Influence of Spatial Rainfall Gradients on River Longitudinal Profiles and the Topographic Expression of Spatially and Temporally Variable Climates in Mountain Landscapes

Mountain landscapes have dynamic climates that, together with tectonic processes, influence their topographic evolution. Spatial and temporal variations in rainfall are ubiquitous in these settings as orographic precipitation patterns evolve with climate change and topography. Despite important implications such changes have for river incision, their influence is understudied. Here, we investigate how changes in rainfall pattern should affect both the steady state form and transient evolution of river profiles at the catchment scale using the stream power model. We find that spatially varied rainfall patterns can complicate steady state relationships between mean rainfall, channel steepness and fluvial relief, depending on where rainfall is concentrated in catchments, and lead to unexpected transient behavior if they are neglected. Specifically, changes in rainfall pattern cause multi-stage transient responses that differ from responses to uniform changes in rainfall. Disparate responses by rivers that experience different rainfall conditions, particularly trunk and tributary rivers, are also an important factor in understanding catchment-wide responses to climate change. Accounting for such disparities in sampling strategies and topographic analyses may, therefore, be vital for detecting and quantifying climate's role in landscape evolution. Lastly, we show how explicitly accounting for rainfall patterns in channel steepness indices, and thus spatial variations in erosional efficiency, may advance understanding of landscape sensitivity to climate. These results have important implications for detecting transient responses to changes in rainfall pattern (and more broadly climate), interpretation of morphometrics in steady state and transient landscapes, and quantifying the sensitivity of landscapes and erosion rates to climate. Plain Language Summary Orographic rainfall patterns are produced when air is forced to rise over mountains, resulting in variations in rainfall with elevation. Rivers that sculpt these landscapes rely on rainfall to erode through rock. Intuitively, more rainfall means more water and typically greater erosive power. The amount of rainfall also affects how steep these rivers are, which in turn affects the steepness of the topography around them. Despite how common orographic rainfall patterns are in mountain landscapes, their effect on rivers is understudied. Here, we present a study describing how concentrating rainfall at higher and lower elevations-representing two common orographic rainfall patterns that may be enhanced or relaxed by climate change-influences the steepness mountain rivers, erosion patterns, and thus the evolution of mountain topography. We show that these orographic rainfall patterns complicate expected relationships among metrics commonly used to quantify the role of rainfall (and more broadly climate) on the topography of mountain landscapes. Further, we show that rivers respond in unexpected ways to changes in orographic rainfall patterns, as would occur following a change in climate, suggesting that common wisdom about how rivers and mountain landscapes respond to changing climates is incomplete.