Secrets Of The Lake Floor Of Lake Maggiore

Flavio Anselmetti, a limnogeologist, is highly enthusiastic: topographical surveys of Swiss lake floors have now been performed in unprecedented detail. On a 2-week excursion in May, a team of Eawag scientists aboard the research vessel Thalassa criss-crossed the Swiss part of Lake Maggiore, using special sonar equipment to scan the lake floor. Although this method has been employed for some time by coastal states to produce bathymetric charts of the seabed, in Switzerland it has only been used in a pilot project. So far, parts of Lakes Lucerne and Geneva have been surveyed, and now it was the turn of Lake Maggiore.

Using modern sonar equipment, in contrast to conventional echo sounders, it is possible to determine a large number of depth points with each measurement. Depth values can thus be calculated for each square metre of the lake floor, and the data can then be processed to produce three-dimensional images. The images now available even reveal underwater cables or – as in the delta of the Verzasca river off Tenero – small craters from which methane is released.

Fascinating historical insights

Researchers can use these precise records to trace the historical development of the lake. It is striking, for example, that while no lake floor channel can now be seen on the slopes of the Maggia delta, such channels are visible in the vicinity of former river mouths near Ascona. This allows conclusions to be drawn concerning the frequency and composition of sediment deposits from the Maggia river. It is also notable that, unlike the Maggia, the Ticino and Verzasca rivers do not form fan‑shaped deltas; instead, their deposits extend westward into the lake along an almost straight line.

Two lakes 1000 years from now?

Particularly evident from the new charts is the fact that, sooner or later, the Maggia delta will cut off the uppermost part of the lake. The toe of the delta which has advanced as far as the foot of the Gambarogno is already higher than the floor of the eastern part of the lake. It is, however, difficult to say how long it will be before the lake off San Nazzaro is replaced by a river: “That certainly won’t happen in the next 500 years,” says Anselmetti.

Risk management and monitoring

As well as providing answers to questions about the lake’s history, the new images can help to predict future events: in areas where deposits are now visible on steep slopes, underwater landslides could be triggered by a future earthquake, giving rise to a tsunami-like wave. In addition, the charts can be used to monitor sediment deposition in lakes, since alterations in flow regimes associated with climate change will also lead to changes in sediment transport and deposits. Initially, however, the Eawag scientists intend to investigate individual structures even more closely. They are focusing, for example, on the small, round depressions observed in the northern part of the Ticino/Verzasca delta. These pockmarks are a sign of gas seepage in this area. Anselmetti comments: “I don’t think the methane gas can be usefully exploited, but the seeps do indicate where the slope could become unstable.”

The possible applications of precise bathymetric images are wide-ranging. One conceivable application is the monitoring of underwater gravel extraction at river mouths. On Lake Lucerne, where similar charts have been produced by Eawag researchers, interest has been expressed by archaeologists hoping to uncover evidence of settlements from earlier periods, when lake water levels were lower. Enquiries have also been received from the army as to whether the new method could possibly be used to pinpoint dumped munitions.

Swisstopo among the sponsors

The latest survey is part of a pilot project involving Eawag and the University of Geneva. It was made possible with technical support from the Geological Survey of Norway and from the University of Ghent in Belgium. Financial support was originally provided by swisstopo (Federal Office of Topography), the Federal Office for the Environment, and the Federal Department of Defence, Civil Protection and Sport. The reasons for swisstopo’s interest are clear: to date, the depth contours shown for lakes on official maps have generally been based on soundings taken in some cases a hundred years ago. The new bathymetric relief map is several orders of magnitude more accurate than the contours on 1:25,000 maps.