SEDIMENT CORE CORRELATION AND MAPPING OF SEDIMENT ACCUMULATION RATES IN LAKE GENEVA (SWITZERLAND, FRANCE) USING VOLUME MAGNETIC-SUSCEPTIBILITY

Studies of the relation between a lake and its watershed often need an accurate knowledge of sediment accumulation rates over large areas. W e report here the combined method of sediment core correlation through Volume Magnetic Susceptibility profiles and sediment absolute dating, which permits drawing sediment accumulation rate maps. The Rhone Rive r is the principal tributary to Lake Geneva, both in terms of water di scharge and sediment load. Due principally to the construction of hydr oelectric dams on major tributaries of the river, there has been an im portant modification to the yearly distribution of discharge. Although the total annual discharge is not affected the sediment load should b e reduced because of the lower summer flows and sediment entrapment in dam reservoirs. To confirm this hypothesis, we analyze 82 sediment co res, recovered in the Rhone delta region in Lake Geneva. Volume Magnet ic Susceptibility (VMS) profiles are used to successfully correlate 52 cores, using a ''shifting and stretching'' method. Two VMS peaks are dated as 1961 and 1943 using time markers (Cs-137 and Hg contamination ) in seven cores. The numerous correlated VMS profiles enable maps of sediment accumulation rate to be constructed for two different periods . These maps show a clear decrease in sedimentation after 1961. From t hese maps, we estimate the sediment deficit to be 250,000 t y(-1). Thi s value is comparable to the mass of sediment trapped in reservoirs lo cated in the watershed.