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In the Négron River catchment area (162 km 2 ), surface-sediment stores are composed of periglacial calcareous 'grèze' (5 ð 10 6 t) and loess (21 ð 10 6 t), and Holocene alluvium (12Ð6 ð 10 6 t), peat (0Ð6 ð 10 6 t) and colluvium (18Ð5 ð 10 6 t). Seventy-five per cent of the Holocene sediments is stored along the thalwegs. Present net sediment yield, calculated from solid discharge at the Négron outlet, is low (0Ð6 t km 2 a 1 ) due to the dominance of carbonate rocks in the catchment. Mean sediment yield during the Holocene period is 7Ð0 t km 2 a 1 from alluvium stores and 7Ð6 t km 2 a 1 from colluvium stores. Thus, the gross sediment yield during the Holocene period is about 18Ð7 t km 2 a 1 and the sediment delivery ratio 3 per cent. The yield considerably varies from one sub-basin to another (3Ð9 to 24Ð5 t km 2 a 1 ) according to lithology: about 25 per cent and 50 per cent of initial stores of periglacial grèze and loess respectively were reworked during the Holocene period. Sediment yield has increased by a factor of 6 in the last 1000 years, due to the development of agriculture. The very high rate of sediment storage on the slope during that period (88 per cent of the yield) can be accounted for by the formation of cultivation steps ('rideaux'). It is predicted that the current destruction of these steps will result in a sediment wave reaching the valley floors in the coming decades. Subboreal and Subatlantic sediments and pollen assemblages in the Taligny marsh, where one-third of the alluvium is stored, show the predominant influence of human activity during these periods in the Négron catchment.

Quantification in grams per metres squared per year of the sediment accumulation in a flood plain ('marsh') located in the southwestern Parisian basin showed that there is no close relationship between the accumulation of organic matter (OM) and mineral matter (MM) during the late Holocene, and provided an accurate view of the distinct yield and storage conditions of both sediment components. Endogenic OM accumulation in peaty sediments is not related to the climate but to felling of the alder forest and its substitution by Cyperaceae and paludal taxa in the marsh (Iron Age and Middle Ages). MM accumulation expresses mainly the sediment yield on the slopes, determined by landuse. During an initial phase (from the late Neolithic to the early Middle Ages), land-use change from crop cultivation to pastureland, possibly related to climate deterioration, led to a decrease in the sediment yield. During a second phase, since the early Middle Ages, the considerable development of crop cultivation over pasturing, even during periods of climate deterioration (such as the 'Little Ice Age'), led to a sharp increase in sediment yield. However, although sediment yield was high, the hydrodynamics in the fen did not favour particle retention. Thus, since the Neolithic, yield and storage of OM and MM sediment have been marked by human activities, initially with high climatic stress, but since the Middle Ages without significant climatic stress.

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