Ground of Lake Maggiore in 3D

New sonar produces the first three-dimensional and detailed image of the lake bottom

The lake bottom of the upper Lago Maggiore. © Eawag
Read out

Thanks to a sophisticated sonar method, Swiss researchers have for the first time succeeded in obtaining detailed surveys of the bottom of Lake Maggiore. The new pictures not only show sediment deposits, undersea gullies and other structures centimeter-accurate and in 3D, they also suggest that the lake will divide into two smaller lakes in another thousand years.

The Limno geologist Flavio Anselmetti from the Swiss water research institute Eawag is thrilled: Such detailed detailed surveys of the Swiss Lakes are a novelty. For two weeks in May 2009, a team from the water research institute with the research ship Thalassa drove to and fro on the Swiss part of Lake Maggiore and sampled the bottom of the lake with a special sonar. Coastal states have been using the process for a long time to display the reason for their seas on bathymetric maps. In Switzerland, it is only being used as part of a pilot project. So far, parts of Lake Lucerne and Lake Geneva were measured, now it was also the Lago Maggiore turn.

In contrast to conventional echosounders, modern sonar devices make it possible to determine a large number of depth points for each measurement. Thus, depth values ​​can be determined for each square meter of the lake floor and finally generated with the computer three-dimensional maps of the reason. Now the pictures are there: even underwater cables or - as in the Verzasca Delta before Tenero - small funnels, from which methane gas emerges, are visible on it.

Relics of earlier estuaries

From the exact records the researchers can read stories about the sea development. It is noticeable, for example, that there is no undersea channel on the slope of the Maggiadeltas today, but it is certainly in the continuation of earlier estuaries off Ascona. This allows conclusions to be drawn on the frequency and composition of the feeding system of the Maggia. It also stands out that unlike the Maggia, Ticino and Verzasca do not form fan-shaped deltas, but slowly advance their deposits along an almost straight line westwards into the lake.

In 1, 000 years two lakes in?

Very clearly visible on the new underwater maps, how the Maggiadelta will sooner or later separate the uppermost part of the lake. Even today, the threshold, which has pushed itself to the foot of the Gambarogno, is higher than the lake floor in the eastern part of the lake. It will be difficult to say how long it will be before San Nazzaro sees no more waves breaking, but a river flowing. "Certainly not in the next 500 years, " says researcher Anselmetti. display

The new pictures can not only answer questions about what happened in the past in the lake. They are also used for prognoses: Where deposits are now visible on steep flanks, the lake bottom could slip in an earthquake and trigger a tsunami-like wave. Or the maps can be used to monitor the loading of sludge into the lakes. After all, with changing climate, the flow of water on the banks changes, and so does bedload transport and sedimentation.

Signs for unstable dependencies?

For the time being, however, Eawag researchers want to examine individual structures even more closely. Among other things, they focus on the small rounded depressions in the northern part of the Ticino / Verzascadeltas. These "pockmarks" are signs of gas leaks in the area. "But I hardly believe that methane gas can be usefully used, " says Anselmetti, "but the gas sources are a sign where the slope could become unstable."

The possible uses of the exact bathymetric recordings are wide-brimmed. It is conceivable to monitor the gravel degradation at the river mouths. At the Vierwaldst ttersee, where Eawag has created a similar map, archaeologists have expressed interest in the pictures: they hope to find evidence of early settlements when the lake levels were even lower. And the military has knocked on whether possibly in the lake sunken ammunition could be caught up with the new procedure.

(Swiss Water Research Institute Eawag, 07.10.2009 - NPO)