynamics, examining these changes through the Drivers–Pressures–States–Impacts–Responses framework. Since the early Holocene, the GBM delta has evolved in response to a combination of tectonics, geology, changing river discharge and sea level rise, but the dynamics observed today are driven by a complex interplay of anthropogenic interferences and natural background processes. Contemporary geomorphic processes such as shoreline change, channel migration, sedimentation and subsidence can increase flooding and erosion, impacting biodiversity, ground and water contamination and local community livelihoods. Continued human disturbances to the GBM delta, such as curtailing sediment supplies, modifying channels and changing land use, could have a more direct influence on the future geomorphic balance of the delta than anthropogenic climate change and sea level rise. In order to contribute to long-term delta sustainability, adaptation responses must therefore be informed by an understanding of geomorphic processes, requiring increased transdisciplinary research on future delta dynamics at centennial timescales and collaboration across all governing bodies and stakeholders. The Ganges–Brahmaputra–Meghna delta is home to more than 170\u2009million people, but is vulnerable to sea level rise, subsidence and direct human disturbance. This Review examines geomorphic change in the delta and its broader impacts. The interplay between long-term tectonic and eustatic sea level changes, sudden earthquake perturbances and large-scale man-made management schemes in the Ganges–Brahmaputra–Meghna (GBM) delta are the key drivers that shaped its evolution. This review provides a spatial representation of the sediment budget, which is necessary for delta management decisions, including the potential for harnessing natural sedimentation processes to enhance land generation. Mapping the spatio-temporal extent of documented geomorphic processes revealed gaps in understanding at the centennial scales and into the future, which are both critical to delta management decisions, as most infrastructures are expected to be effective for up to 100 years into the future. Only 40% of the 427 reviewed publications assess geomorphic processes as interconnected, potentially resulting in a fragmented understanding of dynamics. Geomorphic processes are mostly absent from models of flooding and water security in the GBM delta. These omissions undermine the validity of longer-term projections and call into question the appropriateness of management decisions that are based upon these models. Anthropogenic disturbances could have a more direct influence on the future geomorphic balance of the GBM delta than climate change and sea level rise. The interplay between long-term tectonic and eustatic sea level changes, sudden earthquake perturbances and large-scale man-made management schemes in the Ganges–Brahmaputra–Meghna (GBM) delta are the key drivers that shaped its evolution. This review provides a spatial representation of the sediment budget, which is necessary for delta management decisions, including the potential for harnessing natural sedimentation processes to enhance land generation. Mapping the spatio-temporal extent of documented geomorphic processes revealed gaps in understanding at the centennial scales and into the future, which are both critical to delta management decisions, as most infrastructures are expected to be effective for up to 100 years into the future. Only 40% of the 427 reviewed publications assess geomorphic processes as interconnected, potentially resulting in a fragmented understanding of dynamics. Geomorphic processes are mostly absent from models of flooding and water security in the GBM delta. These omissions undermine the validity of longer-term projections and call into question the appropriateness of management decisions that are based upon these models. Anthropogenic disturbances could have a more direct influence on the future geomorphic balance of the GBM delta than climate change and sea level rise.