A study on multifactorial riverbed degradation

Abstract

This research focuses on analyzing the multiple causes of riverbed degradation along the lower reach of the Bernesga River, a gravel-bed river that drains 1,140 km2 of the South-ern Cantabrian Range, in Northwestern Iberian Peninsula. Among the causes of the riverbed deg-radation in the Bernesga River are extensive gravel mining during the second half the XX century and a history of continuous channelization and hydraulic works starting around 1900. Average degradation in the lower 16 km of the Bernesga River, estimated after comparing historical lon-gitudinal profiles, exceeds 7.5 m, although at some locations bed incision is about 9 m. Quaternary alluvial deposits in the area have been eroded during the degradation process therefore exposing the underneath bedrock, which in turn, has not stop the incision progress. The severe incision occurred simultaneously to a dramatic change in planform pattern: from a wide (~ 450 m) and highly mobile braided morphology around mid XX century to a single-thread, narrow (~ 36 m) and mostly straight channel in 2017. By using simple algebraic expressions based on equilibrium principles, we are able to assign what fraction of the recorded degradation is due to each of the originating causes. This methodology allows us to estimate the non-linear effects and the positive feedbacks between the diverse causes of riverbed degradation acting in the lower Bernesga River.