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  1. Abstract

    The riverine transport and deposition of mud is the primary agent of landscape construction and evolution in many fluvial and coastal environments. Previous efforts exploring this process have raised uncertainty regarding the effects of hydrodynamic and chemical controls on the transport and deposition of mud, and thus the constructions of muddy coastal and upstream environments. As such, direct measurements are necessary to constrain the deposition of mud by river systems. Here, we combine laboratory evidence and a field investigation in the Mississippi River delta to explore the controls on the riverine transport and deposition of mud. We show that the flocculation of mud, with floc diameters greater than 10 μm, in freshwater is a ubiquitous phenomenon, causing the sedimentation of mud to be driven by changes in local hydrodynamics, and thus providing an explanation for how river systems construct landscapes through the deposition of mud in both coastal and upstream environments.

     
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  2. Abstract

    The Mississippi River is the largest commercial waterway in North America and one of the most heavily engineered rivers in the world. Future alteration of the river’s hydrology by climate change may increase the vulnerability of flood mitigation and navigation infrastructure implemented to constrain 20thcentury discharge conditions. Here, we evaluate changes in Lower Mississippi River basin hydroclimate and discharge from 1920–2100 C.E. by integrating river gauge observations and climate model ensemble simulations from CESM1.2 under multiple greenhouse gas emissions scenarios. We show that the Lower Mississippi River’s flood regime is highly sensitive to emissions scenario; specifically, the return period of flood discharge exceeding existing flood mitigation infrastructure decreases from approximately 1000 years to 31 years by the year 2100 under RCP8.5 forcing, primarily driven by increasing precipitation and runoff within the basin. Without aggressive reductions in greenhouse gas emissions, flood mitigation infrastructure may require substantial retrofitting to avoid disruptions to industries and communities along the Lower Mississippi River.

     
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  3. At a global scale, delta morphologies are subject to rapid change as a result of direct and indirect effects of human activity. This jeopardizes the ecosystem services of deltas, including protection against flood hazards, facilitation of navigation, and biodiversity. Direct manifestations of delta morphological instability include river bank failure, which may lead to avulsion, persistent channel incision or aggregation, and a change of the sedimentary regime to hyperturbid conditions. Notwithstanding the in‐depth knowledge developed over the past decades about those topics, existing understanding is fragmented, and the predictive capacity of morphodynamic models is limited. The advancement of potential resilience analysis tools may proceed from improved models, continuous observations, and the application of novel analysis techniques. Progress will benefit from synergy between approaches. Empirical and numerical models are built using field observations, and, in turn, model simulations can inform observationists about where to measure. Information theory offers a systematic approach to test the realism of alternative model concepts. Once the key mechanism responsible for a morphodynamic instability phenomenon is understood, concepts from dynamic system theory can be employed to develop early warning indicators. In the development of reliable tools to design resilient deltas, one of the first challenges is to close the sediment balance at multiple scales, such that morphodynamic model predictions match with fully independent measurements. Such a high ambition level is rarely adopted and is urgently needed to address the ongoing global changes causing sea level rise and reduced sediment input by reservoir building.

     
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  4. Abstract

    Upon avulsion, abandoned deltaic distributary channels receive water and sediment delivered by a tie channel, overbank flow, and by tidal inundation from the receiving basin. The transport and deposition of sediment arising from this latter input have important impacts on delta development yet are not well constrained from field observations or numerical models. Herein, the Huanghe (Yellow River) delta, China, is used as a case study to evaluate how marine‐sourced sediment impacts abandoned channel morphology. For this system, artificial deltaic avulsions occur approximately decadally; the abandoned channels are inundated by tides, and deposition of sediment transforms the channel into a mudflat. Field data were collected from a channel abandoned 20 yr ago and included cores that penetrated the tidally deposited mud and antecedent fluvial channel sediment, topography, bathymetry surveys, and detailed time series monitoring of hydrodynamic conditions within the tidal channel and adjacent mudflat. These data are used to validate a model that predicts the rate of accumulation and grain size of sediment delivered from the tidal channel to the mudflat. The thickness of the marine‐sourced mud differs spatially by an order of magnitude and is primarily impacted by antecedent channel topography. Sediment has aggraded to an elevation approaching mean high tide, which is likely the limit of fill. As this elevation is below antecedent levees, assuming stationary relative sea level, the abandoned channel will remain a topographic low on the delta landscape and is therefore susceptible to reoccupation during future avulsions.

     
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