THE INFLUENCE OF CHANGES IN PRECIPITATION PATTERNS ON RIVER LONGITUDINAL PROFILES – A COMPARISON OF MODELS AND TOPOGRAPHIC ANALYSES IN SOUTHERN PERU

A fundamental requirement for understanding the pace of landscape evolution is a quantitative understanding of climate’s influence on the efficiency of erosional processes. The widely applied stream-power family of models describing fluvial erosion all accommodate this sensitivity explicitly through a combination of assumptions and site-specific parameterization that varies with the particular formulation used, available data, and questions addressed. However, data quantifying the sensitivity of erosional processes to climate for any site or application are invariably sparse. Therefore, data collected specifically for this purpose are critically needed. The setting of one of the most dramatic proposed examples of climate-induced landscape change is the Eastern Andean Plateau in Southern Peru, where canyons carved into the plateau margin have been argued to be a response to Pliocene climate change. Today, this landscape is characterized by an extreme orographic rainfall gradient that ranges from <400 to >6000 mm/yr over a ~30 km distance. Rainfall in the Pliocene is thought to have been 2-3 times less. Here, we present a 2-dimensional conceptual model that describes how climate-driven incision is expressed in a landscape, accompanied by a numerical model that uses a stream-power based approach to quantify the effect of hypothesized changes in precipitation on erosion rate and channel steepness patterns. Then, we compare these results with a detailed topographic analysis in Southern Peru. We show that channel steepness patterns exhibit a distinct linear decrease from the plateau edge to the basin across a range of intensities in rainfall gradient implying either adjustment to a complex spatial gradient in uplift or that fluvial erosion may be relatively less sensitive to the modern rainfall gradient than commonly assumed. Finally, we show how analyses of catchment-averaged erosion rates now in progress will be leveraged to explore whether the sensitivity of the landscape in southern Peru to hypothesized changes in climate is compatible with those assumed in our stream-power based approach.