Avulsion Dynamics Research Articles

Avulsion dynamics determine fluvial fan morphology in a cellular model

Abstract Fluvial fans are large, low-gradient depositional systems that occur in sedimentary basins worldwide. Fluvial fans can represent much of the geologic record of foreland basins, create hazards, and record paleoclimate and tectonic signals. However, we lack an understanding of how fluvial fans grow into the variety of shapes observed around the world. We explored this aspect using a cellular model of foreland basin landscape evolution with rules for sediment transport, river avulsion, and floodplain processes. We tested the hypothesis that avulsion dynamics, namely, avulsion trigger period and abandoned channel dynamics, are a primary control on fluvial fan development. We found that shorter trigger periods lead to rounder planform fluvial fan shapes because, between avulsions, channel aggradation (and thus avulsion setup) propagates shorter distances from the upstream boundary along channel pathways. This prioritizes lateral sediment dispersion, creating shorter, rounder fans, over sediment delivery further into the basin, which would create elongated fans. Modeled fans with abandoned channel attraction (but not repulsion) generated a commonly observed abrupt fan boundary marked by a transition from distributary to tributary channel patterns. While fluvial fans are thought to be linked to climate, they can occur anywhere that rivers aggrade, lose lateral confinement, and preserve alluvial topography. Instead, fluvial fans might be more recognizable in environments that frequently trigger avulsions and preserve abandoned channels that capture future avulsions.

The push and pull of abandoned channels: how floodplain processes and healing affect avulsion dynamics and alluvial landscape evolution in foreland basins

Abstract. River avulsions are an important mechanism by which sediment is routed and emplaced in foreland basins. However, because avulsions occur infrequently, we lack observational data that might inform where, when, and why avulsions occur and these issues are instead often investigated by rule-based numerical models. These models have historically simplified or neglected the effects of abandoned channels on avulsion dynamics, even though fluvial megafans in foreland basins are characteristically covered in abandoned channels. Here, we investigate the pervasiveness of abandoned channels on modern fluvial megafan surfaces. Then, we present a physically based cellular model that parameterizes interactions between a single avulsing river and abandoned channels in a foreland basin setting. We investigate how abandoned channels affect avulsion setup, pathfinding, and landscape evolution. We demonstrate and discuss how the processes of abandoned channel inheritance and transient knickpoint propagation post-avulsion serve to shortcut the time necessary to set up successive avulsions. Then, we address the idea that abandoned channels can both repel and attract future pathfinding flows under different conditions. By measuring the distance between the mountain front and each avulsion over long (106 to 107 years) timescales, we show that increasing abandoned channel repulsion serves to push avulsions farther from the mountain front, while increasing attraction pulls avulsions proximally. Abandoned channels do not persist forever, and we test possible channel healing scenarios (deposition-only, erosion-only, and far-field-directed) and show that only the final scenario achieves dynamic equilibrium without completely filling accommodation space. We also observe megafan growth occurring via ∼100 000-year cycles of lobe switching but only in our runs that employ deposition-only or erosion-only healing modes. Finally, we highlight opportunities for future field work and remote sensing efforts to inform our understanding of the role that floodplain topography, including abandoned channels, plays on avulsion dynamics.

Quantifying river avulsion activity from satellite remote sensing: Implications for how avulsions contribute to floodplain stratigraphy in foreland basins

ABSTRACTThe rarely witnessed process of river avulsion repositions channels across floodplains, which influences floodplain geomorphology and stratigraphic architecture. The way avulsions redirect water and sediment is typically generalized into one of two styles. Avulsions proceeding through rapid channel switching and producing little to no floodplain disturbance are annexational, while those that involve sequential phases of crevassing, flooding, and eventual development of a new channel are progradational. We test the validity of these avulsion style categories by mapping and characterizing 14 avulsion events in Andean, Himalayan, and New Guinean foreland basins. We use Landsat data to identify how avulsions proceed and interpret the possible products of these processes in terms of geomorphic features and stratigraphy. We show that during annexation the avulsion channel widens, changes its meander wavelength and amplitude, or increases channel thread count. During progradation, avulsion channels are constructed from evolving distributary networks. Often beginning as crevasse splays, these networks migrate down the floodplain gradient and frequently create and fill ponds during the process. We also see evidence for a recently defined third avulsion style. Retrogradation involves overbank flow, like progradation, but is marked by an upstream-migrating abandonment and infilling of the parent channel. Avulsion belts in this study range from 5 to 60 km in length, and from 1 to 50 km in width. On average, these events demonstrate annexational style over 22.4% of their length. Eleven of 13 events either begin or end with annexation, and seven both begin and end with annexation. Only one event exhibited progradation over the entire avulsion-belt length. While there are many documented examples of purely annexational avulsions, we see little evidence for completely progradational or retrogradational avulsions, and instead suggest that a given avulsion is better envisioned as a series of spatiotemporal phases of annexation, progradation, and retrogradation. Such hybrid avulsions likely produce significantly greater stratigraphic variability than that predicted by the traditional end-member model. We suggest that a time-averaged, formation-scale consideration of avulsion products will yield more accurate characterizations of avulsion dynamics in ancient fluvial systems.

