Bottom morphology and shallow structures in the northwestern part of Lake Kinneret

The bottom morphology and shallow structures in the northwestern part of Lake Kinneret, between Tabgha in the north and Fuliya in the south, were studied in detail during a high-resolution shallow geophysical survey conducted in April 2002. The bottom morphology was mapped with side scan sonar and the shallow sub-bottom section (up to 13 m deep) was mapped with CHIRP II sub-bottom profiler. Four different bottom morphologies were recorded and mapped: (i) areas with smooth bottom, usually at deeper water, indicating very low sedimentary transport; (ii) areas with high sedimentary transport at shallow water, showing ripple mark features; (iii) areas of gas release, probably methane, in localized "chimney-like" diffusion or as non-localized "bubble-like" diffusions; and (iv) areas with large numbers of holes, most of them unknown before, that range in size from 1 to 30 m in diameter and 0.3 to 8 m in depth. In the 1960s a few of them (e.g., the Barbutim and Maagan group) were found to be the site of underwater salt springs. The origin of the holes in the study area is not yet known. The shape, depth, and location of these holes suggest different origins: We think that the majority of the holes are created by gas release. These are very shallow, they appear or disappear seasonally, and intensify when water levels are low. The deeper holes, probably created by water flow (salt or fresh), seem to last longer and do not change position as rapidly as those resulting from gas release. In the Tabgha region, the deepest holes were found close to the active faults. To validate our hypothesis, future studies are needed that will measure and monitor the activity (e.g., flow rate) and changes in dimension and depth of these holes. As in previous shallow geophysical surveys (mainly 3.5 kHz), most of the lake bottom in the study area shows no internal seismic reflectors (probably no acoustic penetration) while in localized areas, usually close to the coast, good acoustic penetration was found. These shallow seismic characteristics may represent in some places the acoustic effect (masking) of gas/no gas in the upper sediments, while in other places they may be associated with lithological changes in the upper sub-bottom. This can be homogeneous silty lithology (no internal reflectors) vs. silt with sand overlaying layers of conglomerates and boulders (showing internal reflectors). Several active normal faults that offset the upper sub-bottom reflectors by a few meters and also in some cases influence the bathymetry were mapped in the study area. The shallow penetration of the CHIRP sub-bottom profiler limited the ability to describe their properties and to connect them to deeper faults, but it seems that most of them are related to the continuation of faults on land. In Tabgha, the major faulting trend is NE-SW and the minor is NE-SW, while in Fuliya the major trend is NE-SW and the minor is NE-SW.