Sediment dispersal and sequence development along a tectonically active margin: Late Quaternary evolution of the Ganges-Brahmaputra River delta

Situated in the Bengal Basin, the Ganges-Brahmaputra river delta comprises one of the largest fluviodeltaic systems in the world, comprising ∼ 100,000 km2 of floodplain and delta plain and a 40,000 km2 subaqueous delta on the shelf. Sediment load of the Ganges-Brahmaputra river is 109 t/yr, and seasonal flooding may inundate \u3e70% of the delta during large events. Active tectonic processes have resulted in both uplift and subsidence in this structurally complex region. These general characteristics suggest that the Ganges-Brahmaputra delta represents a heretofore undescribed delta end-member, forming along a high-yield, high-energy, tectonically active margin. to investigate this view, stratigraphic, sedimentologic, and geochronologic data are used to evaluate processes, controls, and development of the system over different spatial and temporal scales in the Late Quaternary.;Results of a century-scale sediment accretion study using radioisotope geochronology indicate that ∼ 30% of fluvial sediment load is sequestered to the delta and not reaching the coastal ocean as previously assumed. A Holocene-scale sediment budget generated from radiocarbon-dated stratigraphy also reveals ∼ 30% of sediment discharge was sequestered to the delta during this time. Considered with offshore data, these sediment budgets indicate contemporaneous highstand strata formation across floodplain, shelf, and deep-sea depocenters. Radiocarbon-dated stratigraphy was used to reconstruct the Late Quaternary history of delta formation. Growth of the Ganges-Brahmaputra delta began ∼ 10,000--11,000 cal yr BP, notably 2000--3000 years prior to most of the world\u27s deltas. During subsequent rapid sea-level rise, the immense sediment discharge was sufficient to maintain relative shoreline stability while most margin systems experienced major transgression. offset of radiocarbon dates from eustatic sea level indicate 2--4 mm/yr of subsidence in several areas of the delta, suggesting tectonic control on deltaic sediment trapping and sequence formation. Shallow vibracore stratigraphy from the delta reveals a cap of muddy sediments overlying largely sandy material, reflecting differences in preservation between floodplain and channel deposits. Over longer time frames, floodplain sediments are eroded through channel migration and avulsion, thus preferentially preserving channel sands. Overall, the role of tectonics in controlling deltaic processes and product in the Ganges-Brahmaputra river delta implies a fundamental distinction for deltas forming along active margins

Observations and Scaling of Tidal Mass Transport Across the Lower Ganges-Brahmaputra Delta Plain: Implications for Delta Management and Sustainability

The landscape of southwest Bangladesh, a region constructed primarily by fluvial processes associated with the Ganges River and Brahmaputra River, is now maintained almost exclusively by tidal processes as the fluvial system has migrated east and eliminated the most direct fluvial input. In natural areas such as the Sundarbans National Forest, year-round inundation during spring high tides delivers sufficient sediment that enables vertical accretion to keep pace with relative sea-level rise. However, recent human modification of the landscape in the form of embankment construction has terminated this pathway of sediment delivery for much of the region, resulting in a startling elevation imbalance, with inhabited areas often sitting \u3e1 m below mean high water. Restoring this landscape, or preventing land loss in the natural system, requires an understanding of how rates of water and sediment flux vary across timescales ranging from hours to months. In this study, we combine time series observations of water level, salinity, and suspended sediment concentration with ship-based measurements of large tidal-channel hydrodynamics and sediment transport. To capture the greatest possible range of variability, cross-channel transects designed to encompass a 12.4 h tidal cycle were performed in both dry and wet seasons during spring and neap tides. Regional suspended sediment concentration begins to increase in August, coincident with a decrease in local salinity, indicating the arrival of the sediment-laden, freshwater plume of the combined Ganges–Brahmaputra–Meghna rivers. We observe profound seasonality in sediment transport, despite comparatively modest seasonal variability in the magnitude of water discharge. These observations emphasize the importance of seasonal sediment delivery from the main-stem rivers to this remote tidal region. On tidal timescales, spring tides transport an order of magnitude more sediment than neap tides in both the wet and dry seasons. In aggregate, sediment transport is flood oriented, likely as a result of tidal pumping. Finally, we note that rates of sediment and water discharge in the tidal channels are of the same scale as the annually averaged values for the Ganges and Brahmaputra rivers. These observations provide context for examining the relative importance of fluvial and tidal processes in what has been defined as a quintessentially tidally influenced delta in the classification scheme of Galloway (1975). These data also inform critical questions regarding the timing and magnitude of sediment delivery to the region, which are especially important in predicting and preparing for responses of the natural system to ongoing environmental change

