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Alluvial fans are constructed when river deposition raises local elevations until the river "avulses" to an alternative lower elevation alignment. Avulsions may be triggered by exogenous events, such as floods that locally overtop the river's banks; they may then cause extreme flooding on a fan and have major impacts. Rivers draining the Himalayas into the Gangetic plain have behaved historically in this way; for example, the Kosi River's historical westward migration (1731–1963), and its recent eastward shift (1980). In the major fan‐delta of the Bengal Basin, the Brahmaputra and its tributary, the Tista, have both undergone historical avulsions, with some suggestion that these may have been causally linked. Between the Rennell survey of Bengal (1765–77) and the partial re‐survey by Wilcox and Ommanney (1828–35), the Brahmaputra River substantially changed its course. In Rennell's time it turned south‐east into the Sylhet Basin leaving the higher land of the Madhupur Tract on its right; by 1830, it flowed due south with the Tract on its left. One hypothesis for the timing and cause of this avulsion relates it to the impact of a major flood and avulsion in the River Tista in 1787. This paper examines the evidence of the Rennell maps, the East India Company archives of correspondence between District Collectors about flooding and avulsion in the Tista, and the diaries of Francis Hamilton Buchanan who travelled along the Tista in 1807–11. From these contemporary sources it is possible to re‐construct the changes in the River Tista in the late 18th century, and to conclude that its avulsion had little direct influence on that of the Brahmaputra, although it went through some radical rearrangements of its own. [ABSTRACT FROM AUTHOR]

Issues of disaster and governance in the Brahmaputra Basin of India have been part of different debates within the epistemic boundaries. This study of a social-ecological regime shift, from a prosperous paddy cultivated region within the north bank of the Upper Brahmaputra Valley to a sediment deposited wasteland, unravels the complex relations between narratives of a problem and purposeful action. It is found that policy and practice solutions may not always be grounded in the problem situation but can be shaped by wider discourses and social learning. The study illustrates the continuation of engineering solutions without deeper understanding of their influence on social dynamics, the typification of community behaviour with an ignorance of cultural legacies, and a lack of prioritization of adaptation needs in the novel social-ecological conditions of the region. The conceptual frameworks of flood control debates contribute to distinct discourses influencing policy and praxis, while the discontented riparian community is motivated, by actors influenced by identity and space politics, towards political autonomy. Such influences of opposing discourses may reinforce the narrative of a trust gap between the Indian polity and its Northeast Region. The study further identifies a latent capacity for flexibility in the community, which requires more attention from policy and praxis to explore management solutions in such a complex social-ecological system. The paper argues for science, policy and practice engagements in such contexts and the use of transdisciplinary heuristics for their design to facilitate shared understanding. Copyright © 2017 John Wiley & Sons, Ltd and ERP Environment [ABSTRACT FROM AUTHOR]

Acquiring freshwater resources is a necessary component of sustainable human settlement subject to increasing pressure from population and climate changes. This sometimes scarce resource primarily comes from rivers, and international river basins ( IRBs), where watersheds and watercourses cross political boundaries, are often spaces of great political tension and conflict worldwide. Such conflict potential has garnered interest from a wide range of research communities, and each emphasises public access to hydrologic data as integral to successful international management of IRBs. However; these hydrologic data, especially measurements of river flow rate, are often closely guarded state secrets. Satellites have been cited as a key technology set to challenge this data monopoly that have yet been unable to calculate river flow rate without some form of guarded ancillary data. Now, at-many-stations hydraulic geometry ( AMHG) offers a means of circumventing data limitations without any a priori information, and the forthcoming NASA/ CNES Surface Water and Ocean Topography ( SWOT) satellite also promises to estimate flow rates solely from its novel measurements following launch. In this paper, we explore how these newly available estimates of river flow rate could reconfigure water-management and interstate relations in IRBs, and demonstrate AMHG, for two cases: the Ganges- Brahmaputra and Mekong. For these basins we find that satellite flow rate retrievals will likely reinforce and favour state-level negotiations of water resource governance. Also, satellite flow retrievals can have the direct, concrete effect of improving hydrologic understanding of the upstream Ganges- Brahmaputra, a sorely needed advance that will positively benefit millions of Bangladeshis and affect state-level interactions between India, China, and Bangladesh. Finally, we avoid offering prescriptive water management solutions for each case, as local stakeholders will ultimately determine if and how such satellite retrievals will be used. [ABSTRACT FROM AUTHOR]

