Climate and anthropogenic factors affecting alluvial fan development during the late Holocene in the central Ebro Valley, northeast Spain

Abstract

The late Holocene in the central Ebro Depression is characterized by a number of morphodynamic stages that affected hillslopes and channels. Climate fluctuations and human activities together caused severe erosion and floods, resulting in alternating stages of accumulation and incision in the valley floors. Seven accumulation levels (N6-N0) correlated with Ebro River fluvial terraces have been identified in the secondary fluvial network of the central Ebro Depression. The main accumulation level (N3) corresponds to a long period of increased hillslope erosion and sedimentation in the valleys, from the Neolithic to the late Roman periods. The accumulation levels are evident throughout the valleys and extend to the valley mouths, where many connect with alluvial fans. The study of several sedimentological profiles, 14C AMS dating, and many archaeological remnants, have enabled detailed assessment of sedimentation characteristics in the alluvial fans and the relationship thereof to the presence of human settlements. Four sedimentation stages have been identified: (i) pre-Bronze Age (> 3800 years BP), (ii) Bronze Age/Iron Age (3800-2500 years BP), (iii) Roman/post-Roman period (2500-1500 years BP), and, (iv) the early Middle Ages, mainly during the Visigothic period (sixth century). The sedimentation dynamics of the alluvial fans are related to: (i) climate variability, (ii) human activities in the watershed, and (iii) the meandering character of the Ebro River, which has lateral displacements that change the local base level of tributaries and fans. Level N3 in the valley floors corresponds to stages i-iii of the alluvial fans, which are mostly related to human-induced transformation of characteristics of the secondary basins, coinciding with enhanced erosion under favorable (warm and humid) climatic conditions. In contrast, the Medieval stage, which corresponds to level N2 in the valleys, was caused by floods during a period of high climatic variability.