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[1] Using numerical experiments, we investigate how river-dominated delta channel networks are likely to respond to changes in river discharge predicted to occur over the next century as a result of environmental change. Our results show for a change in discharge up to 60% of the initial value, a decrease results in distributary abandonment in the delta, whereas an increase does not significantly affect the network. However, an increase in discharge beyond a threshold of 60% results in channel creation and an increase in the density of the distributary network. This behavior is predicted by an analysis of an individual bifurcation subject to asymmetric water surface slopes in the bifurcate arms. Given that discharge in most river basins will change by less than 50% in the next century, our results suggest that deltas in areas of increased drought will be more likely to experience significant rearrangement of the delta channel network. Motivation[2] An immediate impact of climate change and human modifications to river catchments is that the long-term average discharge of most rivers will change appreciably over the next century [Nohara et al., 2006;Palmer et al., 2008]. On rivers that terminate in marine or lacustrine basins, this hydrological change also will impact their deltas, and yet no framework exists for predicting the adjustments deltas may undergo as discharge changes. Relative sea-level rise is already threatening the world's deltas [Blum and Roberts, 2009;Syvitski et al., 2009], and that problem could be further compounded if permanent changes in discharge also dramatically alter delta landscapes, which provide wetlands, biodiversity,, and homes to a significant part of the world's population [Coleman, 1988;Day et al., 2007]. Studies to date have focused on identifying which deltas are at risk [Day et al., 1995;Ericson et al., 2006] and how they might respond to perturbations such as sea-level rise [Jerolmack, 2009]. But, how deltaic distributary networks will respond to changes in river discharge remains unknown.[3] As the long-term average river discharge (Q) changes, the most dramatic response of a deltaic channel network, apart from a full avulsion caused by delta-lobe switching [Coleman, 1988], is abandonment or initiation of distributary channels. This scenario could be expected because the number of bifurcations in a distributary network correlates positively with the input Q and inversely with the power in the marine environment [Syvitski and Saito, 2007]. However, it is unclear whether an individual delta's response to changes in Q would follow the same trend between Q and channel number observed from the Syvitski and Saito delta compilation.[4] Past work has shown that the splitting of discharges at channel bifurcations is subject to fairly stringent stability conditions. Two distributaries with identical downstream boundary conditions and a given upstream Shields stress (Q) distribute water and sediment asymmetrically because this is a stable equilibrium configuration [Wang et al., 1995;S...

Sediment is the most valuable natural resource for deltaic environments because it is required to build new land. For land building to occur, sediment must be retained in the delta instead of being transported offshore. Despite this, we do not know what controls sediment retention within a delta. Here we use a calibrated numerical model of Wax Lake Delta, Louisiana, USA to analyze sediment retention for different riverine flood magnitudes, tidal amplitudes, and vegetation extents. Our results show that as riverine flood magnitude increases, areally averaged vertical accretion increases from 0.33 to 2 cm per 60-day flood, but sediment retention decreases from 72% to 34%. For the uniform vegetation characteristics considered, the buffering effect, defined as the reduction of sediment flux onto the islands in the presence of vegetation, reduces the sediment flux onto the islands 14 to 22% on a fully vegetated delta. When sediment is transported onto the islands, vegetation enhances retention, which we refer to as the trapping effect, bỹ 10%. But, this does not offset the buffering effect, and vegetation decreases vertical accretion and retention in the delta up to 6% (or~0.5 cm per 60-day flood). We suggest that vegetation will increase sedimentation only when trapping compensates for buffering. Finally, greater tidal amplitude at higher discharges enhances vertical accretion by~0.5 cm per 60-day flood compared to smaller tidal amplitudes. These results provide insight on the mechanisms behind coastal systems growth, and suggest how sediment diversions might be operated more efficiently in deltas with reduced sediment supply.

Mutual adjustment between process and form shapes the morphology of alluvial river channels, including channel banks. The tops of banks define the transition between the channel and adjacent floodplain, which corresponds to the level of incipient flooding. Despite the geomorphological and hydrological importance of this transition, few, if any, studies have extensively examined spatial variability in bank elevations and its influence on bankfull stage. This study uses an objective method to explore this variability at two spatial resolutions along three alluvial lowland meandering rivers. Results show that variability in bankfull stage is inherent to all three rivers. The mean variability of bankfull stage about the average downstream gradient in this stage is 10% to 20% of mean bankfull depth. Elevations of channel banks exhibit similar variability, even after accounting for systematic variations in heights of inner and outer banks associated with river meandering. Two‐dimensional hydraulic simulations show that the elevation range of mean variability in bankfull stage overlaps considerably with the elevation range of high curvature on rating curves, confirming that variability in bankfull stage influences the shape of these curves. The simulations verify that breaks in channel banks allow flow to extend onto the floodplain at stages below the average bankfull stage. The findings provide fundamental insight into the variable nature of bankfull conditions along meandering rivers and the role of this variability in channel‐floodplain connectivity. The results also inform river‐restoration efforts that seek to re‐establish the natural configuration of channel banks.

River deltas sit at the interface of the terrestrial and ocean environments where relative sea-level rise causes flooding and possibly shoreline retreat. Since 1993, relative sea-level rise has averaged ∼3.1 mm yr −1 globally (Thompson et al., 2019), and this is predicted to cause more frequent flooding in low-lying, densely populated deltas due to storm surges (Edmonds et al., 2020;Hirabayashi et al., 2013;Muis et al., 2016). To counteract relative sea-level rise and potentially mitigate flooding or shoreline retreat, river deltas can adjust their surface elevation though deposition of organic and inorganic sediment in the interdistributary islands that host the existing wetlands (Paola et al., 2011). Sediment deposition on a deltaic surface is non-uniform at short time scales (e.g., Bevington et al., 2017;Nardin et al., 2016;Nienhuis et al., 2018), but at some point, the entire surface must aggrade by some fixed amount to counteract relative sea-level rise.

Levees are commonly found along every kind of river system, yet there are no widely accepted models for where along the channel they form and what controls their shape. In this study, we investigated whether levee growth is driven by sediment transfer from the channel adjacent to the levee or by inundation dynamics in the flood basin. To test these ideas, we conducted empirical analyses and numerical modeling of levees on the fine-grained, meandering Muscatatuck River, IN. Using LiDAR data, we found no statistical relationship between the levee and the adjacent channel planform, which suggests levees are not genetically related to their adjacent channel. On the contrary, modeling experiments of a simplified Muscatatuck River show that levee initiation can be genetically related to the adjacent channel because bed shear stress on the floodplain is low where channel curvature is high. But after levees initiate, the genetic connection to the adjacent channel is obscured because levee shape is modified by inundation dynamics. For instance, tall mature levees are not inundated regularly and instead obstruct floodplain flow, creating flow shadows on the downstream side. Sediment is preferentially deposited in the flow shadow, which moves the location of maximum deposition from the levee crest to the toe. This causes levees to prograde down-valley, which reshapes the levee and genetically disconnects it from the channel. We propose that this morphodynamic mechanism of levee growth is characteristic of fine-grained rivers in narrow floodplains where flood basins can act as conveyance channels that transport sediment down-valley before deposition. Key Points:• Empirical analyses of LiDAR data on the Muscatatuck River, Indiana, USA, show that levee size and shape are unrelated to channel planform • Numerical modeling reveals that levee initiation is related to the channel planform but subsequent growth is influenced by inundation dynamics in the flood basin • On tall levees, the levee toe grows faster than the levee crest because it is inundated more frequently, and this causes levees to prograde down-valley over time

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