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Acoustic surveys collected six months before and after the passing of a major tropical cyclone over the Swatch of No Ground submarine canyon (Bay of Bengal) revealed the formation of widespread mass failures 30 km offshore of the Ganges-Brahmaputra river delta. Mass sediment flows several kilometers wide with minimal run-out distances (1 km wide x 5-60 m thick) developed in areas with preexisting subsurface sediment deformation and evidence for active submarine fluid discharge. In contrast, narrow, steep-walled (7[degrees]-10[degrees]) gullies present before the storm did not fail, suggesting that the gully walls may be shear hardened by the preferential funneling of gravity flows from the Bengal shelf. Combined, the widespread mass failures and gullies are part of a rapidly accreting (5-50 cm/yr), net-aggradational canyon system that supports multiple mechanisms for sediment transport from the active river mouth to the canyon. doi: 10.1130/G31181.1

Extreme spatial heterogeneity has emerged as a salient characteristic of groundwater arsenic in many complex fluviodeltaic environments. Here we examine patterns of arsenic heterogeneity in the shallow ( Keywords: arsenic, groundwater, Bangladesh, floodplain, stratigraphy.

Rivers are the main source of terrigenous sediment delivered to continental margins and thus exert a major control on coastal evolution and sequence development. However, little is known about past changes in fluvial sediment loads despite the recognition of significant variation under changing climatic regimes. In this study we present the first quantified estimate of sediment discharge for a major river system under conditions of an intensified early Holocene monsoon. Development of the Ganges-Brahmaputra River delta began ca. 11000 yr B.P., when rising sea level flooded the Bengal basin, thereby trapping most of the river's discharge on the inner margin. Chronostratigraphic data from these deltaic deposits are used to calculate the rates of sediment storage on the margin, which provide a minimum estimate of the river's past sediment load. Results reveal that ~5 x [10.sup.12] [m.sup.3] of sediment was stored in the Bengal basin from ca. 11000 to 7000 yr B.P., which corresponds to a mean load of 2.3 x [10.sup.9] t/yr. In comparison, modern sediment load of the Ganges-Brahmaputra is ~1 x [10.sup.9] t/yr, ranking it first among the world's rivers and underscoring the significance of a two-fold increase sustained over 4 k.y. Furthermore, the timing of immense discharge in the early Holocene strongly suggests its relation to a stronger than present southwest monsoon in South Asia. Similar patterns of high monsoon-related sediment discharge have been noted throughout the tropics and subtropics, suggesting a widespread fluviosedimentary response, the potential magnitude of which is showcased by the Ganges-Brahmaputra system. Keywords: climate effects, deltaic sedimentation, river discharge, sediment budget, GangesBrahmaputra River.

The partitioning of fluvial sediment load across continental margins is an important control on strata formation and sequence development; however, few quantitative sediment budgets that encompass entire dispersal systems exist. For the Ganges-Brahmaputra river system, sediment discharge is estimated to be [10.sup.9] t/yr at gauging stations [approximately] 300 km inland of the coast, but little has been known of the downstream fate of this material. Geochronological, geophysical, and stratigraphic investigations of the lowland flood plain, delta plain, and shelf help to delineate the extent of Holocene fill and allow calculation of a first-order sediment budget. Results reveal that 1500 x [10.sup.9] [m.sup.3] of sediment fill has been sequestered within the flood plain and delta plain since ca. 7000 yr B.P., or about one-third of the annual discharge. The remaining load appears to be apportioned between the prograding subaqueous delta (1970 x [10.sup.9] [m.sup.3]) and transport to the deep-sea Bengal fan via a nearshore canyon. Modern ([less than] 100 yr) budget estimates based on short-term accretion rates indicate a similar dispersal pattern and show that contemporaneous deposition continues within these disparate depocenters. The roughly equal partitioning of sediment among flood-plain, shelf, and deep-sea settings reflects the respective influence of an inland tectonic basin, a wide shelf, and a deeply incised canyon system. The findings also support new sequence stratigraphic models for these settings and indicate the important insight that modern river deltas can provide for ancient margin systems. Furthermore, results affirm that values of riverine sediment flux to the oceans may be considerably overestimated by not accounting for loss to the flood plains downstream of the gauging stations.

On March 12-13, 1993, the passage of a severe extratropical storm, popularly referred to as the 'Storm of the Century,' resulted in the deposition of storm-suspended sediments along Florida's west-central coastline. In Waccasassa Bay, surge waters near 3 m in height inundated the coast and transported resuspended, near-shore sediments onto the open-marine marshes rimming the embayment. The thickness of the resulting storm deposit reached 12 cm on the levees and up to 2 cm on the marsh surface, and visible sedimentation occurred several hundred metres from the creek banks into the marsh interior. The tan to gray storm layer was composed of mixed clays, silt to very fine sand-sized quartz, and marine biogenic sediments, all similar to those of the underlying marsh sediments. Despite the evidence for severe storm conditions, this event was characterized further by the absence of shoreline erosion along the marsh coast. This condition contrasts with sandy coasts, where strong storms often result in shoreline erosion. As compared to sandy shorelines, the stability of this marsh-fronted coast is enhanced by the cohesive nature of the fine-grained marsh muds, baffling by the marsh-grass canopy, and sediment binding by plant-root matrices. Locally, the Waccasassa Bay coastal system has developed under sediment-poor, sand-starved, and low-energy conditions, and large-scale storm events may be an important component in sediment-transport processes and marsh-surface accumulation. The response of the Waccasassa Bay system to this event offers a different view of storm-related shoreline effects and, in particular, the role that storms may play in long-term shoreline stability.

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., (© 2023. The Author(s).)

We present a review of the processes, morphology, and stratigraphy of the Ganges-Brahmaputra-Meghna delta (GBMD), including insights gained from detailed elevation data. The review shows that the GBMD is best characterized as a composite system, with different regions having morphologic and stratigraphic attributes of an upland fluvial fan delta; a lowland, backwater-reach delta; a downdrift tidal delta plain; and an offshore subaqueous-delta clinoform. These distinct areas of upland and lowland fluvial reaches and tidal dominance vary in time and space, and we distinguish late-Holocene phases of delta construction, maintenance, and decline similar to delta-lobe cycling in other systems. The overall stability of the GBMD landform, relative to many deltas, reflects the efficient, widespread dispersal of sediment by the large monsoon discharge and high-energy tides that affect this region. However, we do identify portions of the delta that are in decline and losing elevation relative to sea level owing to insufficient sediment delivery. These areas, some of which are well inland of the coast, represent those most at risk to the continued effect of sea-level rise.