Records of riverbome turbidity currents and indications of slope failures in the Rhone delta of Lake Geneva

The sublacustrine topography of the Rhone delta in Lake Geneva is characterized by a channel that can be traced about 9 km beyond the mouth of the river. Two instrumented moorings were deployed from June to August 198 5 in the channel axis at depths of 150 and 20 1 m to detect underflows of dense, sediment-laden river water. Each array consisted of three current meters and a thermistor chain. Data were recorded every 15 min (currents) or 30 min (temperatures). During 78 d of effective measurement, 3 1 welldefined downslope events were recorded, five with speeds >50 cm s-l. Major events correlate with discharge peaks of the Rhone, and current activity is characterized by single pulses lasting a few hours. Some flow events were accompanied by temperature increases of up to 3°C suggesting two different current origins: the first is intrusion of sediment-laden, warm river water into the cold bottom water of the lake as a dense turbidity underflow, and the second is turbidity-current flow triggered by sliding or slumping of delta deposits. Damage caused to instruments and breakage of mooring links suggests sliding of large amounts of deltaic sediments or avalanchelike turbidity currents. surface. This process has been described by Fore1 (188 5), whose classic work includes many basic observations and theoretical considerations concerning the sinking-river phenomenon. He also attributed the origin of the channels and levees in the Rhone delta to the underflow of dense, sedimentladen alpine meltwater. Surprisingly, Forel’s scientific conclusions have never been tested by direct current measurements in Lake Geneva itself, although his description of the “cascading river” is often quoted as one of the earliest scientific treatises on turbidity-underflow phenomena. Several studies in different alpine lakes (Lambert et al. 1976) as well as in glaciolacustrine environments (see Weirich 1986) and in reservoirs (Bruk 1985) have indicated that densitycurrent activity is a common process in lakes with sediment-laden tributaries. Maximum velocities of 120 cm s-l have been measured in the Rhine delta area of Lake Constance (Lambert 1982). The alpine Rhone River is the main tribTwo different modes of river water inflow utary and by far the most important source occur in thermally stratified lakes: under of solid matter to Lake Geneva, where it normal flow conditions, the river water will has built a large delta covering 100 km2, be injected at the interface between the or about a sixth of the lake surface (Houbolt warm, upper layer (epilimnion) and the cold, and Jonker 1968). For most of the year, the lower layer (hypolimnion); the river water cold, sediment-laden Rhone-as a result of will form an interflow close to the thermoits higher density-plunges below the lake cline. During flood surges the concentration