Sediment composition in modern fluvial settings is commonly assessed regarding spatial but rarely temporal variability, potentially leading to a bias of unknown extent. Here, we present the grain‐size distribution, bulk chemical and mineralogical composition of a time‐series set of 36 suspended sediment samples from the Brahmaputra river, as well as clay and heavy mineral analysis of selected samples. Sampling covers the June–November 2021 period, which included two major flooding events. We show that the two flooding events are characterized by contrasting grain size, with the first event characterized by a grain‐size minimum and the second by a grain‐size maximum. Although grain sizes of the first flood and the period after the second are similar, their compositions differ significantly, highlighted by a factor‐two decrease of biotite largely compensated by an increase in quartz. By contrast, the content of garnet, clinopyroxene, sillimanite, and rutile increased compared to epidote and amphibole during the second flood event. By relating the results to spatio‐temporal rainfall and discharge patterns and basin morphology, we conclude that the first flooding primarily mobilized hydraulically pre‐sorted sediments from the exposed sandbars of the floodplains, while those sandbars are already submerged during the second flooding in a single‐channel system, resulting in higher sediment contributions from highland tributaries draining igneous and high‐grade metamorphic rocks. Such temporal variations pose constraints on the interpretation of compositional differences between individual samples regarding sediment provenance and dispersal and should be considered in studies of modern drainage basins as well as ancient sediment routing systems. Plain Language Summary: Sediment provenance, which refers to where the sediment in a river comes from, is important to understand because it can tell us about the geology of an area, various earth‐surface processes and how the landscape is changing over time. However, sediment provenance is typically studied at a spatial scale in present day river basins, and temporal variability is rarely considered. This study examines the physical, chemical and mineralogical properties of sediment in the Brahmaputra river during two major flooding events that occurred in the same season. The results show that the sediment composition varies between the events, indicating a change in the relative proportions of distinct sources. This emphasizes the importance of considering temporal variations in sediment composition when interpreting sediment provenance signals. Key Points: Time‐series analysis of sediment composition during two major flooding events of a single monsoon season is presentedThe two flooding events show contrasting grain‐size, chemical and mineralogical compositionTemporal variations in sediment composition pose constraints on the interpretation of provenance and dispersal based on individual samples [ABSTRACT FROM AUTHOR]

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Large rivers can be significantly impacted upon by both tectonics and human activities; yet, there are very few studies where these natural and anthropogenic impacts combine. A new river island called Dibru‐Saikhoa has appeared very recently on the Brahmaputra valley map in Assam. More than 200 km2 in area, it is situated in the extreme upstream reach of the Brahmaputra River and represents the second biggest relict island after Majuli (with a present area of ~500 km2). The cause of the formation of this island is generally attributed to river dynamics, with only a minor anthropogenic role. However, our study, considering morphological changes from 1915 to 2019 and subsurface geophysical data, suggests that the anthropogenic factor was an important 'trigger' of island formation, acting on a system made sensitive to change by the combination of local tectonics and rapidly changing sediment influx. Making use of the satellite‐borne Bouguer gravity data and ground survey‐based seismic sections, we located some of the prominent blind faults. Taking into consideration recent activation and interplay of these faults, the mode of accommodation space generation, and aggradation characteristics due to fluvial processes, we suggest a multi‐order mechanism of formation of this relict island. The first‐order trigger is due to the unstable river morphology of the Siang, which is responding to tilting of the valley floor as a co‐seismic phenomenon, and subsequently influences two other rivers—the Dibang and the Lohit. Second‐order forcing comes from intra‐plate residual pop‐up that comes into play due to uneven sediment loading. The third‐order forcing is inter‐plate suturing, which causes arching and fracturing of the basement. [ABSTRACT FROM AUTHOR]

ABSTRACT Climate variability has major impacts on crop yields and food production in South Asia. The spatial differences of the impact are not, however, well understood. In this study, we thus aim to analyse the spatio-temporal relationship between precipitation and rice yields in the Ganges-Brahmaputra-Meghna region. The effects of rainfall variation on yields were analysed with regression models using the Standardized Precipitation Index ( SPI) as an explanatory variable. Our results indicate that in large part of the study region, a strong relationship between precipitation and rice yields exists and the SPI at various lags chosen as the predictor variable performed well in describing the inter-annual yield variability. However, the study demonstrated large spatial variations in the strength of this relationship or optionally in the suitability of the chosen methodology for investigating it. In the mid-plains of the Ganges, which represent very important agricultural areas, precipitation variability has a strong impact on rice yields, while in downstream Ganges as well as in Brahmaputra, where precipitation is more abundant, the relationship was less pronounced. Where the performance of the regression models was weaker, it is likely that yield variation depended on other factors such as management practices or on other climate factors such as temperature. The results further showed that the SPI at 1, 3, 6 and 12 month lags calculated for the monsoon time (June-October) are most commonly the best at explaining the rice yield variability. The SPI can thus be considered a very useful predictor of rice yield variability in some parts of the study region, demonstrating that they could be used for agricultural applications and policy decisions to improve the region's food security. [ABSTRACT FROM AUTHOR]