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

The river Jarama is a medium-sinuosity meandering river with gravel be dload. Throughout the Quaternary it developed a large number of terrac es in a repetitive sequence of aggradation-stability-degradation-stabi lity stages, which are studied in the middle reach of the river, apply ing geomorphological and sedimentological methods, as well as radiocar bon dating techniques. The Upper Terrace System of earliest Pleistocen e to mid Pleistocene age forms isolated fragments vertically disconnec ted and colluviated, and their sequential distribution of facies impli es lateral shifting of channels and a well-developed flood plain. The Middle Terrace System of mid-to-late Pleistocene age has differentiate d 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 clear ly differentiated in them. A major degradation stage follows this soup of terraces, with an incision of up to 30 m. Then, during a relativel y long period of stability, a forest of riparian species developed, pr ior to the last aggradational stage of the Lower Terrace System. The a ggradation resulted in a fining-upward sequence where lateral accretio n is clearly visible in the gravels, due to migration of the medium- t o low-sinuosity river. Sedimentation continued and was active up to th e 1950s, but since then, intense anthropogenic modifications led to re newed incision of the river.