Spatiotemporal morphodynamics of an ephemeral Himalayan River impacted by sand mining: A process-response framework.

In-channel sediment mining significantly disrupts reach-scale sediment connectivity and channel geometry, causing immediate and intense geomorphological responses. River systems perturbed by anthropogenic stress, like sand and gravel mining, tend to respond within a shorter timescale, making the study of feedback mechanisms important. 'Sensitive' rivers display dramatic change via a positive feedback mechanism, exacerbating the change in the system. This paper investigates the geomorphic impacts of in-channel sand mining on the Gaula River, an ephemeral piedmont zone Himalayan River in northwest India. This river is characterized by high sediment flux from the tectonically active hinterland and flows primarily during the monsoon, making it suitable for sediment mining. Most geomorphological studies on sand mining in India have focused on peninsular rivers, leaving a knowledge gap in understanding the impacts on Himalayan systems. This study used satellite-based planform data (1976 to 2021) to develop morphometric indices coupled with the valley bottom confinement index, longitudinal profile and total stream power. Our results indicate that sand mining has altered flux boundary conditions, causing channel narrowing, incision, thalweg fixing, bed armouring, and a severe decline in geomorphic diversity. We have integrated the morphological response to sand mining to develop an evolutionary pathway of the Gaula River from barren conditions (1976) to underfit channel formation (2021). We finally propose a process-response framework that highlights how sand mining triggers geomorphic degradation through a positive feedback mechanism.