Discharge Determines Avulsion Regime in Model Experiments With Vegetated and Unvegetated Deltas

AbstractThe dynamics and morphological evolution of deltas and their channel networks involve interactions between many factors, including water and sediment discharge and cohesion from fine sediment and vegetation. These interactions are likely to affect how strongly vegetation influences deltas. Altering water or sediment discharge may affect channel mobility, magnitude of deposition and erosion events, and may result in the delta being less suitable for vegetation colonization. Using the numerical model DeltaRCM Vegetation, we explore how water and sediment discharge affects delta evolution and under which conditions vegetation exerts a stabilizing effect on the channel network. We propose new insights into delta evolution under different discharge conditions. First, we observe a regime shift in avulsion dynamics, driven by delta‐scale water surface slopes, with increasing water discharge: from a few active channels undergoing occasional complete, global avulsions (with low discharge) to many active channels experiencing frequent partial, local avulsions (with high discharge). Second, with vegetation, increased sediment discharge results in more frequent switching of the dominant channels but also prevents vegetation from establishing in non‐dominant channels, resulting in more frequent channel reoccupation and therefore greater stability in channel network planform. These insights have important implications for understanding the distribution of water, sediment, and nutrients on deltas in the face of future changes in climate and human modifications of fluxes of sediment and water to the coast, especially for restored or engineered deltas with controlled water or sediment discharges.

Influences on Discharge Partitioning on a Large River Delta: Case Study of the Mississippi‐Atchafalaya Diversion, 1926–1950

AbstractThe modern Mississippi River (M.R.) Delta is plumbed by the Mississippi and Atchafalaya rivers, setting water and sediment dispersal pathways for Earth’s fifth‐largest river system. The Atchafalaya River’s (A.R.) partial annexation of discharge from the M.R., particularly between 1926 and 1950, prompted warnings of a rapid river avulsion and the construction of the Old River Control Structure to regulate flow. Natural and anthropogenic causes of this discharge annexation are difficult to disentangle. Here, we develop and validate a hydrodynamic model of flow partitioning through the historic channel network. We then isolate how several key changes to the system affected discharge partitioning and stage at the A.R.‐M.R. diversion. Simulations show that erosion of the upper A.R. can account for 73% of the water discharge increase. Dredging in the lower A.R. between 1932 and 1950 can account for 35% of the water discharge increase, and was also an important control on shear stress distribution. The lower M.R. was slightly erosional during this period, and therefore hindered the A.R. discharge increase. Significant lacustrine delta deposition in A.R. had little effect on partitioning. These findings highlight the importance of A.R. enlargement processes on avulsion dynamics at this site. Given the essential nature of this river junction to the society, transportation, and commerce of the United States, improved attribution of discharge increases may lead to future management strategies that are broadly impactful.

Modeling long-term delta dynamics reveals persistent geometric river avulsion locations

