Spatial analysis of stream length-gradient (SL) index for detecting hillslope processes: A case of the Gállego River headwaters (Central Pyrenees, Spain)

A morphometric analysis of drainage networks and relief using geomorphic indices and geostatistical analyses of topographical data are useful tools for discussing the morphoevolution of a given area. Among the geomorphic indices, the stream length-gradient (SL) index represents a practical tool to highlight anomalous changes in river gradients. Perturbations of SL are usually indicative of (1) differences in the resistance of outcropping lithological units to erosion, (2) sub-surface processes, such as active faulting, and (3) slope failures that directly reach the stream channels, particularly in small catchments. In this work, the SL index was calculated for the upstream sector of the Gállego River basin in the central Spanish Pyrenees to test its accuracy and sensitivity for detecting the imprints of different surface processes. A geostatistical procedure is proposed to obtain SL index maps through the interpolation and filtering of the values estimated along the drainage network. This method allows computing of SL, validation and assessing of its spatial distribution with robust statistical accuracy, and objectively defining the anomalies in SL. The anomalies in the SL map of the study area, which coincide with knickpoints and knickzones, were analyzed in detail. The results indicate (1) perturbation in the drainage network caused by differences in the resistance to erosion of outcropping lithological units and (2) hillslopes affected by large landslides, earth flows, and rock falls directly reaching the stream bed. This study indicates that the SL index has strong potential to solve geomorphological problems in different geological settings, especially in detecting the role of active, large-scale features that influence landscape evolution.