How Drainage Capture Restructures Incision and Uplift in the Eastern Himalayan Syntaxis

The Eastern Himalayan Syntaxis (EHS) hosts some of Earth’s deepest gorges and rapid exhumation, yet the mechanisms sustaining its extreme relief remain an active area of geomorphic research. Classic interpretations invoke the tectonic aneurysm model, in which rapid fluvial incision feeds back with crustal deformation to concentrate uplift around the Namche Barwa massif. However, emerging evidence suggests that large-scale drainage reorganization may also set the stage for this focused topography. The well-documented diversion of the Lohit headwaters into the Siang system provides a natural experiment in how river capture can redistribute erosional power across the landscape. To assess this interaction, we combine geomorphic analysis of the Siang–Dibang–Lohit network with landscape evolution modeling to explore how capture-driven changes in drainage area and discharge propagate through the channel system. Our simulations show that the addition of Lohit drainage to the Siang following upstream reorganization of the Yarlung system enhances incision along the main valley and establishes long-lived disequilibrium at adjacent divides. This response persists under uniform uplift but becomes markedly subdued when localized uplift is introduced, aligning with expectations from tectonic aneurysm hypothesis and with patterns observed in the present landscape. Taken together, these results indicate that drainage capture acted not merely as an isolated geomorphic event but as a primary perturbation that initiated transient incision and set up conditions favourable for subsequent focused uplift. We argue that the interplay between channel reorganization, erosional feedbacks, and crustal flow offers a more flexible framework for understanding how the EHS evolved—and why it remains one of the most dynamically responsive mountain regions on the planet.