The position of the sidescan sonar vehicle, relative to the 
ship, was measured with a Benthos DS 7000 acoustic ranging 
system. The acoustic ranges were merged with the ship’s GPS 
navigation to compute a sidescan vehicle navigation. 
Because of the wide swathwidths of the SIS-1000 sidescan 
sonar system and the depth of the lake, the entire 370 km? 
section of the lake that is deeper than 10m was surveyed in 11 
days. The thermocline in the lake caused some degradation of 
the sidescan image in some places, and future surveys perhaps 
should be conducted in winter or early spring when lake would 
be more isothermal. 
The sonar images revealed a wide variety of processes that are 
shaping the lake floor. A few examples of these different 
processes are summarized here and are illustrated with sonar 
images. 
The Sarca river, the lake inlet, supplies a large volume of 
sediment to the northern end of Lake Garda, and extending 
7km southward along the lake floor from the delta at the 
river’s mouth are several channels with large sand waves in 
their floors (see figure 2). The presence of sand waves in the 
channel floors suggests active transport of sand through these 
channels, and high-backscatter areas at the end of the channels 
are interpreted to be deposits transported to the deep lake floor 
by these flows. 
Along the steep sides of the northern part of the lake, and 
especially on the western side of he lake are several large 
debris aprons (see figure 3). These aprons are particularly 
large off the mouths of rivers, and they are interpreted to be 
grain-flow deposits comprised of sand and gravel derived from 
rivers that drain into the lake. Large boulders are seen on some 
of these debris aprons, and they probably continue to be active 
sites of sediment accumulation otherwise they would be 
partially buried under sediments that are accumulating in the 
central part of the lake. : 
The tectonic history of the lake is still reflected in the lake 
floor geology. Small faults were seen in a few places along the 
eastern wall of the lake and the steep western wall of the lake 
probably represents a series of large fault scarps that were 
associated with the original formation of the lake. Whether 
these faults are still active is unknown, however, the presence 
of slump scarps (see figure 4) and slump deposits in the 
Holocene sediments of the lake floor suggests that the lake 
continues to be a tectonically active area. 
Evidence of anthropogenic activity in the lake is present in the 
sonar images as well. A pipeline or cable crossing the deep 
part of the lake ( see figure 5 ) appears to be partially buried 
by one of the debris aprons which suggests that understanding 
the geological processes acting on the lake floor will be 
important for proper environmental management of the lake. 
The presence of two sunken ships, a Venetian galleon and a 
military vessel (Austrian from 1st World War ?), on the lake 
floor suggests that a more detailed analysis of the sonar image 
may show it to be valuable as an archaeological tool as well. 
5.0 Merging of digital orthophotos with sidescan data 
The high altitude photography around the area of Riva ,at the 
northern end of the lake, was used to create a digital 
orthophoto at the scale of 1:10000 and a ground pixel 
resolution of 2m to make it comparable with that of the final 
mosaic of the sidescan data. The DTM used in the 
orthorectification was a grid with a cell size of 40m and 
elevations derived from contour lines and spot elevations 
digitized from existing maps at the scale of 1:10000. The 
software employed in the orthophoto production and the 
DTM calculation was, respectively, the Phodis of Carl Zeiss 
  
Figure 4 - Sidescan sonar images of slump scarps seen of the 
eastern side of upper lake Garda 
  
Figure 5 - Sidescan sonar image of pipelines or cables cros- 
sing upper lake Garda 
291 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996