The qualities of a laser diffraction grain size analyser, the Coulter LS-100 (range claimed by the manufacturer: 0.4-900 mum in a single measurement), are evaluated on sediments of fluvial and lacustrine origin. Accuracy and resolution of measurement on standard latex spheres are excellent. Reproducibility of the results on natural sediments appears to be satisfactory, but the method underestimates the fraction of clay particles with an efficiency of detection (36-70%) proportional to the clay content determined from pipette analysis. This efficiency is somewhat higher than those reported from other instruments of the same generation. Comparison of the Coulter LS-100 with other sizing techniques shows good agreement with the sieving method but some differences appear with the electroresistance particle technique: median and mean size values measured by the Coulter Counter TA1 are systematically lower than those obtained by the Coulter LS-100. Analyses show good correlation with those of a Malvern Laser particle analyser but a discrepancy appears with very fine silt and clay sized sediments. The Coulter LS-100 detects a higher clay content than that measured with the Malvern Laser Sizer 2600. Except when precise measurements of clay content are needed, the Coulter LS-100 produces precise and accurate results in size ranges required for geological and environmental studies

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During this century, the characteristics of the Upper Rhone River discharge, flowing into Lake Geneva, have been altered in response to the changes which have occurred in its watershed. Principally, numerous hydroelectric dams have been constructed on the course of the Rhone River tributaries. At present the major reservoirs can hold about 1220 10(6) m(3) of water, which represents 1/5 of the total annual Rhone River flow. Flow regime characteristics of the river have been modified by the dam operations. Water is released from reservoirs during winter and stored in summer. Large floods have also been reduced in amplitude and frequency. From the available literature data, sediment rating curves have been calculated and used to estimate the evolution of the sediment load from the Rhone River to Lake Geneva. They show that sediment input has decreased by at least a factor 2. This reduction has impacted the occurrence of underflows along the lake bottom, due to the reduction of sediment-laden floods, which in turn may have considerable negative effects on the reoxygenation of Lake Geneva deep waters

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