Grain-size distributions and their associated percentiles are a key geomorphic metric of gravel-bed rivers. Traditional measurement methods include manual counting or photo sieving, but these are typically achievable only at the patch (1 square meter) scale. With the advent of unmanned aerial vehicle systems and increasingly high-resolution cameras, we can now generate orthoimagery over large areas at resolutions of <1 cm. These scales, along with the complexity of many natural environments in high-mountain rivers, necessitate different approaches for photo sieving. Here, a new open-source algorithm is presented: PebbleCounts. As opposed to other image segmentation methods that use a watershed approach and automatically segment entire images, PebbleCounts relies on k-means clustering in the spatial and spectral (color) domain and rapid manual selection of well-outlined grains. This results in improved estimates for complex river-bed imagery without the need for post-processing.
This data publication is supplementary to a study on the effect of large boulders and bedrock fracture patterns on hillslope denudation rates in the Chilean Coastal Cordillera, by Lodes et al. (submitted). Hillslope denudation rates are primarily determined by tectonic uplift rates, but landscape morphology is also controlled by climate and lithological properties such as bedrock fractures. Fracture patterns can influence the locations of ridges and valleys in landscapes through lowering surface grain sizes in fractured areas, and therefore the residence time of fractured hillslope material, dictating differential denudation rates. In this project, we used 10Be cosmogenic nuclide analysis to quantify the denudation rates of fractured bedrock, boulders, and soil on hillslopes, and compared the orientations of surrounding streams and faults, to understand the effects of fracturing and faulting on denudation rates, fluvial incision, and grain size in three field sites along a climate gradient in the Chilean Coastal Cordillera. In the humid and semi-arid climate zones, we found that denudation rates for unfractured bedrock and large hillslope boulders (10 to 15 m Myr-1) are lower than for soil (15 to 20 m Myr-1), indicating that exposed bedrock and boulders retard hillslope denudation rates. In the mediterranean climate zone, hillslope denudation rates are higher (40-140 m Myr-1) and show a less consistent pattern, likely due to steeper slopes. LiDAR-derived stream orientations support a fracture-control on landscape denudation in the three field sites, which we link with fracture density. Together, our results thus provide new insights into how fracture patterns can dictate topographic highs and valleys through grain size reduction. The main objective of this data publication is to provide our 10Be dataset which we used to calculate denudation rates for bedrock, boulders, and soils.