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Absent clear lithologic control, the presence of elevated, low-relief topography in upland landscapes has traditionally been interpreted as a signature of relative surface uplift and incision of a paleo-landscape. Such interpretations are commonly supported and quantified using analyses of river longitudinal profiles under the assumption of a static drainage network topology. Drainage networks, however, are not static, and it has been proposed recently that divide migration and drainage capture can lead to the generation of low-relief upland topography that mimics that of incised paleo-landscapes and that might be falsely interpreted as recording surface uplift and/or the onset of accelerated incision. Indeed, the interpretation of the incised southeastern Tibetan Plateau, and thus the associated geodynamic implications, have been called into question. Here we use theory and one- and two-dimensional landscape evolution models to develop a set of morphometric criteria to distinguish these alternative mechanisms of low-relief upland formation. Application to the southeastern Tibetan Plateau illustrates the utility of these metrics and demonstrates that the topography is in no way consistent with the drainage network dynamics mechanism and is fully consistent with incision into an elevated, preexisting low-relief landscape.

We used measurements of cosmogenic 10 Be in alluvium to estimate erosion rates on a 10 3 –10 4 yr time scale for small (0.01–47 km 2 ), unglaciated basins in northern Colorado, southern Wyoming, and adjacent western Nebraska (western United States). Basins formed in Proterozoic cores of Laramide ranges are eroding more slowly (23 ± 7 mm k.y. –1 , n = 19) than adjacent basins draining weakly lithified Cenozoic sedimentary rocks (75 ± 36 mm k.y. –1 , n = 20). Erosion rates show a relationship to rock resistance and, for granitic rocks, to basin slope, but not to mean annual precipitation. We estimated longer-term (>10 5 yr time scale) erosion rates for the granitic core of the Front Range by measuring the concentration of 10 Be and 26 Al produced mainly by muon interactions at depths 1.7–10 m below the surface. Concentrations imply erosion rates of 9–31 mm k.y. –1 , similar to shorter-term erosion rates inferred from alluvial sediment. The spatial distribution of erosion rates and stratigraphic evidence imply that relief in the southern Rocky Mountains increased in the late Cenozoic; modern relief probably dates from post-middle Miocene time.

Numerous empirical and model-based studies argue that, in general, hillslopes and river channels increase their gradients to accommodate high rates of base-level fall. To date, however, few data sets show the dynamic range of both these relationships needed to test theoretical models of hillslope evolution and river incision. Here, we utilize concentrations of 10 Be in quartz extracted from river sand on the eastern margin of the Tibetan Plateau to explore relationships among short-term (10 2 –10 5 a) erosion rate, hillslope gradient, and channel steepness. Our data illustrate nonlinear behavior and a threshold in the relationship between erosion rate and mean hillslope gradient, confi rming the generalization that hillslopes around the world are limited by slope stability and cease to provide a metric for erosion at high rates (>~0.2 mm/a). The relationship between channel steepness index and erosion rate is also nonlinear, but channels continue to steepen beyond the point where threshold hillslopes emerge up to at least 0.6 mm/a, demonstrating that channel steepness is a more reliable topographic metric than mean hillslope gradient for erosion rate and that channels ultimately drive landscape adjustment to increasing rates of base-level fall in tectonically active settings.