Effects of tectonic deformation and sea level on river path selection: Theory and application to the Ganges-Brahmaputra-Meghna River Delta

Abstract

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.

Subsidence and human influences in mega deltas: The case of the Ganges–Brahmaputra–Meghna

“…This data, any newly published data (e.g. Reitz et al, 2015), plus the results presented in this paper, could form part of an additional study to determine the more detailed reasons as to why subsidence has occurred and what factors could control future rates. Lastly, these methods could be applied to other deltas where subsidence is important, but systematic studies are unavailable.…”

How much subsidence is enough to change the morphology of river deltas?

“…Understanding subsidence-induced changes is relevant to coastal restoration design, flood mitigation, and interpretation of sedimentary records. The theory and its extensions have been recently applied to experimental deltas [Kopp and Kim, 2014] and natural systems [Reitz et al, 2015]. The "channel-to-tectonic timescale ratio" theory [Hickson et al, 2005;Kim et al, 2010;Straub and Esposito, 2013] compares the rate at which tectonic subsidence generates lateral gradients to the rate at which channels rework the fluvial surface.…”

The Planform Mobility of River Channel Confluences: Insights from Analysis of Remotely Sensed Imagery

“…The Ganges-Jamuna junction is perhaps the type example of this type of environment. This river system has high discharges and sediment loads driven by high uplift rates in the Himalayas, monsoonaldominated floods, coupled with ongoing subsidence in the Bengal Foredeep (Goodbred and Kuehl, 1999;Goodbred et al, 2003;Reitz et al, 2015) promoting basin wide deposition . High rates of channel and bar migration are present, with the Jamuna River being particularly dynamic even where kilometre scale bars are extremely mobile, which may migrate up to 3 km yr − 1 .…”

“…However, some case studies, such as the confluence of the Ganges and Jamuna rivers, Bangladesh, show junction migration over distances of several kilometres in a year (Best and Ashworth, 1997). In addition, the course of the Jamuna River has also been shown to avulse on centennial to millennial timescales Pickering et al, 2014;Reitz et al, 2015), thus changing the location of its confluence with the Ganges River by hundreds of kilometres. High-angle confluences in meandering rivers have also been demonstrated to adjust their confluence planform over decadal timescales (Riley, 2013).…”