EFFECTS OF BANK STRENGTH ON TERRACE FORMATION IN A NUMERICAL MODEL OF MEANDERING RIVERS WITH MIXED BEDROCK AND ALLUVIAL BANKS

Vertically incising rivers leave a landscape record in the form of sediment (fill-cut) and bedrock (strath) terraces. River terraces are often interpreted to form when a river transitions from a period of slow vertical incision and valley widening to a period of fast vertical incision and terrace abandonment. A defining characteristic of actively meandering rivers is the unsteadiness of their lateral erosion rates in space and time, yet the influence of unsteady lateral erosion rates on terrace formation under steady vertical incision is largely untested. We developed a simple numerical model of valley evolution by a vertically incising, meandering river, to characterize the age and geometry of river terraces formed under simple vertical incision scenarios. The model further incorporates differences in bank strength that alter the course of meandering channels in valleys with mixed bedrock and alluvial banks. Terraces commonly form under model conditions of constant bank strength and constant vertical incision rates. Moreover, terrace attributes can resemble those commonly attributed to cyclic incision driven by climate change, including millennial-scale age differences, terrace occurrence at similar levels on opposites sides of a valley (i.e., pairing), and terrace extent along-valley. For model runs with bank strength differences between bedrock and sediment, bank strength coevolves with channel migration in three distinct regimes, including: (1) channel entrenchment in bedrock, with terrace abandonment; (2) highly confined meandering with no terrace formation; and (3) loosely confined meandering with terrace development. Overall bank strength differences reduce terrace formation frequency and length, favor pairing, and can explain sublinear terrace margins at valley boundaries that are commonly observed for natural cases. The coevolution of channel migration with bank materials can amplify the inherent unsteadiness of lateral erosion rates and produce a variety terrace shapes without changes in external forcing driven by base-level or climate change.