Deltas are complex systems that emerge where rivers meet the sea. The dynamic flow of water and sediment through deltaic landscapes makes them among the most fertile places on earth, attracting densely populated communities in search of places to live, work, or trade. Today, deltas are home to over half a billion people globally. Throughout history, humans have attempted to manage the inherent risks of living in deltas through infrastructure. Flood control measures such as dams, diversions, and embankments greatly influence physical dynamics, resulting in the redistribution of land and water through sediment deposition and erosion. These human modifications have expanded arable land, navigable waterways, and protection for settlements; but also threaten human well-being via the intensification of flood risk, riverbank erosion, land subsidence, and soil degradation. Communities in low-lying deltas face tough choices between strategies that maximize short-run economic opportunity, and long-term risk resulting from human modifications.

Using the Bengal Delta as a case study, this project integrated social and earth science data to investigate how human mobility and economic livelihoods are both outcomes and drivers of environmental change. The research site offered an opportunity to study these processes in one of the largest, most dynamic, and densely populated deltas in the world while advancing scientific understanding in a globally significant way. The research was guided by three main objectives: (1) to assess how river and coastal processes affect the livelihood opportunities and migration patterns of families living in deltas, (2) to better understand how livelihood activities and migration, in turn, change physical dynamics, and (3) to gain insight into how these relationships have unfolded over time and may change in the future.

We used household surveys, interviews, water and sediment analysis, geochronology, remote-sensing, and computational modeling. Our findings reveal that the introduction of earthen embankments, locally known as polders, were associated with a period of agricultural growth and relatively less short-term outmigration in the second half of the 20^th century. However, embankments have contributed to the emergence of new and complex risks. Several tidal channels have reduced in width over the last 30+ years as sediment-laden water that was once dispersed across the delta plain is now restricted. As water levels from the tides continue to increase in the years to come (known as effective sea level rise), flood risk will correspondingly increase by ~70%. We find that the sediment supply in the river system is sufficient to keep the delta in equilibrium with sea-level rise only if appropriate land-use measures are adopted. However, the Indian River Interlinking Project threatens opportunities to adapt through a reduction in sediment supply. The sediment filling tidal channels can also be used by local communities to adapt to flood risk. On a delta-wide scale, if all infilled tidal channels were dredged to 10% of their original widths, nearly 550,000 houses could be raised above the flood levels and ~500 km of embankments could be elevated—positively impacting ~2.4 million people living in vulnerable coastal regions of the delta.

Geophysical change also contributes to migration. We find that the intensification of riverbank erosion is associated with an increase in outmigration, primarily among young landless males. However, we find no change in migration associated with deposition along riverbanks (called accretion). Across multiple analyses, we find that extreme weather, landlessness, and economic inequality are major drivers of migration—pushing many individuals and families into crowded urban areas. Yet, migration is also an important mechanism by which families cope with environmental change. For example, remittances are a significant source of income for many families and communities. We also find that, when migrant sons are away, households are significantly less likely to switch or diversify livelihoods outside of agriculture.

This project provided professional training opportunities for ten graduate students, three postdoctoral fellows, and several early career scientists. It supported the development of two new graduate-level courses and the implementation of 3-day experiential education program for >100 high school students from historically disadvantaged neighborhoods in Baton Rouge, LA. Stakeholders were also reached through collaborative field visits, story maps, guest lectures, publicly accessible data archives, peer reviewed articles, and presentations to representatives from multiple institutions including the US State Department, World Bank, United Nations, and Bangladeshi Government.

Last Modified: 02/05/2024
Modified by: Amanda Carrico