Title: Correlations of fluvial knickzones with landslide dams, lithologic contacts, and faults in the southwestern Annapurna Range, central Nepalese Himalaya
Abstract: We investigate the role of landslide dams, spatial changes in lithology, and rock uplift on faults in the formation of knickzones on bedrock rivers. We focus our analysis in the southwestern Annapurna Range of the central Nepalese Himalaya where detailed geologic maps, topographic data, field observations, and aerial photographs are available. We identified knickzones in our study area from normalized river steepness indices (k sn values) extracted from river longitudinal profiles derived from a 25 m digital elevation model we interpolated from digitized topographic map contours. We compared the location of these knickzones with (1) lithologic contacts and faults from a detailed geologic map of the Modi Khola valley and (2) inferred ancient landslide dam features mapped from field observations and aerial photographs. The steepest location on the Modi Khola occurs near the same latitude as the steepest reach on the Mardi Khola located directly to the east, potentially highlighting a major topographic transition across the Annapurna. However, we find that landslide dams once blocked the flow of the Modi Khola, and damming followed by incision after landslide breaching can explain the location of these knickzones without the need for active faulting near the Main Central thrust. We also conclude that (1) knickzones do not correlate with any spatial changes in lithology and (2) knickzones generated by rock uplift on unmapped faults cannot be ruled out. We emphasize that disentangling the processes responsible for knickzone formation remains challenging even when high‐resolution geologic and topographic data are available.