Downstream grain-size changes using PebbleCounts in the south-central Andes: Relations to channel steepness and SAR observations

Grain-size data are imperative for understanding erosional and fining processes in steep terrain as rivers are the primary conduits for sediment transport. However, collecting hundreds of pebble measurements at multiple channel bed survey sites in steep and dynamic high-mountain river settings remains a challenging roadblock in studying downstream variations in grain-size. Using PebbleCounts (https://github.com/UP-RS-ESP/PebbleCounts), we survey large (~1,000+ m²) channel cross sections and measure thousands of grains per survey to build robust grain-size distributions in the Quebrada del Toro, Northwest Argentina. Because of imagery resolution considerations, we only examine the grain sizes in the ≥ 2.5 cm fraction of the distribution. We gather measurements via a careful counting and validation process to constrain uncertainties, which highlights the dominant over- and under-segmentation errors that occur in PebbleCounts in this challenging geographic setting. Despite uncertainties, we are able to study downstream changes in grain-size percentiles at seven survey sites along a 100-km stretch of the trunk stream, which traverses a steep topographic and environmental gradient. We find that only the upper-most percentiles (≥ 95th) are sensitive, whereas the 50th and 84th percentiles show little downstream variability in this rapidly eroding catchment. In particular, we note a strong relation between increases in these upper percentiles and the along-channel junctions with large, oversteepened tributaries, where extreme channel steepness reaches are > 200 m^0.8 (θ=0.4). Furthermore, independent spaceborne synthetic aperture radar (SAR) coherence and amplitude observations show clear relations to mass transfer and channel bed roughness changes, which also relate to the grain-size variability that we find.