QUATERNARY EVOLUTION OF A MEANDERING GRAVEL-BED RIVER IN CENTRAL SPAIN

The river Jarama is a medium-sinuosity meandering river with gravel bedload. Throughout the Quaternary it developed a large number of terraces in a repetitive sequence of aggradation-stability-degradation-stability stages, which are studied in the middle reach of the river, applying geomorphological and sedimentological methods, as well as radiocarbon dating techniques. The Upper Terrace System of earliest Pleistocene to mid Pleistocene age forms isolated fragments vertically disconnected and colluviated, and their sequential distribution of facies implies lateral shifting of channels and a well-developed flood plain. The Middle Terrace System of mid-to-late Pleistocene age has differentiated levels in the upstream reaches while downstream they condense giving a thick multi-episodic series, corresponding to the tectonic tilting of the basin. Classical point bar fining-upward sequences of a medium- to high-sinuosity river (with gravels and sands as bedload) are clearly differentiated in them. A major degradation stage follows this soup of terraces, with an incision of up to 30 m. Then, during a relatively long period of stability, a forest of riparian species developed, prior to the last aggradational stage of the Lower Terrace System. The aggradation resulted in a fining-upward sequence where lateral accretion is clearly visible in the gravels, due to migration of the medium- to low-sinuosity river. Sedimentation continued and was active up to the 1950s, but since then, intense anthropogenic modifications led to renewed incision of the river.