A synthesis of aquatic science for management of Lakes Mead and Mohave: U.S. Geological Survey Circular 1381

Lake Mead provides many significant benefits that have made the modern development of the southwestern United States possible. The lake also provides important aquatic habitat for a wide variety of wildlife including endangered species, and a diversity of world-class water based recreational opportunities for more than 8 million visitors annually. It is one of the most extensively used and intensively monitored reservoirs in the United States. The largest reservoir by volume in the United States, it supplies critical storage of water supplies for more than 25 million people in three western states (California, Arizona, and Nevada). Storage within Lake Mead supplies drinking water and the hydropower to provide electricity for major cities including Las Vegas, Phoenix, Los Angeles, Tucson, and San Diego, and irrigation of greater than 2.5 million acres of croplands. Due to the importance of Lake Mead, multiple agencies are actively involved in its monitoring and research. These agencies have a long history of collaboration in the assessment of water quality, water-dependent resources, and ecosystem health. In 2004, the National Park Service obtained funds from the Southern Nevada Public Lands Management Act to enhance this partnership and expand monitoring and research efforts to increase the overall understanding of Lake Mead and Lake Mohave. Participating agencies included the National Park Service, Southern Nevada Water Authority, U.S. Geological Survey, Nevada Department of Wildlife, Bureau of Reclamation, U.S. Fish and Wildlife Service, University of Nevada, Las Vegas, and University of Nevada, Reno. Results of these important efforts have been presented in Lake Mead Science Symposia conducted in 2009 and 2012. The relationships forged by the collaboration led to the development in 2012 of the Lake Mead Ecosystem Monitoring (LaMEM) Work Group, which has formalized the partnership and documented an interagency purpose and mission statement with common objectives for protection of Lake Mead and Lake Mohave water quality and water-dependent resources. This Circular has been developed to summarize the state of the knowledge related to the interests and objectives of the LaMEM Work Group, to inform management and the public of current lake conditions, and identify future needs for monitoring and research. It is hoped that this report will provide a framework for continued long-term investigations and analysis of the environmental health of Lakes Mead and Mohave

Assessment of Endocrine and Gonadal Condition of Male Largemouth Bass from Lake Mead, Nevada

Las Vegas Bay (LVB) of Lake Mead receives combined flows of tertiary treated wastewater effluent, urban runoff, and groundwater from the Las Vegas metropolitan area. This study examined the potential for endocrine disrupting effects of these anthropogenic inputs on male largemouth bass (Micropterus salmoides). Adult male bass were collected at two sites within Lake Mead: Overton Arm (OA, reference site), and Las Vegas Bay (LVB). Post-spawn fish were collected in July 2007 (n = 6-10 per site) and pre-spawn fish in March 2008 (n = 13 per site). Post-spawn fish were characterized by regressed testes whereas pre-spawn bass had full-grown gonads. Mean fish lengths and weights did not vary between sites or sampling times. Pre-spawn LVB males had lower plasma 11-ketotestosterone (KT), higher estradiol-17β (E2), higher E2/KT ratio, higher hepatosomatic index (ratio of liver to body weight), higher hematocrit values, and higher condition factor compared to OA males. However, no significant differences were evident in the gonadosomatic index (ratio of testes to body weight) of either pre-or post-spawn males from the two sites. In post-spawn males, no significant site-associated differences were detected for any of the parameters measured. Overall, these results suggest the existence of site-specific environmental influences on several indices of endocrine condition and health of pre-spawn male largemouth bass in Lake Mead, and are generally consistent with outcomes from previous studies that suggested the occurrence of altered endocrine and reproductive condition in male common carp from LVB

