title: Exceptional river gorge formation from unexceptional floods.
abstract: An understanding of rates and mechanisms of incision and knickpoint retreat in bedrock rivers is fundamental to perceptions of landscape response to external drivers, yet only sparse field data are available. Here we present eye witness accounts and quantitative surveys of rapid, amphitheatre-headed gorge formation in unweathered granite from the overtopping of a rock-cut dam spillway by small-moderate floods (∼100-1,500 m(3) s(-1)). The amount of erosion demonstrates no relationship with flood magnitude or bedload availability. Instead, structural pattern of the bedrock through faults and joints appears to be the primary control on landscape change. These discontinuities facilitate rapid erosion (>270 m headward retreat; ∼100 m incision; and ∼160 m widening over 6 years) principally through fluvial plucking and block topple. The example demonstrates the potential for extremely rapid transient bedrock erosion even when rocks are mechanically strong and flood discharges are moderate. These observations are relevant to perceived models of gorge formation and knickpoint retreat.
authors.affiliation: Departamento de Ciencias Analíticas, Facultad de Ciencias, Universidad Nacional de Educación a Distancia (UNED), Senda del Rey 9, Madrid 20840, Spain.
authors.forename: L
authors.initials: L
authors.lastname: Anton
authors.affiliation: School of Geography, Earth and Environmental Sciences, Plymouth University, Drake Circus, Devon PL4 8AA, UK.
authors.forename: A E
authors.initials: AE
authors.lastname: Mather
authors.affiliation: School of Geography, Earth and Environmental Sciences, Plymouth University, Drake Circus, Devon PL4 8AA, UK.
authors.forename: M
authors.initials: M
authors.lastname: Stokes
authors.affiliation: Grupo de Investigación en Tectonofísica Aplicada, Departamento Geodinámica, Universidad Complutense de Madrid, and Instituto de Geociencias (CSIC, UCM), Madrid 28040, Spain.
authors.forename: A
authors.initials: A
authors.lastname: Muñoz-Martin
authors.affiliation: Grupo de Investigación en Tectonofísica Aplicada, Departamento Geodinámica, Universidad Complutense de Madrid, and Instituto de Geociencias (CSIC, UCM), Madrid 28040, Spain.
authors.forename: G
authors.initials: G
authors.lastname: De Vicente