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.

OBJECTIVES: Prone position ventilation is a potentially life-saving ancillary intervention but is not widely adopted for coronavirus disease 2019 or acute respiratory distress syndrome from other causes. Implementation of lung-protective ventilation including prone positioning for coronavirus disease 2019 acute respiratory distress syndrome is limited by isolation precautions and personal protective equipment scarcity. We sought to determine the safety and associated clinical outcomes for coronavirus disease 2019 acute respiratory distress syndrome treated with prolonged prone position ventilation without daily repositioning. DESIGN: Retrospective single-center study. SETTING: Community academic medical ICU. PATIENTS: Sequential mechanically ventilated patients with coronavirus disease 2019 acute respiratory distress syndrome. INTERVENTIONS: Lung-protective ventilation and prolonged protocolized prone position ventilation without daily supine repositioning. Supine repositioning was performed only when Fio 2 less than 60% with positive end-expiratory pressure less than 10 cm H2O for greater than or equal to 4 hours. MEASUREMENTS AND MAIN RESULTS: Primary safety outcome: proportion with pressure wounds by Grades (0–4). Secondary outcomes: hospital survival, length of stay, rates of facial and limb edema, hospital-acquired infections, device displacement, and measures of lung mechanics and oxygenation. Eighty-seven coronavirus disease 2019 patients were mechanically ventilated. Sixty-one were treated with prone position ventilation, whereas 26 did not meet criteria. Forty-two survived (68.9%). Median (interquartile range) time from intubation to prone position ventilation was 0.28 d (0.11–0.80 d). Total prone position ventilation duration was 4.87 d (2.08–9.97 d). Prone position ventilation was applied for 30.3% (18.2–42.2%) of the first 28 days. Pao 2:Fio 2 diverged significantly by day 3 between survivors 147 (108–164) and nonsurvivors 107 (85–146), mean difference –9.632 (95% CI, –48.3 to 0.0; p = 0·05). Age, driving pressure, day 1, and day 3 Pao 2:Fio 2 were predictive of time to death. Thirty-eight (71.7%) developed ventral pressure wounds that were associated with prone position ventilation duration and day 3 Sequential Organ Failure Assessment. Limb weakness occurred in 58 (95.1%) with brachial plexus palsies in five (8.2%). Hospital-acquired infections other than central line–associated blood stream infections were infrequent. CONCLUSIONS: Prolonged prone position ventilation was feasible and relatively safe with implications for wider adoption in treating critically ill coronavirus disease 2019 patients and acute respiratory distress syndrome of other etiologies.

The Holocene stratigraphy of Sylhet basin, a tectonically influenced sub-basin within the Ganges-Brahmaputra-Meghna delta (GMBD), provides evidence for autogenic and allogenic controls on fluvial system behaviour. Using Holocene lithology and stratigraphic architecture from a dense borehole network, patterns of bypass-dominated and extraction-enhanced modes of sediment transport and deposition have been reconstructed. During a~3-kyr mid-Holocene occupation of Sylhet basin by the Brahmaputra River, water and sediment were initially (~7.5-6.0 ka) routed along the basin's western margin, where limited downstream facies changes reflect minimal mass extraction and bypass-dominated transport to the basin outlet. Sedimentdispersal patterns became increasingly depositional~6.0-5.5 ka with the activation of a large (~2250 km 2 ) splay that prograded towards the basin centre while maintaining continued bypass along the western pathway. Beginning~5.0 ka, a second splay system constructed an even larger (~3800 km 2 ) lobe into the most distal portions of the basin along the Shillong foredeep. This evolution from a bypass-dominated system to one of enhanced mass extraction is well reflected in (i) the rapid downstream fining of deposited sand and (ii) a change in facies from amalgamated channel deposits to mixed sands and muds within discrete depositional lobes. The persistence of sediment bypass suggests that seasonal flooding of the basin by local runoff exerts a hydrologic barrier to overbank flow and is thus a principal control on river path selection. This control is evidenced by (i) repeated, long-term preference for occupying a course along the basin margin rather than a steeper path to the basin centre and (ii) the persistence of an underfilled, topographically low basin despite sediment load sufficient to fill the basin within a few hundred years. The progradation of two 10-20-m thick, sandy mega-splays into the basin interior reflects an alternative mode of sediment dispersal that appears to have operated only in the mid-Holocene (~6.0-4.0 ka) during a regional weakening of the summer monsoon. The reduced water budget at that time would have lowered seasonal water levels in the basin, temporarily lessening the hydrologic barrier effect and facilitating splay development into the basin interior. Overall, these results place basin hydrology as a first-order control on fluvial system behaviour, strongly modifying the perceived dominance of tectonic subsidence. Such coupling of subsidence, fluvial dynamics and local hydrology have been explored through tank experiments and modelling; this field study demonstrates that complex, emergent behaviours can also scale to the world's largest fluvial system.Deciphering the response of depositional systems to changes in sediment transport dynamics, climate and Correspondence: R. Sincavage,

The set of active rivers of the Ganges-Brahmaputra-Meghna (GBM) Delta in Bangladesh overlies an active plate boundary that continually modifies the landscape of the delta by deformation. The response of rivers to spatially variable subsidence, from tectonic tilting or other causes, has been thought to include preferred occupation of regions of higher subsidence. In this paper, we develop further the theoretical framework for analysis of the interplay of tectonics and river dynamics, and apply this model to conditions in the GBM Delta. First, we examine the overall competition between variable subsidence and channel dynamics, and find that tilting in Bangladesh should be strong enough to influence river path selection. We then present new theory for the effect of subsidence that is spatially (not temporally) variable. We find a constant residence timescale on different parts of the delta, and differing frequencies of avulsion to these locations, and describe the effects of incision or floodplain deposition on these quantities. We present estimates of the channel residence timescale of the Jamuna (Brahmaputra) River reconstructed from the lithology, provenance, and dating of sediment cores. We apply our framework to a map of regional subsidence to predict the effects on avulsion for the Jamuna River. Comparison between our predicted (2150 years) and our stratigraphically based estimates of avulsion timescale (1800 years) shows encouraging consistency.

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