Measuring longitudinal profiles and hydraulic geometry of rivers in NW Argentina

Abstract

In actively uplifting mountain belts, landscape evolution is largely limited by bedrock river incision. Over the last twenty years, progress has been made in modelling the dynamics of bedrock channels using the stream-power (or shear-stress) incision model (Howard and Kerby, 1983) and modified versions incorporating sediment supply (e.g., Sklar and Dietrich, 2008). Although the bedrock erosion equation is incorporated into landscape evolution models, its parameterisation arises from the assumption that rivers are in equilibrium with uplift rates or climate. Elevation data of river longitudinal profiles in northwest Argentina were collected using a Leica SR530 differential-GPS system in order to quantify how the river profiles are adjusting to tectonic and climatic perturbations, and to capture the fluvial morphology in its transient state. Additionally, the profiles of abandoned terraces are compared to the active river channels to gauge the evolution of river profiles through time. Our previous research conducted in northwest Argentina’s Quebrada de Humahuaca has produced a chronological framework of basin behaviour over the last ~120,000 years and the calculation of rates of denudation through time. This framework, and the elevation data collected as part of this study, allow us to further evaluate the response times and behaviour of the river profiles to uplift and/or climate perturbations. This high resolution topographical dataset permits a test of theoretical models of fluvial incision; bed slope and catchment area data were analysed and rivers draining the western side of the valley (“equilibrium rivers”) define linear relationships between slope and area with concavity indices (0.31 ± 0.05 to 0.48 ± 0.05) that are similar to those calculated in previous studies. However, the slope-area relationships for eastern catchments are composite “equilibrated zones” where sub-basins with alluvial rivers are in the process of being reactivated by faults, and completely non-linear in zones where slope rises and falls over bedrock knickpoint which traverse faults. Concavity indices deviate from the equilibrium range of values and demonstrate that the stream-power model would be an unsuitable predictor of profile evolution for these rivers.