USING GIS TO ESTIMATE CLIMATIC INFLUENCES ON RIVER INCISION

Over the past decade, an appreciation for the influence of climate change on river incision has been increasing. Deep canyons and exhumed mountain ranges in tectonically quiet settings have been traditionally interpreted as evidence for late Quaternary epeirogenic rock uplift. Yet river incision cannot be easily apportioned among climatic and tectonic driving forces. We approach the problem of isolating the climatic influences of river incision through a spatial analysis of 40 large watersheds on the eastern American High Plains spanning a steep precipitation gradient in a tectonically stable landscape. The basins were selected to minimize variability in base level, relief, vegetation, rock type, and basin area. Nineteen of these watersheds are underlain primarily by carbonate rocks while the remaining 21 basins are underlain primarily by shale, mudstone, and sandstone. In a GIS, we generated an area-normalized stream concavity index (SCI) for the trunk channel and a corresponding basin hypsometry for each watershed from a 3-arcsec DEM. A positive correlation exists between the SCI and climatic variables such as mean annual precipitation intensity and mean maximum annual discharge. These relationships are largely independent of bedrock, surficial cover, local base level, or vegetation. Cast in the context of watersheds with similar hypsometry, the results can be interpreted in terms of the range of river incision that can be expected by a climatically driven, but transient change in profile concavity. We conclude that in tectonically stable settings, rainfall intensity, perhaps associated with a stormier climate or more seasonal distribution of precipitation, leads to transient changes in profile concavity and incision over Pleistocene time scales. Generally speaking, a doubling of rainfall intensity on the Great Plains leads to a tripling in the SCI that is manifest as tens of meters of incision for watersheds of similar size and relief as those studied.