Timescales of formative discharge and principal controls on the channel width of the Jamuna River, Bangladesh

Abstract

To understand the relationship between variable flood flow and channel width of the Jamuna River, we developed a five-decade-long, nearly annual temporal resolution delineation of channel width from analysis of satellite imagery and compared this with a daily discharge time series. We show narrow channel conditions in the 1970s and early 1980s, rapid widening in the late 1980s through early 1990s, and a period of channel narrowing since about 2010 that corresponds to decadal-scale shifts in the hydroclimate. We identify the maximum 91-day average discharge for the single previous season's monsoon flood, the average of which is approximately the long-term geomorphically effective discharge for the river, as a strong control over channel width. This empirically determined relation closely fits a hydraulic geometry prediction of active channel width for the discharge divided into three to five principal anabranches. We also show that the inherited width (the width from 1 year previous) and the intensity of revetments explain some variability in the observed channel width. This analysis outlines a compelling alternative hypothesis to the dominant narrative that Jamuna River widening was a response to the Great Assam Earthquake sediment wave. These findings have major implications for ongoing efforts to understand and manage morphodynamically active rivers around the world: they suggest researchers and managers in such environments must consider the potential effects of variable hydroclimate on channel morphology over annual to decadal timescales. Of course, they are also especially relevant to the management of the Jamuna River, where these findings suggest efforts to reclaim land and return the channel to narrower widths observed in the 1970s, a period of notably low flood flows, may increase flood risk by increasing exposure of densely settled areas to channel migration and related embankment-failure flood hazards in wetter hydroclimate periods resulting from natural variability or climate change.