Numerical Model Predictions of Intrinsically Generated Fluvial Terraces and Comparison to Climate-Change Expectations

Abstract:

Terraces eroded into sediment (cut-fill) and bedrock (strath) preserve a geomorphic record of river activity. River terraces are often thought to form when a river switches from a period of low vertical incision rates and valley widening to high vertical incision rates and terrace abandonment. Consequently, terraces are frequently interpreted to reflect landscape response to changing external drivers, including tectonics, sea-level, and most commonly, climate. In contrast, unsteady lateral migration in meandering rivers may generate river terraces even under constant vertical incision and without changes in external forcing. To explore this latter mechanism, we use a numerical model and an automated terrace detection algorithm to simulate landscape evolution by a vertically incising, meandering river and isolate the age and geometric fingerprints of intrinsically generated river terraces. Simulations indicate that terraces form for a wide range of lateral and vertical incision rates, and the time interval between unique terrace levels is limited by a characteristic timescale for relief generation. Surprisingly, intrinsically generated terraces are commonly paired, an attribute that is thought to be diagnostic of climate change. For low ratios of vertical-to-lateral erosion rates, modeled terraces are longitudinally extensive and typically dip toward the valley center, and terrace slope is proportional to the ratio of vertical to lateral erosion. Evolving, spatial differences in bank strength between bedrock and sediment reduce terrace formation frequency and length, and can explain sub-linear terrace margins at valley boundaries. Comparison of model predictions to natural river terraces indicates that terrace length is the most reliable indicator of terrace formation by pulses of vertical incision, and may contain the imprint of past climate change on landscapes.