[1] River delta complexes are built in part through repeated river-channel avulsions, which often occur about a persistent spatial node creating delta lobes that form a fan-like morphology. Predicting the location of avulsions is poorly understood, but it is essential for wetland restoration, hazard mitigation, reservoir characterization, and delta morphodynamics. Following previous work, we show that the upstream distance from the river mouth where avulsions occur is coincident with the backwater length, i.e., the upstream extent of river flow that is affected by hydrodynamic processes in the receiving basin. To explain this observation we formulate a fluvial morphodynamic model that is coupled to an offshore spreading river plume and subject it to a range of river discharges. Results show that avulsion is less likely in the downstream portion of the backwater zone because, during high-flow events, the water surface is drawn down near the river mouth to match that of the offshore plume, resulting in river-bed scour and a reduced likelihood of overbank flow. Furthermore, during low-discharge events, flow deceleration near the upstream extent of backwater causes enhanced deposition locally and a reduced channel-fill timescale there. Both mechanisms favor preferential avulsion in the upstream part of the backwater zone. These dynamics are fundamentally due to variable river discharges and a coupled offshore river plume, with implications for predicting delta response to climate and sea level change, and fluvio-deltaic stratigraphy.

[1] Sediment flux from rivers to oceans is the fundamental driver of fluvio-deltaic morphodynamics and continental margin sedimentation, yet sediment transport across the river-to-marine boundary is poorly understood. Coastal rivers typically are affected by backwater, a zone of spatially decelerating flow that is transitional between normal flow upstream and the offshore river plume. Flow deceleration in the backwater zone, as well as spreading of the offshore plume, should render rivers highly depositional near their mouths, leading to sedimentation and eventual elimination of the backwater zone at steady state. This reasoning is counter to observations of riverbed scour, erosional bed forms, and long-lived backwater zones near the mouths of some coastal rivers (e.g., Mississippi River, United States). To explain these observations, we present a quasi-2-D model of a coupled fluvial backwater and offshore river plume system and apply it to the Mississippi River. Results show that during high-discharge events the normal-flow depth can become larger than the water depth at the river mouth resulting in drawdown of the water surface, spatial acceleration of flow, and erosion of the riverbed. As proposed by Lane (1957), the transition to drawdown and erosion is ultimately forced by spreading of the offshore river plume. This points to the need to model coupled river and river plume systems with a dynamic backwater zone under a suite of discharges to accurately capture fluvio-deltaic morphodynamics and connectivity between fluvial sediment sources and marine depositional sinks.Citation: Lamb, M. P., J. A. Nittrouer, D. Mohrig, and J. Shaw (2012), Backwater and river plume controls on scour upstream of river mouths: Implications for fluvio-deltaic morphodynamics,

Where rivers near the coastline, the receiving basin begins to infl uence fl ow, and gradually varied, nonuniform fl ow conditions arise. The section of the river affected by nonuniform fl ow is typically referred to as the backwater segment, and for large lowland rivers, this portion of the river can extend many hundreds of kilometers above the outlet. River morphology and kine matics vary in the backwater segment; however, these channel properties have not been explicitly related to properties of the fl ow and sediment-transport fi elds. This study examines the infl uence of spatially and temporally varying fl ow velocity and sediment transport on channel properties for the lower 800 km of the Mississippi River, a section of the river that includes the backwater segment. Survey transects (n = 2650) were used to constrain the cross-sectional area of water fl ow every ~312 m along the Mississippi River channel for eight successive intervals of water discharge. Assuming conservation of water discharge, the local fl ow velocity was calculated at each transect by dividing water discharge by the local measurement of cross-sectional fl ow area. Calculated fl ow velocity was converted to total bed stress using a dimensionless friction coeffi cient that was determined by optimizing the match between a predicted and a measured water-surface profi le. Estimates for the skin-friction component of the total bed stress were produced from the values for total shear stress using a form-drag correction. These skin-friction bed-stress values were then used to model bed-material transport. Results demonstrate that in the lower Mississippi River, cross-sectional fl ow area increases downstream during low-and moderate-water discharge. This generates a decrease in calculated water-fl ow velocity and bed-material transport. During highwater discharge, the trend is reversed: Crosssectional fl ow area decreases downstream, producing an increase in calculated waterfl ow velocity and bed-material transport. An important contribution of this work is the identifi cation of a downstream reversal in the trend for channel cross-sectional area due to variable water discharge. By accounting for the spatial divergences in sediment transport predicted over an average annual hydrograph, we demonstrate the tendency for channel-bed aggradation in much of the backwater reach of the Mississippi River (150-600 km above the outlet); however, a region of channel-bed erosion is calculated for the fi nal 150 km. These results help to explain the spatial variability of channel morphology and kinematics for the lower Mississippi River, and they can be extended to other lowland river systems near the coastline.

