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This Article Full Text (PDF) All Versions of this Article: B26482.1v1 121/7-8/1123    most recent Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Lamb, M. P. Articles by Dietrich, W. E. Search for Related Content GeoRef GeoRef Citation

The persistence of waterfalls in fractured rock

¹ Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA
² Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA

Although the upstream translation of waterfalls is often thought to occur by undercutting of resistant strata, collapse, and headwall retreat (e.g., Niagara Falls), many propagating waterfalls maintain a vertical face in the absence of undercutting. To explain this observation, we propose that bedrock-fracture geometry exerts a fundamental control on knickpoint morphology and evolution such that vertical waterfalls can persist during retreat due to toppling in bedrock with near horizontal and vertical sets of joints (e.g., columnar basalt). At a waterfall, rock columns are affected by shear and drag from the overflowing water, buoyancy from the plunge pool at the foot of the waterfall, and gravity. We used a torque balance to determine the stability of a rock column and any individual blocks that comprise the column. Results indicate that rotational failure should occur about the base of a headwall (and therefore preserve its form during upstream propagation) where columns are tilted in the downstream direction or slightly tilted in the upstream direction, depending on the plunge-pool depth. Flume experiments were performed to test the model, and the model provides a good prediction of the flow necessary to induce toppling and the morphology of the headwall. Waterfall-induced toppling explains the morphology of canyon headwalls in the volcanic terrain of the northwestern United States, where catastrophic paleofloods (e.g., Bonneville Flood) have carved steep amphitheater-headed canyons in columnar basalt. This model may also explain similar landforms elsewhere on Earth and Mars, and it can be used to predict the minimum flow discharge needed to create these features.

Key Words: knickpoint • amphitheater • plucking • toppling • bedrock erosion • Mars