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.
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Rivers in reverse: Upstream-migrating dechannelization and flooding cause avulsions on fluvial fans
Abstract The process of river avulsion builds floodplains and fills alluvial basins. We report on a new style of river avulsion identified in the Landsat satellite record. We found 69 examples of retrogradational avulsions on rivers of densely forested fluvial fans in the Andean and New Guinean alluvial basins. Retrogradational avulsions are initiated by a channel blockage, e.g., a logjam, that fills the channel with sediment and forces water overbank (dechannelization), which creates a chevron-shaped flooding pattern. Dechannelization waves travel upstream at a median rate of 387 m/yr and last on average for 13 yr; many rivers show multiple dechannelizing events on the same reach. Dechannelization ends and the avulsion is complete when the river finds a new flow path. We simulate upstream-migrating dechannelization with a one-dimensional morphodynamic model for open channel flow. Observations are consistent with model results and show that channel blockages can cause dechannelization on steep (10−2 to 10−3), low-discharge (~101 m3 s−1) rivers. This illustrates a new style of floodplain sedimentation that is unaccounted for in ecologic and stratigraphic models.
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- Award ID(s):
- 1911321
- PAR ID:
- 10342906
- Date Published:
- Journal Name:
- Geology
- Volume:
- 50
- Issue:
- 1
- ISSN:
- 0091-7613
- Page Range / eLocation ID:
- 37 to 41
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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ABSTRACT The 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.more » « less
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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–
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Abstract Coastal rivers that build deltas undergo repeated avulsion events—that is, abrupt changes in river course—which we need to understand to predict land building and flood hazards in coastal landscapes. Climate change can impact water discharge, flood frequency, sediment supply, and sea level, all of which could impact avulsion location and frequency. Here we present results from quasi‐2D morphodynamic simulations of repeated delta‐lobe construction and avulsion to explore how avulsion location and frequency are affected by changes in relative sea level, sediment supply, and flood regime. Model results indicate that relative sea‐level rise drives more frequent avulsions that occur at a distance from the shoreline set by backwater hydrodynamics. Reducing the sediment supply relative to transport capacity has little impact on deltaic avulsions, because, despite incision in the upstream trunk channel, deltas can still aggrade as a result of progradation. However, increasing the sediment supply relative to transport capacity can shift avulsions upstream of the backwater zone because aggradation in the trunk channel outpaces progradation‐induced delta aggradation. Increasing frequency of overbank floods causes less frequent avulsions because floods scour the riverbed within the backwater zone, slowing net aggradation rates. Results provide a framework to assess upstream and downstream controls on avulsion patterns over glacial‐interglacial cycles, and the impact of land use and anthropogenic climate change on deltas.
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Abstract The displacement of a river to a new position within its adjacent floodplain is called avulsion, and here we examine how a newly recognized style, called retrogradational avulsion, affects the surrounding floodplain in tropical rainforests using remote sensing. Retrogradational avulsions begin with a channel blockage that causes self‐propagating upstream dechannelization and flooding. While this flooding results in vegetation die‐off and floodplain sedimentation, few quantitative measurements of disturbance by retrogradational avulsions exist. Here, we first focus on land‐cover change following a single retrogradational avulsion in Papua New Guinea from 2012 to 2021. During the avulsion, the river dechannelized 892 m upstream, and the parent channel width doubled. Using maximum likelihood image classification, we observed healthy vegetation fluctuated around 4.3 km2, vegetation regrowth peaked in 2017 at 3.2 km2, dead vegetation peaked in 2013 at 2.1 km2, and visible extent of deposited sediment was greatest in 2015 at 0.44 km2. We also examined 19 other retrogradational avulsions in Papua New Guinea and South America using NDVI. The area of floodplain disturbance (i.e., vegetation die‐off and possible sedimentation) for each avulsion ranged from <1 to >13 km2and scaled with the dechannelization area. Comparing our plan‐view disturbance results with FABDEM digital‐elevation data and ICESat‐2 surface elevation measurements, we hypothesize floodplain disturbance extent is a function of topographic relief. Our results also suggest that retrogradational avulsions, on average, perturb larger areas of forest compared to blowdowns, suggesting this might be an important disturbance regime that influences gap‐filling regeneration in tropical rainforests.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/G49318.1
