Geomorphic imprint of high mountain floods: Insight from the 2022 hydrological extreme across the Upper Indus terrain in NW Himalayas

Abstract

Abstract. The interaction of tectonics, surface processes, and climate extremes impacts how the landscape responds to extreme hydrological events. An anomalous precipitation event in 2022 occurred during the monsoon season along the lower reaches of the Upper Indus River, resulting in short-lived high-magnitude flooding and socioeconomic disruption downstream. To understand the spatial relationship between the geomorphic response and climatic controls of this flood event, as well as their primary triggers, we performed a landscape analysis using topographic metrics and quantified the causal association between hydro-climatic variables. Temperature anomalies in upstream glaciated sub-catchments had a considerable impact on snow cover distribution, based on our observations. As snow cover changed, glacial melt runoff rose, contributing to increased fluvial stream power after traversing higher-order reaches. The higher-order reaches of the Upper Indus River received an anomalously high amount of precipitation, which, when combined with substantial glacial and melt discharge, contributed to an extreme flood across the high-relief steep gradient channels. The flood-affected regions had a high mean basin ksn and SL-index, including numerous spikes in their magnitudes along their channel profiles downstream. To determine how the lower reaches of the Upper Indus River responded to this flood event, we employed the Enhanced Vegetation Index (EVI) and Normalized Difference Water Index (NDWI) as change indicator metrics. We observed an inverse causal influence of NDWI on EVI and a statistically significant relationship between anomalous stream power and relative EVI, suggesting that downstream channel morphology changed rapidly during this episodic event and highlighting EVI as a useful indicator of geomorphic change. We suggest that this extreme flood event is a result of the interaction of anomalous glacial melt and anomalous precipitation over a high-relief landscape, with a certain causal connection with anomalous temperature over the event duration. The synoptic observations suggest that this meteorological condition involves the interaction of the Indian Summer Monsoon (ISM) and Western Disturbance (WD) moisture fluxes. However, the geomorphic consequences of such anomalous monsoon periods, as well as their influence on long-term landscape change, are still unclear.