River gorge eradication by downstream sweep erosion

Rivers can respond to local surface uplift or depositional blockage by incision of a narrow channel, but the resulting gorges are often temporary features. Although fluvial downcutting, channel narrowing and gorge formation have been studied over a range of conditions, the eradication of bedrock gorges and the transformation of an incised gorge into a broad floodplain are poorly understood and have not been documented in a natural river. Here we mapped channel width using aerial photographs and Lidar data from 2004 to 2010 of the Daan River bedrock gorge in Taiwan-formed in response to coseismic uplift in 1999-to evaluate mechanisms of gorge eradication. We identify a mechanism, which we term downstream sweep erosion, that is rapidly transforming the gorge into a bevelled floodplain through the downstream propagation of a wide erosion front located where the broad upstream channel abruptly transitions into a narrow gorge. We estimate that gorge erosion will remove the uplifted topography in as little as 50 years. We suggest that downstream sweep erosion can remove bedrock gorges, impose local valley widths based on upstream conditions, remove evidence of tectonic activity or depositional river blockages and therefore complicate the interpretation of channel morphology in terms of tectonic or climatic forcing.