A side‐looking sonar was used to study bottom features in Lake Mendota, Wisconsin. Interferences between direct and surface‐reflected signals gave a tiger‐striped pattern in the data. This pattern could be approximately interpreted as being the tracings of isoangle contours. However, this simple analysis was precluded by the strong gradients of sound‐velocity profile. Ray traces were used to construct a profile of the interference field. The profile was used to interpret the data. Features having lateral dimensions of 10 m and a few meters depth were contoured. The data was taken when the sonar was about 100 m from the features.