River deltas grow through repeated stacking of sedimentary lobes, the location and size of which are determined by channel avulsions (relatively sudden changes in river course). We use a model coupling fluvial and coastal processes to explore avulsion dynamics under a range of wave energies and sea-level-rise rates and find that the primary control on avulsion location and delta lobe size in our model is the critical superelevation ratio (SER), the amount of channel aggradation relative to the surrounding floodplain that is required to trigger an avulsion. The preferred avulsion location arises because of geometric constraints – a preferential avulsion node occurs at the break in floodplain slope that develops as the river progrades and/or sea level rises. This concavity develops in our model because the river profile aggrades and erodes via linear diffusion, whereas the diffusion of the floodplain topography is limited to episodic crevasse splays. These results are in contrast to recent modeling work, which was motivated by laboratory experiments and assumes a union between river channel and floodplain aggradation rates, and where avulsion nodes are driven by backwater hydrodynamics. The preferred avulsion length in our model scales well with laboratory, field, and model results without including hydrodynamic backwater effects. This work suggests an alternative mechanism to explain avulsion locations on deltas where floodplain topography aggrades and/or diffuses more slowly than the river channel profile, and it points to the need to elucidate river channel and floodplain connectivity over large space and time scales, and how the connectivity varies from one type of delta to another.

Comparing Aggradation, Superelevation, and Avulsion Frequency of Submarine and Fluvial Channels

Constraining the avulsion dynamics of rivers and submarine channels is essential for predicting the distribution of sediment, organic matter, and pollutants in alluvial, deltaic, and submarine settings. We create a geometric channel-belt framework relating channel, levee, and floodplain stratigraphy that allows comparative analysis of avulsion dynamics for rivers and submarine channels. We utilize 52 channel-overbank cross sections within this framework to provide avulsion criteria for submarine channels, and how they differ from rivers. Superelevation and a new channel-floodplain coupling metric are two key parameters that control channel-belt thickness in both rivers and submarine channels. While rivers only superelevate 1 channel depth above the floodplain prior to avulsion, submarine channels are more stable during aggradation, with superelevation values commonly >3 channel depths. Additionally, channel-floodplain coupling in rivers is often weak, with floodplain aggradation negligible compared to channel aggradation, making rivers avulsion-prone. However, floodplain aggradation is more significant for submarine channels, resulting in stronger channel-floodplain coupling and thus a decreased potential for avulsion. The combination of enhanced superelevation and strong channel-floodplain coupling results in submarine channel-belts that can be as thick as ~ 10 channel depths, while fluvial channel belts are limited to 2 channel depths. Submarine channels are more stable because turbidity currents have ~50x lower density contrast between flow and ambient fluid as compared to rivers. This density contrast creates far less potential energy for avulsion, despite the much greater relief of submarine levees compared to fluvial levees. The modern Amazon submarine channel showcases this stability, with a channel belt that is ~ 5 channel-depths thick for more than 400 streamwise km, which is more than twice the superelevation that a river is capable of. We interpret that enhanced floodplain aggradation and levee aggradation (and thus superelevation) in submarine channel belts are promoted by unique submarine flow characteristics, including turbidity current overspill, flow-stripping, and hemipelagic processes. We emphasize that rivers and submarine channels display very different avulsion dynamics and frequencies, profoundly affecting the stratigraphic architecture of channel-belt and downstream distributary deposits.

Modeling Deltaic Lobe‐Building Cycles and Channel Avulsions for the Yellow River Delta, China

River deltas grow by repeating cycles of lobe development punctuated by channel avulsions, so that over time, lobes amalgamate to produce a composite landform. Existing models have shown that backwater hydrodynamics are important in avulsion dynamics, but the effect of lobe progradation on avulsion frequency and location has yet to be explored. Herein, a quasi‐2‐D numerical model incorporating channel avulsion and lobe development cycles is developed. The model is validated by the well‐constrained case of a prograding lobe on the Yellow River delta, China. It is determined that with lobe progradation, avulsion frequency decreases, and avulsion length increases, relative to conditions where a delta lobe does not prograde. Lobe progradation lowers the channel bed gradient, which results in channel aggradation over the delta topset that is focused farther upstream, shifting the avulsion location upstream. Furthermore, the frequency and location of channel avulsions are sensitive to the threshold in channel bed superelevation that triggers an avulsion. For example, avulsions occur less frequently with a larger superelevation threshold, resulting in greater lobe progradation and avulsions that occur farther upstream. When the delta lobe length prior to avulsion is a moderate fraction of the backwater length (0.3– ), the interplay between variable water discharge and lobe progradation together set the avulsion location, and a model capturing both processes is necessary to predict avulsion timing and location. While this study is validated by data from the Yellow River delta, the numerical framework is rooted in physical relationships and can therefore be extended to other deltaic systems.

How does the downstream boundary affect avulsion dynamics in a laboratory bifurcation?

