Floodplain Sediments of the Tagus River, Portugal: Assessing Avulsion, Channel Migration and Human Impact

A study of the Tagus River floodplain (Portugal) has been carried out using a variety of methods including sedimentological, geochemical and geochronological analyses, as well as geomorphological and hydrological studies, performed in order to characterize the flood sediments and the dynamics of the river during the Holocene. Until the 19th century, the Tagus was an anastomosed river, with multiple channels separated by large areas of floodplain; today, it is a single channel river with alternate bars, mainly as a result of anthropogenic modification. In order to study its behaviour during the Holocene, four cores ranging in length from 3.70 to 8.04 m were obtained from the floodplain and 232 samples were analysed. Detailed textural analysis was necessary owing to the lack of preservation of sedimentary structures in the cores. The sediments of the present-day geomorphological elements of the floodplain (bars, natural levees, crevasse-splay deposits, flood basin and abandoned channels) were also studied in order to compare their textural characteristics with those of the cored samples. Both the present analogues and core sediments were well discriminated using mean diameter versus standard deviation and average mean diameter versus average mud percentage graphical correlations. The textural parameters defined (sand/mud ratio, mean, standard deviation, skewness) and particularly the interparameter correlations, together with 12 C numerical ages of organic matter obtained, allowed the evaluation of: (i) sedimentological changes in the floodplain (channel migration, avulsion and crevasse-splay development); and (ii) the chronological evolution of the different energetic environments of the floodplain for the past 4 kyr. These approaches permitted the determination of sedimentation rates for the different alluvial plain environments. The highest sedimentation rates occurred in the flood basin and channelfill domains, with values ranging from 2.23 mm yr to 4.72 mm yr and the lowest in the channel (0.27 mm yr). Values from 0.81 to 1.60 mm yr were recorded in sedimentary environments proximal to the channel, where several crevasse-splay episodes have been recognized. In the period common to the four cores, i.e. the past 4000 kyr, the sedimentation rates decreased towards the present. In spite of increasing human intervention in the hydrographic basin during this time, the increasing aridity of the climate is considered to have outweighed the sediment availability, which resulted in a decreasing sedimentation rate.