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

We analyze the TSS distribution over the whole Seine basin (67 500 km2, stream order 8) from: 1. a set of 236 stations sampled quaterly to bimonthly from the French national water quality monitoring network over the 1971-1997 period, 2. four stations sampled daily over 3 and 4 years located on stream orders 1, 5, 6 and 8, 3. a weekly survey of the exceptional 1994/95 high 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 year, 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 maximum TSS barely reach 300 mg l-1. Due to similar relief distributions, runoff patterns and geology in all sub-basins, major tributaries have near-identical long-term TSS regimes and seasonal variations during the 1994/95 flood stage. Second order TSS variations are linked to lithology: streams draining argillaceous and marl terrains are up to 3 and 4 times more turbid than 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 levels C10% to C75% double from order 3 to 8. Human impacts on TSS levels are quite limited: Greater Paris influence on longitudinal profiles is not observed; 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, the 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 mouth from 1971 to 1997. Comparison with a previous daily survey in 1863-1866 showed present marked decrease of average TSS and TSS yearly range attributed mostly to locks.