Massive incision and outcropping of bedrock in a former braided river attributed to mining and training

Abstract

This research analyses multiple anthropogenic pressures in the Bernesga River, a gravel-bed river in Northwestern Iberian Peninsula. Among the causes of river incision in Bernesga River are extensive mining during the second half of the twentieth century, a ∼100 years-long history of channelization and river training starting around 1900, and to a lesser extent, land-use changes in the basin. As a result of these impacts, reach-averaged incision exceeds 8.5 m. Severe incision occurred simultaneously with a dramatic change in channel pattern: from ∼450 m-wide braided morphology around middle of the twentieth century to a narrow single-thread (∼40 m wide) channel in 2017. Quaternary alluvial deposits have been mostly depleted exposing the underlying bedrock. Unlike other rivers, bedrock outcrops have not stopped incision. Extremely narrow channels (∼12 m wide) have developed and incised at rates of ∼0.11 m/yr into bedrock materials, headcuts have formed on bedrock deposits and have rapidly retreated (at 33 m/yr) and the channel has incised forming adverse slopes under the presence of bed sills. Cause-effect relationships of cumulative impacts have been established between the recent evolutionary trajectory of the Bernesga River and multiple and simultaneous anthropogenic pressures.