High-mountain floods and landscape perturbation: Geomorphic and hydroclimatic insights of extreme flood events across the Upper Indus catchment in the NW Himalayas

The short-lived, high-magnitude events have had a significant impact on the geomorphic evolution of the bedrock catchments, but the relative contribution of these episodic events over the high-relief regions is not well understood. The Upper Indus River in the western syntaxial region has witnessed several infrequent episodic and outburst flood incidences. However, the geomorphic imprints of these catastrophic events and their influence on the long-term fluvial processes in the Upper Indus region have not been clearly understood due to a lack of discharge information from these instances of flooding. In this study, we estimate the stream power proxy driven by the channel gradient-discharge product to identify areas of possible anomalous channel erosion and the geomorphic response of the Upper Indus River during two recent anomalous flooding events in the Upper Indus catchment, which occurred in the 2010 and 2022 monsoon periods. The synoptic observations during these two flood events, derived from the HYSPLIT model using the backward trajectory with different heights, indicate that the anomalous precipitation triggering these floods is brought about by a meteorological disturbance. This disturbance involves the interaction of two distinct moisture fluxes, namely the southward moving mid-latitude westerlies troughs and eastward advancing southwestern monsoon circulation. We used topographic metrics to conduct a landscape analysis and calculated the causal relationship between hydroclimatic variables to understand the spatial relationship between the geomorphic response, climatic controls, and primary triggers of these flood events. The topographic analysis indicates that the trunk channel of the Upper Indus River exhibits significant variations in the ranges of the k_sn anomaly, χ-gradient, and SL-index, along with frequent sudden rises in stream power profiles across the flooded zone over the low-relief region of Ladakh. Then, when the river traverses through the Nanga Parbat- Harmosh Massif (NP-HM) region along a rapidly exhumed region of the north-western (NW) Himalaya, there is a progressive rise in the local relief and channel gradient, which is also reflected in the spatial distribution of stream power. The spike or transition in the magnitude of the stream power from the Ladakh terrain to the NP-HM region corresponds to the zone of progressive erosion across the active structures. Our study uncovers several significant event characteristics of the Upper Indus catchment's 2010 and 2022 anomalous flood events. Our analysis shows that the 2022 flood originated across elevated glacial channels due to the anomalous temperature rise, which increased the glacial runoff. This increase in runoff across glaciated catchments after traversing through fluvial reaches enhanced the fluvial discharge and likely increased the stream power in the anomalous precipitation region.  We observe a statistically significant relationship between the anomalous stream power and relative EVI range across the lower reaches, which serves as a significant geomorphic indicator of change in the channel morphology. These extreme floods illustrate how causal connections between temperature and precipitation across high relief-gradient channels can magnify the impacts. Such hydrological events may play significant roles as efficient geomorphic agents of erosion and, therefore, in the coupling of climate extremes, topography, and surface processes.