Abstract. Bifurcations play a major role in the evolution of landscapes by controlling how fluxes such as water and sediment are partitioned in distributary and multithread channel networks. In this paper, we present the first experimental investigation on the effect of the downstream boundary on bifurcations. Our experiments in a fixed-wall Y-shaped flume consist of three phases: progradation, transitional, and bypass; the first two phases are net depositional, whereas during the third the sediment flux exiting the downstream boundary matches the input on average. We find that deposition qualitatively changes bifurcation dynamics; we observe frequent switching in the discharge partitioning under net depositional conditions, whereas bypass results in long periods of time where one branch captures most of the flow. We compare our results with a previously developed model for the effect of deposition on bifurcation dynamics. The switching dynamics we observe are more irregular and complex than those predicted by the model. Furthermore, while we observe long periods of time where one branch dominates under bypass conditions, these are not permanent, unlike in the model. We propose that the range of switching timescales we observe arises from a complex interplay of downstream-controlled avulsion and the effect of bars in the upstream channel, including previously unrecognized long-term dynamics associated with a steady bar. Finally, we describe bifurcation experiments conducted with sand but no water. These experiments share the essential feedbacks of our fluvial bifurcation experiments, but do not include bars. In these experiments, we find that the sandpile grows symmetrically while it progrades, but bypass leads to one branch permanently capturing all avalanches. We conclude that the downstream control of deposition vs. bypass is likely a major influence on bifurcation dynamics across a range of physical systems, from river deltas to talus slopes.

Using bar preservation to constrain reworking in channel-dominated fluvial stratigraphy

Fluvial deposits comprising more than 80% channel facies are often thought to have accumulated during intervals of relatively slow subsidence in sedimentary basins. This interpretation stems from the conceptual model that migrating and avulsing rivers rework their own deposits during times of limited accommodation creation, preferentially removing and bypassing fine floodplain deposits. Alternatively, channel-dominated stratigraphy may reflect avulsion patterns that favor channel preservation over floodplain preservation, or channel networks fed by sand-rich sediment sources that never deposited significant floodplain sediments. These potential origins of channel-dominated stratigraphy cannot be differentiated without a way of independently assessing how ancient rivers eroded and reworked their own alluvium. Here we propose a new method that uses fluvial-bar preservation as a proxy for reworking in channel-dominated stratigraphy. We apply this approach to the lower Castlegate Sandstone (Upper Cretaceous, Utah, USA) and use geometric modeling to investigate the degree to which sediment supply and avulsion dynamics influence fluvial deposit preservation. Castlegate exposures in central Utah show up to 80% bar preservation in some localities, suggesting that, in contrast to previously published interpretations, Castlegate channel deposits are not heavily reworked and accommodation-creation rates during Castlegate deposition might have been relatively high. Model comparisons indicate that well-preserved Castlegate deposits could have resulted from random avulsions in a rapidly aggrading basin with a sand-dominated sediment supply, or under lower aggradation conditions if river avulsions avoided previously occupied locations. This approach represents a new advance for interpreting the relationships between basin accommodation, sediment supply, and avulsion dynamics on ancient landscapes and provides a new method for estimating sediment reworking and bypass from ancient river deposits.

Avulsion Dynamics in a River with Alternating Bedrock and Alluvial Reaches, Huron River, Northern Ohio (USA)

The Huron River consists of alternating bedrock reaches and alluvial reaches. Analysis of historical aerial photography from 1950-2015 reveals six major channel avulsion events in the 8-km study area. These avulsions occurred in the alluvial reaches but were strongly influenced by the properties of the upstream bedrock reach (“inherited characteristics”). The bedrock reaches aligned with the azimuth of joint sets in the underlying bedrock. One inherited characteristic in the alluvial reach downstream is that the avulsion channels diverged only slightly from the orientation of the upstream bedrock channel (range 2 ° - 38 °, mean and standard deviation 12.1 ° ± 13.7 °). A second inherited characteristic is that avulsion channels were initiated from short distances downstream after exiting the upstream bedrock channel reach (range 62 - 266 m, mean and standard deviation 143.7 ± 71.0 m), which is a fraction of the meander wavelength (1.2 km). Field evidence shows that some avulsion channel sites were re-occupied episodically. In addition, two properties were necessary for channel avulsions: 1) avulsion events were triggered by channel-forming hydrologic events (5-year recurrence interval flows), but not every channel-forming hydrologic event resulted in an avulsion, and 2) channel sinuosity (P) increased to 1.72 - 1.77 prior to an avulsion then decreased to 1.65 - 1.70 following an avulsion, suggesting that P ≥ 1.72 is the “critical sinuosity” or triggering value for avulsions on the Huron River. In summary, for this river consisting of alternating bedrock and alluvial reaches, the bedrock reaches impose certain parameters on downstream alluvial reaches (including sediment supply, channel direction and avulsion channel position downstream after exiting a bedrock reach) while adjustments in sinuosity and sediment storage occur in the alluvial reaches.

