We analyse average water and particulate chemistry (nutrients, major ions, heavy metals) in the Seine basin at 10 key positions, from stream order one to river mouth (order 8, 67000 km2), and for a population density gradient from one to 20000 people km-2. Particulates are studied on stream deposited sediments and on recent riverflood deposits collected over two years. The impact of Paris megalopolis (10 M people for 2300 km2) is considered both on the main river course and on periurban and urban streams. Average concentrations at each position are normalized for all variables to pristine levels, mostly determined on a set of forested headwaters, in order to define the 'change ratio'. In the main river course these ratios vary from less than 0.5 for dissolved SiO2 to more than 10 for Na+, K+, particulate Hg and Zn, and exceeds 50 for NH4 +,PO4 -3 and NO2 -. They reach over two order of magnitude for NH4 + and PO4 -3 in urban streams now covered and used as sewers, the ultimate anthropogenic impact. Few variables, such as Mg++ and particulate Co, are not affected by human activities, and Ca+ and HCO3 - are regulated by calcite precipitation linked to river eutrophication. The change ratios can be used to describe the spatial structure of impacts. For each variable, the maximum impact position depends on the pollution mode and origin: NO3 maximum is already noted in small agricultural streams but PO4 -3 maximum occurs at the most downstream stations. The maximum impact of the Paris megalopolis is observed more than 75 km downstream of the city centre (proximal impact) but the river water quality is still affected 200 km downstream (distal impact). In addition to this classical longitudinal impact mode, the megalopolis also creates radial impacts, and 'inverse' impacts due to flow regulations in upstream river reaches.
