River braiding caused by rapid floodplain deformation – Insights from Holocene dynamics of the Jarama River in central Spain

In this paper we will use an illustrative example to show the significance of rapid vertical movements inside floodplains as well as varying discharge values for the alteration of channel and sedimentation patterns during the Holocene. Concerning such relationships, well documented and well dated references from sedimentary records are rather rare. The developments within the lower Jarama Valley have been significantly supported by an interaction of tectonic activity, evaporite dissolution, and halokinetics that initiated rapid vertical movements over short distances and caused the formation of knick-points and gradient changes along the longitudinal floodplain profile. Furthermore, the specific Mediterranean climate caused highly variable discharges including extreme runoff peaks.The juxtaposition of different floodplain gradients gave rise to spatial and temporal changes of discharge patterns and channel geometry, which is reflected by varying sedimentation as well as erosion patterns. We have been able to confirm that alluvial thickening due to intense dissolution and subsidence has been a characteristic and frequently recurring feature during the Pleistocene. Moreover, we want to emphasize that also during the Holocene gradient adjustments exercised controlling influence on fluvial dynamics, in particular with regard to channel response. In fact, we found consistency between low and moderate gradients and meandering single channel patterns associated with overbank deposition in distal floodplain positions. Only a section with stronger inclination revealed evidences for multiple channel discharge patterns in form of gravelly braided river deposits. In comparison with recent discharge patterns and information from historical sources it appears that braided river deposition took place just within a short time frame between 5 and 3 ka cal. BP. This matches exactly a period of strong climatic aridity including highly variable precipitation and discharge patterns. From that we conclude, that in the first instance, gradient changes promoted the change of channel and discharge patterns, but however, specific overlap of the parameters gradient and runoff is considered to have been necessary to generate braided discharge pattern. A subsequent formation of braided patterns during the Late Holocene was presumably prevented by river entrenchment as well as a strong increase of sediment availability during the time of Roman colonization.On the basis of this study, we can establish a number of general relationships between floodplain gradient, runoff pattern, and channel response during the Holocene period that may help us to better understand fluvial behavior in various different areas.