Exploring Wave and Sea‐Level Rise Effects on Delta Morphodynamics With a Coupled River‐Ocean Model

Deltas, which are often densely populated, have become more vulnerable to flooding due to increasing rates of sea‐level rise and decreasing sediment supply. These landscapes grow through the successive stacking of delta lobes, as abrupt changes in the river's course episodically alter the river mouth's location. The dynamics of these channel avulsions determine the location and size of delta lobes. We use a newly developed coupled model of river and coastal processes to explore how changing wave climates (varying wave heights and offshore approach angles) and a range of sea‐level rise rates affect avulsion processes and large‐scale delta morphology. Although the relative impacts of the fluvial sediment flux and wave influence have long been studied and more recently quantified, we find that the sign and magnitude of wave climate diffusivity both play important roles in determining not only delta morphology but also avulsion dynamics. We also find, surprisingly, that increasing sea‐level rise rates do not always accelerate avulsions. If the river channel is prograding rapidly enough, relative to the rate of sea‐level rise, then avulsion frequency will not be significantly affected by the rate of sea‐level rise. This finding highlights important differences between wave and river‐dominated deltas, as well as between prototypical deltas and experimental deltas created in the lab. The wave climate, sea‐level rise rate, and critical superelevation required to trigger an avulsion all play important roles in determining the degree of autogenic variability operating in delta systems, which has implications for both avulsion dynamics and interpretation of delta stratigraphy.

Experimental alluvial fans: Advances in understanding of fan dynamics and processes

Alluvial fans are depositional systems that develop because of a disparity between the upstream and downstream sediment transport capacity of a system, usually at the base of mountain fronts as rivers emerge from the constrained mountain area onto the plain. They are dynamic landforms that are prone to abrupt changes on a geomorphological (decades to centuries) time scale, while also being long-term deposition features that preserve sedimentary strata and are sensitive indictors of environmental change. The complexity of interactions between catchment characteristics, climate, tectonics, internal system feedbacks, and environmental processes on field alluvial fans means that it is difficult to isolate individual variables in a field setting; therefore, the controlled conditions afforded by experimental models has provided a novel technique to overcome some of these complexities. The use of experimental models of alluvial fans has a long history and these have been implemented over a range of different research areas utilising various experimental designs. Using this technique, important advances have been made in determining the primary factors influencing fan slope, understanding of avulsion dynamics, identifying autogenic processes driving change on fan systems independent of any change in external conditions, and the mechanics of flow and flood risk on alluvial fans, to name a few. However, experiments cannot be carried out in isolation. Thus, combining the findings from experimental alluvial fans with field research and numerical modelling is important and, likewise, using these techniques to inform experimental design. If this can be achieved, there is potential for future experimental developments to explore key alluvial fan issues such as stratigraphic preservation potential and simulating extra terrestrial fan systems.

