[1] Longitudinal profiles of alluvial and bedrock rivers are typically concave up, and the precise shape of their concavity is known to integrate tectonic, climatic, and channel grain size distribution factors. We isolate climatic/hydrologic effects on profile concavity through a spatial analysis of large watersheds with mixed bedrock-alluvial trunk channels spanning a steep climatic/hydrologic gradient in a tectonically stable landscape. Forty watersheds on the eastern American high plains were selected to minimize variability in base level, relief, vegetation, rock type, and drainage area. We calculate stream concavity by two complementary methods: an area-normalized stream concavity index (SCI) and q, the slope of a line regressed through log-log plot of channel slope and basin area. The SCI and q covary. More importantly, a positive correlation exists between profile concavity and climatic/hydrologic factors such as precipitation intensity and peak annual discharge; however, modeled profile steepness has no correlation to climate or watershed hydrology. We conclude that in tectonically stable settings, higher-intensity rainfall and greater mean annual precipitation lead to more concave profiles. We do not have data to know if the concavity changes reflect primarily bedrock or alluvial (grain size) processes, but generally, a doubling of rainfall intensity on the high plains leads to a tripling in concavity manifest as tens of meters of incision. Such climatically influenced profile concavity could explain some of the often-cited late Cenozoic incision and increase in local relief for the high plains and Rocky Mountains traditionally ascribed to tectonics.

Geomorphic research in the Black Hills and northern High Plains poses an intriguing hypothesis for the Cenozoic evolution of this salient of the Laramide Rockies. Most recently, geologists have appealed to late Cenozoic epeirogenic uplift or climate change to explain the post-Laramide unroofing of the Rockies. On the basis of field mapping and the interpretation of long-valley profiles, we conclude that the propagation of knickzones is the primary mechanism for exhumation in the Black Hills. Long profiles of major drainages show discrete breaks in the slope of the channel gradient that are not coincident with changes in rock-type. We use the term knickzones to describe these features because their profiles are broadly convex over tens of kilometers. At and below the knickzone, the channel is incising into bedrock, abandoning a flood plain, and forming a terrace. Above the knickzone, the channel is much less incised, resulting in a broad valley bottom. Numerous examples of stream piracy are documented, and in each case, the capture is recorded in the same terrace level. These observations are consistent with migrating knickzones that have swept through Black Hills streams, rearranging drainages in their wake. We demonstrate there are two knickzone fronts associated with mapped terraces. Preliminary field evidence of soil development shows that these terraces are time transgressive in nature. Our data strongly suggest that knickzone propagation must be considered a viable mechanism driving late Cenozoic fluvial incision and exhumation of the northern High Plains and adjacent northern Rocky Mountains.

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