Deciphering signals of landscape disturbance in an erosive cohesive-bank creek (late middle ages, southern Spain): The sedimentary sink role of knickpoints

The Yeguas Creek (Guadalquivir Valley, southern Iberian Peninsula) is a dominantly erosive, cohesive-bank creek submitted to bank destabilization, deep channel incision, and several contemporaneous knickpoints along the river profile, where recorded 20 to 200 m3/s floods (1945 to the present) generate hydraulic jumps. Two fill (aggradatonal) terraces were identified and analyzed: (1) Terrace 1 (T1), topping an aggradational sequence of cohesive soils to a fluvio-lacustrine tufa system (Roman Period, Morphosedimentary Unit 1: MU-1); and (2) a confined Terrace 2 (T2), topping Late Medieval (1200–1400 CE) to modern (1960s) river flood deposits (Morphosedimentary Unit 2: MU-2). The Late Medieval deposits (MU-2) constitute a solitary pulse of river aggradation preserved in this dominantly erosive setting over the last two millennia (after Roman Period). Unlike the standard horizontal bedding of aggradational fluvial terraces, the Late Medieval terrace in the Yeguas River exhibits a downstream-dipping, concave-up scour base infilled by poorly sorted and massive deposits. These contain high variability in grain size (gravels, sands, and silts) and grain type (cm-scale charcoal, freshwater fossils, and pottery remains). Sedimentary features, internal structure and the recent hydrogeomorphologic behavior of the Yeguas Creek, allow us to interpret the Late Medieval terrace as an ancient aggradational knickpoint. This cut-and-fill structure (knickpoint) is controlled by the high sediment discharge transported by the stream during Late Middle Age floods. This Late Medieval sediment legacy captured in the aggradational knickpoint of the Yeguas Creek—alongside other reported Late Middle Ages river terraces in the Guadalquivir Valley—is discussed in the context of the river's adjustment over 250 years of military activities impacting a frontier land (between the Christian Kingdom of Castile and the Muslim Nasrid Kingdom of Granada). Human activities in the Late Middle Ages may have accelerated landscape and river degradation in the southern Iberian Peninsula, which began after the Roman Period.