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

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.

Remotely sensed land‐cover change and floodplain disturbance following upstream‐migrating river avulsions in tropical rainforests

“…We found avulsion R11.1, located in Bolivia, as an outlier for both river discharge (yearly max) and drainage basin area, and even removing that or other outliers did not create clear trends in the data. that limits overbank flooding (Valenza et al, 2020(Valenza et al, , 2022. Retrogradational avulsions seem to always have overbank flooding during the dechannelization process, as evident by the large area of floodplain disturbance and associated vegetation die-offs.…”

“…All avulsion styles disturb the floodplain and change land-cover through time by flooding and associated hydrosedimentological processes that cause vegetation die-off and shape floodplain architecture. These disturbances impact the spatiotemporal distribution of surrounding ecological biodiversity (Assine et al, 2016;Richards et al, 2002;Stanford et al, 2005;Tockner et al, 2010;Valente et al, 2013;Valenza et al, 2022;Van Asselen et al, 2017;Ward et al, 2002). Progradational avulsions create widespread floodplain disturbance from flooding and sediment deposition, and annexational or incisional avulsions result in little floodplain disturbance as they only reoccupy pre-existing channels (Valenza et al, 2020(Valenza et al, , 2022.…”

“…These disturbances impact the spatiotemporal distribution of surrounding ecological biodiversity (Assine et al, 2016;Richards et al, 2002;Stanford et al, 2005;Tockner et al, 2010;Valente et al, 2013;Valenza et al, 2022;Van Asselen et al, 2017;Ward et al, 2002). Progradational avulsions create widespread floodplain disturbance from flooding and sediment deposition, and annexational or incisional avulsions result in little floodplain disturbance as they only reoccupy pre-existing channels (Valenza et al, 2020(Valenza et al, , 2022. Comparatively, disturbances from retrogradational avulsions remain underexamined, and while some studies exist (Edmonds et al, 2022;Gullison et al, 1996;Lombardo, 2017;Valenza et al, 2022), a need for basic information regarding the characteristic size of disturbance and the mechanics of the avulsion process remains.…”

Remote Sensing of Riverbank Migration Using Particle Image Velocimetry

“…This disadvantage can be overcome by identifying channel positions based on water-surface presence (Jones, 2015(Jones, , 2019Sylvester et al, 2019), which enables waterlines to be tracked along both vegetated and unvegetated banks (Figures 4e and 4f). Unvegetated banks may migrate significantly faster than vegetated banks (Figure 4f), and can be distinguished from one another based on spectral indices (Kauth & Thomas, 1976;Valenza et al, 2022). Quantifying differences between vegetated and unvegetated banks may be crucial for unraveling the dynamics of different types of multi-thread channels, which are often classified based on whether mid-channel islands are unvegetated (in braided rivers) or vegetated (in anastomosing rivers) (Church & Rood, 1983;Eaton et al, 2010;Makaske, 2001).…”

A method to detect abrupt shifts in river channel position using a Landsat‐derived water occurrence record

“…Another issue with flattened images is that channels frequently avulse or cut off by reoccupying previous channel positions (e.g. Valenza et al, 2020Valenza et al, , 2022, or avulse and then laterally migrate over the old channel position. In such cases, flattened occupancy maps cannot distinguish between these processes.…”