Floods and quaternary sedimentation style in a bedrock-controlled reach of the Bergantes River, Ebro Basin, Northeast Spain

Headwater catchments of the southern tributaries of the Ebro River in north-east Spain are typically steep and sparsely vegetated on highly folded and fractured rock. This makes the river channels and valley floors particularly sensitive to climatic change and anthropogenic disturbance and therefore a valuable source of information on the response of rivers to a changing bioclimatic and flooding regime. This paper describes the key geomorphological features of a 2 km reach of the Bergantes River, an unregulated tributary of the Guadalope-Ebro system. The study reach is strongly affected by the regional tectonics and the Pleistocene alluviation history. Geomorphological mapping, EDM survey and luminescence dating (IRSL) are used to establish the Late Pleistocene and Holocene alluvial history of the Bergantes and to describe the modern sediment transfer processes in three contrasting, but interlinked reaches: (i) a wide alluvial reach dominated by thin lobate bars and gravel sheets, (ii) a steep, bedrock controlled, step-pool sequence with little within-channel deposition, and (iii) a bedrock-walled confined reach with no opportunity for lateral migration, and deposition dominated by fine-grained sediment trapped in rock niches. Interpretation of the terrace and floodplain morphology and sedimentology allows the valley floor to be divided into discrete alluvial units associated with four distinct episodes of fluvial aggradation. The first and third terrace levels are dated at 37.8±4.2 ka and 9.6±1.6 ka. An intermediate level has been identified, and overbank deposits on the terrace surface have been dated at 0.53±0.08 ka. This suggests that selective deposition of fines high above the current channel level can occur in large flood events. Relatively moderate floods are responsible for transporting coarse material as gravel sheets and lobes 3-5 m above the present channel. Dendro-dates obtained from vegetation growing on the active gravel floodplain yield minimum ages of deposition for these lobes of 35 years. This paper suggests that the nature of the Bergantes valley floor is highly influenced by the outcropping bedrock, but also by the flood regime and more recent variations in climate and human activity.