Interpreting Paleo-Avulsion Dynamics from Multistory Sand Bodies

Abstract Channel avulsion plays a significant role in building alluvial stratigraphy, including fluvial sand bodies with through-going erosion surfaces, called multistory sand bodies (MSBs); consequently, MSB characteristics may be useful for interpreting paleo-avulsion dynamics from fluvial deposits. Here, using a combination of literature review, geometric modeling, and field observations, we explore the degree to which MSB characteristics may be useful for interpreting paleo-avulsion dynamics from ancient deposits. We use published interpretations of MSB origins to identify characteristics that are exclusively associated with avulsion-related MSBs and find that vertical story stacking, irregular MSB bounding surfaces, and floodplain deposits that correlate with individual stories are attributes uniquely associated with avulsion-origin MSBs. To evaluate how avulsion patterns may affect MSB properties, we use a 2D geometric model to build synthetic stratigraphy and compare characteristics for MSBs formed under different avulsion patterns at a range of model-aggradation rates. Avulsion patterns include random (equal probability of a channel relocating anywhere across the model domain), compensational (channels avulse to the lowest elevation in the model domain at each time step), and clustered (where channel relocation is restricted to a zone near a previously occupied location). Model results show that at moderate to high aggradation rates, MSBs formed under clustered avulsion patterns can be differentiated from those formed via random and compensational avulsion patterns. Specifically, MSBs formed by clustered avulsion patterns have many stories and commonly contain remnant floodplain deposits. We apply these insights to the well-exposed lower Williams Fork Formation (Upper Cretaceous, Piceance Basin, Colorado, USA) to demonstrate how they can be used in practice. Fifty-four percent of lower Williams Fork MSBs contain clear evidence for avulsion reoccupation, 32% have characteristics suggesting intra-channel-belt origins, and the remaining 20% have ambiguous evidence or insufficient exposure to permit confident process interpretations. Overall this study demonstrates that despite inherent challenges in eliciting information about avulsion dynamics from the stratigraphic record, some MSB characteristics can be interpreted as clear signals of paleo-avulsion processes.

Experimental alluvial fan evolution: Channel dynamics, slope controls, and shoreline growth

River deltas and alluvial fans have channelization and deposition dynamics that are not entirely understood, but which dictate the evolution of landscapes of great social, economic, and ecologic value. Our lack of a process‐based understanding of fan dynamics hampers our ability to construct accurate prediction and hazard models, leaving these regions vulnerable. Here we describe the growth of a series of experimental alluvial fans composed of a noncohesive grain mixture bimodal in size and density. We impose conditions that simulate a gravel/sand fan prograding into a static basin with constant water and sediment influx, and the resulting fans display realistic channelization and avulsion dynamics. We find that we can describe the dynamics of our fans in terms of a few processes: (1) an avulsion sequence with a timescale dictated by mass conservation between incoming flux and deposit volume; (2) a tendency for flow to reoccupy former channel paths; and (3) bistable slopes corresponding to separate entrainment and deposition conditions for grains. Several important observations related to these processes are: an avulsion timescale that increases with time and decreases with sediment feed rate; fan lobes that grow in a self‐similar, quasi‐radial pattern; and channel geometry that is adjusted to the threshold entrainment stress. We propose that the formation of well‐defined channels in noncohesive fans is a transient phenomenon resulting from incision following avulsion, and can be directly described with dual transport thresholds. We present a fairly complete, process‐based description of the mechanics of avulsion and its resulting timescale on our fans. Because the relevant dynamics depend only on threshold transport conditions and conservation of mass, we show how results may be directly applied to field‐scale systems.

Delta distributary dynamics in the Skagit River Delta (Washington, USA): Extending, testing, and applying avulsion theory in a tidal system

Analysis of historical aerial photos shows that Skagit Delta (Washington, USA) distributary dynamics are consistent with the Slingerland and Smith model of avulsion dynamics where the ratio of the water surface slopes of the two branches of a bifurcation predicts avulsion stability. This model was extended to predict distributary inlet (upstream) width and bankfull cross-sectional area. The water surface gradient ratio for a bifurcation pair predicted distributary width well; the lowest R 2 was 0.61 for the 1937 data points, but R 2 ranged from 0.83 to 0.90 for other year-specific regression lines. Gradient ratios were not constant over the historical record; from 1937 to 1972 the mainstem river channel lengthened by 1250 m in the course of marsh progradation, while distributary lengthening was comparatively negligible. Consequently, the gradient advantage of the distributaries increased and their channels widened. After the mainstem river terminus stabilized from 1972 to the present, the distributaries continued to lengthen with marsh progradation, so that distributary gradient advantage steadily declined and the distributaries narrowed. While distributary cross sections were not available for the historical period, they were surveyed in 2007 near the distributary inlets. Gradient ratio was more closely related to distributary inlet bankfull cross-sectional area ( R 2 = 0.95) than to minimum distributary width for any photo year examined. Applying this form of analysis to Skagit Delta distributaries that have been dammed in the course of agricultural development suggests that their restoration to stabilize eroding marshes at their outlets and recover salmon migration pathways would be feasible without significant risk of full river avulsion.