Publication Date:
2022-03-31
Description:
Mixed sand‐ and gravel‐bed rivers record erosion, transport, and fining signals in their bedload size distributions. Thus, grain‐size data are imperative for studying these processes. However, collecting hundreds to thousands of pebble measurements in steep and dynamic high‐mountain river settings remains challenging. Using the recently published digital grain‐sizing algorithm PebbleCounts, we were able to survey seven large (≥ 1,000 m2) channel cross‐sections and measure thousands to tens‐of‐thousands of grains per survey along a 100‐km stretch of the trunk stream of the Toro Basin in Northwest Argentina. The study region traverses a steep topographic and environmental gradient on the eastern margin of the Central Andean Plateau. Careful counting and validation allows us to identify measurement errors and constrain percentile uncertainties using large sample sizes. In the coarse ≥2.5 cm fraction of bedload, only the uppermost size percentiles (≥95th) vary significantly downstream, whereas the 50th and 84th percentiles show less variability. We note a relation between increases in these upper percentiles and along‐channel junctions with large, steep tributaries. This signal is strongly influenced by lithology and geologic structures, and mixed with local hillslope input. In steep catchments like the Toro Basin, we suggest nonlinear relationships between geomorphic metrics and grain size, whereby the steepest parts of the landscape exert primary control on the upper grain‐size percentiles. Thus, average or median metrics that do not apply weights or thresholds to steeper topography may be less predictive of grain‐size distributions in such settings.
Description:
Plain Language Summary:
Rock fragments on hillsides are transported to rivers, eventually becoming pebbles, sand, and mud as they are carried downstream by flowing water. The initial size of the pebbles, the way the size changes downstream, and the overprinting of the sizes with new pebbles from other hills and tributaries all form a complex process that can be difficult to disentangle. Yet studying the size of the pebbles at a given stream location or in a sedimentary deposit can provide insights into the conditions of their transport in terms of local upstream patterns of erosion, tectonics, and climate. We show that just looking at the size of the large pebbles on a riverbed can be used to infer the sources of material, but, since there are fewer large pebbles, they require more measurements to quantify. This necessitates new methods for pebble measurement using modern image‐processing tools.
Description:
Key Points:
Complex grain‐size distributions in dynamic mountain rivers can be computed via thousands of measurements from PebbleCounts.
Many measurements allow robust estimation of higher percentiles and we observe the most significant changes in the ≥95th percentile.
Downstream grain‐size variation is nonlinearly related to variations in topographic steepness and lithology.
Description:
Deutsches Zentrum für Luft‐ und Raumfahrt (DLR)
http://dx.doi.org/10.13039/501100002946
Description:
Deutsche Forschungsgemeinschaft (DFG)
http://dx.doi.org/10.13039/501100001659
Description:
Bundesministerium für Bildung und Forschung (BMBF)
http://dx.doi.org/10.13039/501100002347
Description:
Brandenburger Staatsministerium für Wissenschaft, Forschung und Kultur (MWFK)
http://dx.doi.org/10.13039/501100004581
Description:
https://zenodo.org/record/5089789
Keywords:
ddc:551.3
;
ddc:550
Language:
English
Type:
doc-type:article
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