Terraces eroded into sediment (alluvial) and bedrock (strath) preserve an important history of river activity. River terraces are thought to form when a river switches from a period of slow vertical incision and valley widening to fast vertical incision and terrace abandonment. Consequently, terraces are often interpreted to reflect changing external drivers including tectonics, sea level, and climate. In contrast, the intrinsic unsteadiness of lateral migration in rivers may generate terraces even under constant rates of vertical incision without external forcing. To explore this mechanism, we simulate landscape evolution by a vertically incising, meandering river and isolate the age and geometry of autogenic river terraces. Modeled autogenic terraces form for a wide range of lateral and vertical incision rates and are often paired and longitudinally extensive for intermediate ratios of vertical-to-lateral erosion rate. Autogenic terraces have a characteristic reoccurrence time that scales with the time for relief generation. There is a preservation bias against older terraces due to reworking of previously visited parts of the valley. Evolving, spatial differences in bank strength between bedrock and sediment reduce terrace formation frequency and length, favor pairing, and can explain sublinear terrace margins at valley boundaries. Age differences and geometries for modeled autogenic terraces are consistent, in cases, with natural terraces and overlap with metrics commonly attributed to terrace formation due to climate change. We suggest a new phase space of terrace properties that may allow differentiation of autogenic terraces from terraces formed by external drivers.
Bibliographical noteFunding Information:
This work was supported by the Department of Defense through the National Defense Science and Engineering Graduate Fellowship (NDSEG) Program and NSF grant EAR-1147381 to M.P.L. Acknowledgment is also made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. Model data and digitized terrace maps from this manuscript are available upon request. We thank Bob Anderson, Jean-Philippe Avouac, Patrick Belmont, Noah Finnegan, Dirk Scherler, and Andrew Thompson for their insightful discussions. We acknowledge Alexander Densmore, Nicole Gasparini, Alex Whittaker, Stephen Lancaster, and John Buffington for detailed and constructive formal reviews.
©2016. American Geophysical Union. All Rights Reserved.
- climate change
- landscape evolution
- river terraces