In 1982 Glenfarg Reservoir, Scotland, was drained to permit essential maintenance, exposing the floor of the flooded valley for the first time since damming in 1926. The accumulated deposits and the various, normally subaqueous, bed features were examined. A sidescan sonar survey undertaken after re-impounding was therefore interpreted against an exceptionally high level of ground information. The results of this control experiment, thought to be unique in the deployment of sidescan sonar, are described and discussed.