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Rivers & Sediments

OVERVIEW

The Ganges and Brahmaputra rivers drain 3/4 of the Himalayan mountain range, carrying ~ 1,000,000,000 metric tons of sediment to the Bengal Basin, or ~7% of the world flux of sediment to the oceans. In the country of Bangladesh, these two great rivers combine with the Meghna River to form the Ganges-Brahmaputra-Meghna Delta (GBM), which feeds the Bengal Fan, the largest depositional system in the world. The delta front is so active that it advances across the shelf at 15 meters per year, and since the Eocene (35 Ma) this enormous sediment supply has built the entire continental margin >300 km seaward. The great rivers carrying this sediment load are equally dynamic. Two centuries ago, the course of the Brahmaputra River avulsed westward by up to 100 km from the Old Brahmaputra channel (Fig. 1) and has since captured other major rivers. The primary mouth of the Ganges has shifted 100s of km eastward from the Hooghly River near Calcutta to other channels over the last 400 years, finally joining the Brahmaputra River in the 19th century.

SCIENTIFIC OBJECTIVES

River Avulsions: River avulsion (abrupt lateral changes in channel path) is both a fundamental process in river dynamics and an important geohazard for populated alluvial landscapes. The GBMD is especially appealing in that it provides the opportunity to study multiple recent river avulsions and their resulting stratigraphic architecture in a region with complex, active tectonics. While we will not be able to provide predictions of future avulsions, given the complex interplay of tectonics and channel dynamics in the GBMD, even statistical constraints on avulsion frequency and spatial scales (e.g., jump distance and length) would be valuable in improving risk estimation, especially given how much is at stake in Bangladesh.

Basin Dynamics and Evolution: Sedimentary basins are among the world's most important archives of geological history – a lens through which the complexity of earth-system processes are revealed and understood, including histories of orogenic development, paleoseismology, crustal strain, material fluxes, fluvial drainage patterns, and climate change. Sedimentary basins also host oil, gas, groundwater, and soil resources that sustain large human populations and economies, often in environmentally sensitive and hazard-prone settings. Now humans add a new level of complexity in becoming the dominant agent of change through river damming and diversion, groundwater and petroleum extraction, construction of flood-control devices, and engineering of river channels. Thus, an overarching goal that stems from all of our tectonics and rivers objectives is to improve the general understanding of basin evolution and the complex fluvial-tectonic-sedimentary interactions that define basin architecture and behavior.

Human Dimensions: In addition to these specific research topics, the tectono-sedimentary history we will obtain for the GBMD defines the geological template upon which other hazards play out, including flooding patterns, salt water intrusion, coastal land loss, and arsenic contamination. For example, efforts to model arsenic distribution through groundwater models – and thus guide mitigation strategies – have had to proceed with insufficient constraints on aquifer connectivity. These and other practical issues center around the stratigraphic architecture and development of the GBMD, which are poorly known or understood due to a lack of field data, and of fundamental knowledge on large river response to dynamic forcings of climate and tectonics.

APPROACH

To understand these integrated controls on GBMD behavior, we will compare the sedimentary and geophysical records of abrupt historical events with those millennial-scale environmental changes. Such complete sedimentary-basin records remain virtually unexplored in Bangladesh, or in any other large, tectonically active, braided stream delta. Well-known events like the 1950 Assam earthquake, the late 18th century Brahmaputra avulsion, and the 20-m lowering of groundwater around Dhaka city will demonstrate how such occurrences are manifested in the GBMD stratigraphy, for example by liquefaction features, channel to floodplain transitions, or shallow sediment compaction. Early Holocene megafloods have been documented from the Brahmaputra catchment, but the impact and record of these floods, if any, have not yet been investigated in the GBMD. The record of these abrupt events of the last few hundred years will be compared with the GBMD stratigraphic record of longer-term changes, such as monsoon-driven variations in river discharge and sediment flux, crustal shortening and flexure, and Holocene sea-level rise. Factors such as the ratio of channel:floodplain deposits, the length scale of channel/bar features, and maximum grain size, as well as stratigraphic tilting, deformation, and faulting are targeted indicators of climatic and tectonic forcing mechanisms, respectively.

Drilling. We will drill a series of detailed tubewell transects up to 100 m deep across the GBMD's buried lowstand valleys and tectonic structures using a rapid, inexpensive local drilling technique. The tubewells are constructed with a bamboo fulcrum-and-lever system that is used to lift and drop the drill pipe in 10-ft sections. To drive circulation of the drill fluid, one operator stands on the bamboo frame and uses his hand as a check valve. This hand seal allows the drill fluid to be lifted along with the drill pipe, and then the drill fluid is expelled at the top as the pipe is dropped. This expelled drill fluid contains the sediment cuttings, or ‘wash’ samples, that are transported up the drill string and collected for analysis. The method is surprisingly effective and can recover even coarse sediments with no apparent sorting effects due to the viscous drill fluid

Experiments and theory. The fieldwork will be complemented by a program of experimental and theoretical work on the critical processes: tectonic deformation, subsidence, and fluvial response. Some of the experiments will be conducted at University of Minnesota’s unique Experimental EarthScape subsiding-floor basin (XES or “Jurassic Tank”) that address the effect of spatially variable subsidence and uplift on the evolution of braided rivers, which will be compared with the data from Bangladesh. We are also developing a range of analytical and numerical delta models that address issues such as development of the channel network, avulsion, and the effects of subsidence


 Figure 1. Digital Elevation Model of the Bengal Basin.