Topography extracted from swath profiles along the northern, southern, and eastern margins of the Tibetan Plateau show two end-member morphologies: steep, abrupt margins and longwavelength, low-gradient margins. Because the lack of significant upper crustal shortening across much of the eastern plateau margin implies that the crustal thickening occurs mainly in the deep crust, we compare regional topographic gradients surrounding the plateau to model results for flux of a Newtonian fluid through a lower crustal channel of uniform thickness. For an assumed 15-km-thick channel, we estimate a viscosity for the lower crust of 10 18 Pa•s beneath the low-gradient margins, 10 21 Pa•s beneath the steep margins, and an upper bound of 10 16 Pa•s beneath the plateau. These results indicate that the large-scale morphology of the eastern plateau reflects fluid flow within the underlying crust; crustal material flows around the strong crust of the Sichuan and Tarim Basins, creating broad, gentle margins, and "piles up" behind the basins creating narrow, steep margins. These results imply that this portion of the Eurasian crust was heterogeneous, but largely weak, even prior to construction of the Tibetan Plateau.

A new regional compilation of the drainage history in southeastern Tibet suggests that the modern rivers draining the plateau margin were once tributaries to a single, southward flowing system which drained into the South China Sea. Disruption of the paleo‐drainage occurred by river capture and reversal prior to or coeval with the initiation of Miocene (?) uplift in eastern Tibet, including ∼2000 m of surface uplift of the lower plateau margin since reversal of the flow direction of the Yangtze River. Despite lateral changes in course due to capture and reversal, the superposition of eastward and southward draining rivers that cross the southeastern plateau margin suggests that uplift has occurred over long wavelengths (>1000 km), mimicking the present low‐gradient topographic slope. Thus reorganization of drainage lines by capture and reversal events explains most of the peculiar patterns of the eastern plateau rivers, without having to appeal to large‐magnitude tectonic shear.

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