Cascading hazards of a major Bengal basin earthquake and abrupt avulsion of the Ganges River.

Earthquakes present severe hazards for people and economies and can be primary drivers of landscape change yet their impact to river-channel networks remains poorly known. Here we show evidence for an abrupt earthquake-triggered avulsion of the Ganges River at ~2.5 ka leading to relocation of the mainstem channel belt in the Bengal delta. This is recorded in freshly discovered sedimentary archives of an immense relict channel and a paleo-earthquake of sufficient magnitude to cause major liquefaction and generate large, decimeter-scale sand dikes >180 km from the nearest seismogenic source region. Precise luminescence ages of channel sand, channel fill, and breached and partially liquefied floodplain deposits support coeval timing of the avulsion and earthquake. Evidence for reorganization of the river-channel network in the world's largest delta broadens the risk posed by seismic events in the region and their recognition as geomorphic agents in this and other tectonically active lowlands. The recurrence of comparable earthquake-triggered ground liquefaction and a channel avulsion would be catastrophic for any of the heavily populated, large river basins and deltas along the Himalayan arc (e.g., Indus, Ganges, Brahmaputra, Ayeyarwady). The compounding effects of climate change and human impacts heighten and extend the vulnerability of many lowlands worldwide to such cascading hazards.

Sediment delivery to sustain the Ganges-Brahmaputra delta under climate change and anthropogenic impacts.

The principal nature-based solution for offsetting relative sea-level rise in the Ganges-Brahmaputra delta is the unabated delivery, dispersal, and deposition of the rivers' ~1 billion-tonne annual sediment load. Recent hydrological transport modeling suggests that strengthening monsoon precipitation in the 21st century could increase this sediment delivery 34-60%; yet other studies demonstrate that sediment could decline 15-80% if planned dams and river diversions are fully implemented. We validate these modeled ranges by developing a comprehensive field-based sediment budget that quantifies the supply of Ganges-Brahmaputra river sediment under varying Holocene climate conditions. Our data reveal natural responses in sediment supply comparable to previously modeled results and suggest that increased sediment delivery may be capable of offsetting accelerated sea-level rise. This prospect for a naturally sustained Ganges-Brahmaputra delta presents possibilities beyond the dystopian future often posed for this system, but the implementation of currently proposed dams and diversions would preclude such opportunities.

Anatomy of Mississippi Delta growth and its implications for coastal restoration.

The decline of several of the world's largest deltas has spurred interest in expensive coastal restoration projects to make these economically and ecologically vital regions more sustainable. The success of these projects depends, in part, on our understanding of how delta plains evolve over time scales longer than the instrumental record. Building on a new set of optically stimulated luminescence ages, we demonstrate that a large portion (~10,000 km2) of the late Holocene river-dominated Mississippi Delta grew in a radially symmetric fashion for almost a millennium before abandonment. Sediment was dispersed by deltaic distributaries that formed by means of bifurcations at the coeval shoreline and remained active throughout the life span of this landform. Progradation rates (100 to 150 m/year) were surprisingly constant, producing 6 to 8 km2 of new land per year. This shows that robust rates of land building were sustained under preindustrial conditions. However, these rates are several times lower than rates of land loss over the past century, indicating that only a small portion of the Mississippi Delta may be sustainable in a future world with accelerated sea-level rise.