A mechanistic understanding of river avulsion location and frequency is needed to predict the growth of alluvial fans and deltas. The Huanghe, China, provides a rare opportunity to test emerging theories because its high sediment load produces regular avulsions at two distinct nodes. Where the river debouches from the Loess Plateau, avulsions occur at an abrupt decrease in bed slope and reoccur at a time interval (607 years) consistent with a channel-filling timescale set by the superelevation height of the levees. Downstream, natural deltaic avulsions reoccur at a timescale that is fast (7 years) compared to channel-filling timescale due to large stage-height variability during floods. Unlike the upstream node, deltaic avulsions cluster at a location influenced by backwater hydrodynamics and show evidence for episodic downstream migration in concert with progradation of the shoreline, providing new expectations for the interplay between avulsion location, frequency, shoreline rugosity, and delta morphology.

[1] New methods of data collection and processing are developed to provide quantitative, reach-scale measurements of bedform transport mass within the tidally influenced Mississippi River. A multibeam swath profiler was used to collect daily bathymetry over a range of water discharges, and bed elevation changes induced by dune migration are measured. These values are coupled with bulk physical properties of the bed sediment to constrain mass flux, and annual bedform transport is estimated at 2.2 Â 10 6 metric tons (MT). The total annual sand flux from the Mississippi River, calculated by combining measured bedform transport rates and suspended sediment flux, is estimated to be 20 Â 10 6 MT. Survey data also provide information about the spatial distribution of dunes across the channel bottom. Straight reach segments are commonly mantled by dunes for the entire cross section, while bends are typically areas of focused scour devoid of bedforms. Presumably, any sediments associated with migrating dunes are propelled into suspension within bends before redepositing in the subsequent straight reach. Movement via suspension is therefore an important component for the downriver transport of bed materials in the lower Mississippi River.

River deltas are built by cycles of lobe growth and abrupt channel shifts, or avulsions, that occur within the backwater zone of coastal rivers. Previous numerical models differ on the origin of backwater‐scaled avulsion nodes and their consistency with experimental data. To unify previous work, we developed a numerical model of delta growth that includes backwater hydrodynamics, river mouth progradation, relative sea level rise, variable flow regimes, and cycles of lobe growth, abandonment, and reoccupation. For parameter space applicable to lowland deltas, we found that flow variability is the primary mechanism to cause persistent avulsion nodes by focusing aggradation within the backwater zone. Backwater‐scaled avulsion nodes also occur under less likely scenarios of initially uniform bed slopes or during rapid relative sea level rise and marine transgression. Our findings suggest that flow variability is a fundamental control on long‐term delta morphodynamics.

In this study, the distribution of channel‐bed sediment facies in the lowermost Mississippi River is analysed using multibeam data, complemented by sidescan sonar and compressed high‐intensity radar pulse seismic data, as well as grab and core samples of bed material. The channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel bed and sidewalls. The consolidated substratum is made up of latest Pleistocene and Early Holocene fluvio‐deltaic deposits and is preferentially exposed in the deepest thalweg segments and on channel sidewalls in river bends. The exposed substratum commonly displays a suite of erosional features, including flutes that are quantitatively similar in form to those produced under known laboratory conditions. A total of five bed facies are mapped, three of which include modern alluvial deposits and two facies that are associated with the relict substratum. A radius of curvature analysis applied to the Mississippi River centreline demonstrates that the reach‐scale distribution of channel‐bed facies is related to river planform. From a broader perspective, the distribution of channel‐bed facies is related to channel sinuosity — higher sinuosity promotes greater substratum exposure at the expense of alluvial sediment. For example, the ratio of alluvial cover to substratum is ca 1·5:1 for a 45 km segment of the river that has a sinuosity of 1·76 and this ratio increases to ca 3:1 for a 120 km segment of the river that has a sinuosity of 1·21. The exposed substratum is interpreted as bedrock and, given the relative coverage of alluvial sediment in the channel, the lowermost Mississippi River can be classified as a mixed bedrock‐alluvial channel. The analyses demonstrate that a mixed bedrock‐alluvial channel boundary can be associated with low‐gradient and sand‐bed rivers near their marine outlet.

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