Analysis of grain size, composition of sand-size and <2mu m size fractions of sediments stored in the Lac Chambon watershed during the Postglacial period, using the earlier quantification of sediments, made it possible to precise the effect of watershed parameters on detrital sediment yield. The whole yield of 1/5 pebbles and gravels, 2/5 sands, 2/5 silts and some % clays, taken from volcanic and granitic areas about in proportion to their respective outcrop surfaces, shows effect of non-varying watershed parameters: lithology, relief and tectonics. Fluctuations of climate and vegetation induced variations in grain size of sediment yield. Sediment yield (from 49+ or -15 to 166+ or -50 m3/km 2 /yr) dominated either on volcanic areas (Bolling-Allerod, Younger Dryas. Pre-Boreal periods), either on granitic areas (Boreal period), or was in proportion to the outcrop surfaces of these rocks (first half of the Subatlantic period). During the second half of the Subatlantic period, man made vegetation clearance increased sediment yield from granitic areas.