History of contaminant inputs into Lake Mead derived from sediment cores

Assessing the changes in contaminant inputs (both organic and inorganic) over time is important in determining sources and sinks of these inputs. Variations in contaminant input were assessed in four sediment cores taken in 1998 from three different parts of Lake Mead (two from Las Vegas Bay and one from Overton Arm and Virgin Basin). Sediments were analyzed for major and trace elements, radionuclides, and organic compounds. Anthropogenic contaminant concentrations are greatest in Las Vegas Bay reflecting inputs from the Las Vegas urban area, although concentrations are low compared to sediment quality guidelines and to other USA lakes. One exception to this pattern was higher mercury concentrations in the Virgin Basin core. The Virgin Basin core is in the main body of the reservoir and is influenced by the hydrology of the Colorado River, which changed after completion of Glen Canyon Dam. Major- and trace-elements in the core show pronounced shifts in the early 1960s and, in many cases, gradually return to concentrations more typical of pre-1960s by the 1980s and 1990s, after the filling of Lake Powell upstream. The Overton Arm is the sub-basin least effected by anthropogenic contaminant inputs. Cores from Las Vegas Bay taken in 2007 were analyzed for emerging contaminants and although data are still preliminary, detections of musk fragrances have been found only in the upper 10 – 15 cm of the core, indicating that these compounds either degrade with time or have only been accumulating for the past 10 – 20 years

Late Quaternary Sedimentary Record and Holocene Channel Avulsions of the Jamuna and Old Brahmaputra River Valleys in the Upper Bengal Delta Plain

The first Holocene stratigraphic record of river-channel occupation and switching between the Brahmaputra–Jamuna and Old Brahmaputra paleovalleys is presented here. Motivated by the Brahmaputra River's historic avulsion from the Old Brahmaputra channel to its present-day Jamuna course, we have obtained sediment and radiocarbon samples from 41 boreholes along a 120 km transect crossing these two braided-river valleys. The stratigraphy along this transect reveals sand-dominated Holocene channel systems, each bound by remnant, mud-capped Pleistocene stratigraphy. Using sediment lithology and bulk strontium concentration as a provenance indicator, we define the geometry and channel-occupation history of each paleovalley. The western Brahmaputra–Jamuna valley is broad and somewhat deeper compared with the Old Brahmaputra valley, the latter actually comprising a composite of two narrower sub-valleys bifurcated by an antecedent topographic remnant. The gently sloped valley margins (slope: 0.002 to 0.007) and high width-to-thickness ratio (W/T: ~ 1000) of the Brahmaputra–Jamuna valley suggest that it was filled primarily through lateral channel migration and the reworking of braidbelt and overbank deposits. Conversely, the two Old Brahmaputra sub-valleys have comparatively steeper valley margins (slope: 0.007 to 0.022) and lower width-to-thickness ratios (W/T: ~ 125 and ~ 250), indicating that these were filled primarily through vertical aggradation of channel sands. We attribute this disparity in valley geometry and fill processes to the different occupation histories for each valley. In this case, the much larger Brahmaputra–Jamuna valley represents the principal, if not singular, river course during the last lowstand of sea-level, with a prominent gravel lag underlying the valley. In contrast the smaller Old Brahmaputra valleys do not appear to have been present, or at least well developed, at the last lowstand. Rather these courses were first occupied during the early Holocene transgression, and we infer that the river had been previously excluded from this region by the relatively higher elevation between the Madhupur Terrace and the Shillong Massif. We also demonstrate that the Brahmaputra River experienced 3–4 major avulsions during the Holocene, with considerably longer occupation times within the principal Brahmaputra–Jamuna valley. Together these observations indicate that occupation history and antecedent topography have been important controls on river course mobility and avulsion behavior

The planform mobility of river channel confluences: Insights from analysis of remotely sensed imagery

River channel confluences are widely acknowledged as important geomorphological nodes that control the downstream routing of water and sediment, and which are locations for the preservation of thick fluvial deposits overlying a basal scour. Despite their importance, there has been little study of the stratigraphic characteristics of river junctions, or the role of confluence morphodynamics in influencing stratigraphic character and preservation potential. As a result, although it is known that confluences can migrate through time, models of confluence geomorphology and sedimentology are usually presented from the perspective that the confluence remains at a fixed location. This is problematic for a number of reasons, not least of which is the continuing debate over whether it is possible to discriminate between scour that has been generated by autocyclic processes (such as confluence scour) and that driven by allocyclic controls (such as sea-level change). This paper investigates the spatial mobility of river confluences by using the 40-year record of Landsat Imagery to elucidate the styles, rates of change and areal extent over which large river confluence scours may migrate. On the basis of these observations, a new classification of the types of confluence scour is proposed and applied to the Amazon and Ganges-Brahmaputra-Meghna (GBM) basins. This analysis demonstrates that the drivers of confluence mobility are broadly the same as those that drive channel change more generally. Thus in the GBM basin, a high sediment supply, large variability in monsoonal driven discharge and easily erodible bank materials result in a catchment where over 80% of large confluences are mobile over this 40-year window; conversely this figure is < 40% for the Amazon basin. These results highlight that: i) the potential areal extent of confluence scours is much greater than previously assumed, with the location of some confluences on the Jamuna (Brahmaputra) River migrating over a distance of 20 times the tributary channel width; ii) extensive migration in the confluence location is more common than currently assumed, and iii) confluence mobility is often tied to the lithological and hydrological characteristics of the drainage basins that determine sediment yield

