Epigenetic gorges in fluvial landscapes

W. B. Ouimet, K. X. Whipple, B. T. Crosby, J. P. Johnson, T. F. Schildgen
2008 Earth Surface Processes and Landforms  
Epigenetic gorges form when channels that have been laterally displaced during episodes of river blockage or aggradation incise down into bedrock spurs or side-walls of the former valley rather than excavating unconsolidated fills and reinhabiting the buried paleovalley. Valley-filling events that promote epigenetic gorges can be localized, such as a landslide dam or an alluvial/debris flow fan deposit at a tributary junction, or widespread, such as fluvial aggradation in response to climate
more » ... nge or fluctuating base-level. The formation of epigenetic gorges depends upon the competition between the resistance to transport, strength and roughness of valley-filling sediments and a river's ability to sculpt and incise bedrock. The former affects the location and lateral mobility of a channel incising into valley-filling deposits; the latter determines rates of bedrock incision should the path of the incising channel intersect with bedrock that is not the paleovalley bottom. Epigenetic gorge incision, by definition, post-dates the incision that originally cut the valley. Strath terraces and sculpted bedrock walls that form in relation to epigenetic gorges should not be used to directly infer river incision induced by tectonic activity or climate variability. Rather, they are indicative of the variability of shortterm bedrock river incision and autogenic dynamics of actively incising fluvial landscapes. The rate of bedrock incision associated with an epigenetic gorge can be very high (> > > > >1 cm/yr), typically orders of magnitude higher than both short-and long-term landscape denudation rates. In the context of bedrock river incision and landscape evolution, epigenetic gorges force rivers to incise more bedrock, slowing long-term incision and delaying the adjustment of rivers to regional tectonic and climatic forcing. Figure 1. Conceptual model of how large landslide deposits and landslide dams can lead to the formation of epigenetic gorges. The particular example depicted shows an epigenetic gorge forming as a river incises into valley landslide deposits and becomes entrenched into side-walls of the former valley. Sequence: (a) initiation of a large landslide in an incised valley with bedrock valley walls; (b) landslide deposits fill the valley, forming a landslide dam; (c) river starts to cut down through the landslide debris while eroding the bedrock channel walls; (d) river establishes itself into bedrock, abandons the landslide debris and continues to cut an epigenetic gorge. The first example within the Dadu River catchment lies near the town of Danba in western Sichuan (SW China). Upstream from Danba, there are significant landslide-related knickpoints on the Dadu mainstem and three of its large tributaries, the Ge Sud Za He, Xiaojin Chuan and Dong Gu He (Ouimet et al., 2007) . An excellent example of a landslide-induced epigenetic gorge is located 1 km upstream and SW of Danba along Dong Gu He River (Figure 2 ). The epigenetic gorge here is cut within a mica-schist and the total amount of epigenetic bedrock incision is 80-100 m. Figure 2. Landslide-induced epigenetic gorge located 1 km upstream of Danba along Dong Gu He River (GPS: 30·8787 North, 101·8735 West). (a), (b) Paired photograph and sketch of the epigenetic gorge viewed from upstream. (c) Corona image of Danba region highlighting the large landslide complex above the city. A star marks the epigenetic gorge location. Locations 1 and 2 indicate where photographs (a) and (d) were taken, respectively. (d) Photograph showing the landslide scarp above Danba. This figure is available in colour online at The active channel through the gorge does not show a significant increase in channel gradient or decrease in channel width through the gorge, though bedrock valley width is much smaller than the valleys upstream and downstream. A second example of a landslide-induced epigenetic gorge lies farther north in the Dadu River catchment, on the Do Qu River ~70 km west of Maerkang (Figure 3 ). The epigenetic gorge here is cut within Jurassic flysch and the total amount of epigenetic bedrock incision is 100 -120 m. The active channel through the gorge is steeper and narrower than alluvial reaches upstream and downstream. Both of the examples within the Dadu River catchment were of epigenetic gorges that no longer have a significant waterfall or knickpoint associated with them. A smaller scale landslide-induced epigenetic gorge that is actively Figure 3. (a)-(d) Landslide-induced epigenetic gorge located ~70 km west of Maerkang on the Do Qu River (GPS: 31·7973 North, 101·5150 West). (a), (b) Paired photograph and sketch of the epigenetic gorge viewed from upstream. (c) Map sketch of the epigenetic gorge. The Dadu River flows from upper left to lower right. Locations 1 and 2 indicate where photographs (a) and (d) were taken, respectively. (d) Photograph of the epigenetic gorge from downstream. (e), (f) Paired photograph and sketch of a landslide-induced epigenetic gorge located on the Somang Qu River (a tributary of the Min River) ~25 km NW of Lixian (GPS: 31·5205 North, 102·9198 West). This particular example has a waterfall, indicating that the epigenetic gorge is actively incising its gorge. This figure is available in colour online at www.interscience.wiley.com/journal/espl 6 W. B. Ouimet et al.
doi:10.1002/esp.1650 fatcat:ydirqbt2vvayxaesxb7kxjhmyi