Spatial and temporal variability of Total Suspended Solids in the Seine basin

We analyze the TSS distribution over the whole Seine basin (67 500 km(2), s tream order 8) from: 1. a set of 236 stations sampled quaterly to bimonthly from the french national water quality monitoring network over the 1971-19 97 period, 2. four stations sampled daily over 3 and 4 years located on str eam orders 1, 5, 6 and 8, 3. a weekly survey of the exceptional 1994/95 hig h water stage at 4 stations upstream and downstream of Greater Paris (10 M. people). Due to very low relief and even rainfall distribution over the ye ar, the Seine and its river network from order 3 to 8, are characterized by very low TSS: 79% of medians (C50%) are between 8 and 32 mg l(-1) and maxi mum TSS barely reach 300 mg l(-1). Due to similar relief distributions, run off patterns and geology in all sub-basins, major tributaries have near-ide ntical long-term TSS regimes and seasonal variations during the 1994/95 flo od stage. Second order TSS variations are linked to lithology: streams drai ning argillaceous and marl terrains are up to 3 and 4 times more turbid tha n those draining limestones and chalks. Basin size was also tested: the TSS range (quantiles C1% to C99%) decreases from order 1 to 8, and quantiles l evels C10% to C75% double from order 3 to 8. Human impacts on TSS levels ar e quite limited: Greater Paris influence on longitudinal profiles is not ob served; in periurban streams, where population density reach 1000 p km(-2), TSS levels are twice those observed in rural conditions (40 p km(-2)): C75 % are 32 +/- 12 and 17.5 +/- 9 mg l(-1), respectively. In orders 6 to 8, th e lower TSS quantiles (C10% and C25%) are higher than in orders 3 - 5, this can be attributed to eutrophication and/or to an important fluvial traffic . No significant trend was observed on the TSS distributions at the river m outh from 1971 to 1997. Comparison with a previous daily survey in 1863-186 6 showed present marked decrease of average TSS and TSS yearly range attrib uted mostly to locks.