Synthesis of the distribution of subsidence of the lower Ganges-Brahmaputra Delta, Bangladesh

Deltas, the low-lying land at river mouths, are sensitive to the delicate balance between sea level rise, land subsidence and sedimentation. Bangladesh and the Ganges-Brahmaputra Delta (GBD) have been highlighted as a region at risk from sea-level rise, but reliable estimates of land subsidence have been limited. While early studies suggested high rates of relative sea-level rise, recent papers estimate more modest rates. Our objective is to better quantify the magnitude, spatial variability, and depth variation of sediment compaction and land subsidence in the lower GBD to better evaluate the processes controlling them and the pattern of relative sea level rise in this vulnerable region. We combine subsidence and compaction estimates from hand-drilled tube wells and historic sites (1–5 mm/y), GNSS and river gauges (4–8 mm/y) and RSET-MH and borehole vertical strainmeters (9–10 mm/y) in SW Bangladesh. The differences between the different types of measurements reflect the different timescales, spatial distribution and depth sensitivity of the different observations. Rates are lower for times >300y providing data on the timescale of compaction. We also observe differences related to the degree to which different devices measure shallow and deep subsidence. Higher values reflect a greater component of subsidence from young shallow deposits from soil compaction and organic matter degradation. Thus, we observe different rates for different environments and physical settings. These differences indicate that in planning adaptation for rising sea level, hard construction with a solid foundation may experience different subsidence rates than open fields or reclaimed land with recent natural or anthropogenic sedimentation. Significance statement: Land subsidence increases the impact of sea level rise. We combine six different types of measurements that examine land subsidence in coastal Bangladesh. The results show that causes of subsidence, including compaction of the sediments varies both spatially and with depth, and that compaction and organic matter degradation from young shallow deposits is a significant contribution to subsidence. This suggests that hard construction with a solid foundation, such as buildings and embankments, may experience a lower subsidence rates than open fields or reclaimed land with recent natural or anthropogenic sedimentation

The sedimentology of river confluences

Channel confluences are key nodes within large river networks, and yet surprisingly little is known about their spatial and temporal evolution. Moreover, because confluences are associated with vertical scour that typically extends to several times the mean channel depth, the deposits associated with such scours should have a high preservation potential within the rock record. Paradoxically, such scours are rarely observed, and their preservation and sedimentological interpretation are poorly understood. The present study details results from a physically‐based morphodynamic model that is applied to simulate the evolution and alluvial architecture of large river junctions. Boundary conditions within the model were defined to approximate the junction of the Ganges and Jamuna rivers, Bangladesh, with the model output being supplemented by geophysical datasets collected at this junction. The numerical simulations reveal several distinct styles of sedimentary fill that are related to the morphodynamic behaviour of bars, confluence scour downstream of braid bars, bend scour and major junction scour. Comparison with existing, largely qualitative, conceptual models reveals that none of these can be applied simply, although elements of each are evident in the deposits generated by the numerical simulation and observed in the geophysical data. The characteristics of the simulated scour deposits are found to vary according to the degree of reworking caused by channel migration, a factor not considered adequately in current conceptual models of confluence sedimentology. The alluvial architecture of major junction scours is thus characterized by the prevalence of erosion surfaces in conjunction with the thickest depositional sets. Confluence scour downstream of braid bar and bend scour sites may preserve some large individual sets, but these locations are typically characterized by lower average set thickness compared to major junction scour and by a lack of large‐scale erosional surfaces. Areas of deposition not related to any of the specific scour types highlighted above record the thinnest depositional sets. This variety in the alluvial architecture of scours may go some way towards explaining the paradox of ancient junction scours, that while abundant large scours are likely in the rock record, they have been reported rarely. The present results outline the likely range of confluence sedimentology and will serve as a new tool for recognizing and interpreting these deposits in the ancient fluvial record

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