Generating river bottom profiles with a dual-frequency identification sonar (DIDSON)

Estimating fish passage in rivers with a side-looking sonar positioned near shore requires proper alignment of the transducer with the river bottom. To maximize fish detection, we developed a robust aiming protocol using river bottom profiles generated from standard and long-range dual-frequency identification sonars (DIDSONs). The DIDSON profiles compared well with those generated from range and depth measures or bathymetry methods when the river bottoms were linear or changed from a steeper slope near shore to a flatter slope offshore. If the offshore slope was steeper, the DIDSON profile displayed shallower depths. The actual differences in depth between the DIDSON profiles and those from other methods were often considerably less than the potential differences from beam-spreading error. The maximum depth differences between the same DIDSON profiles marked independently multiple times ranged from 0.08 in for the DIDSON's highest frequency to 0.5 in for the lowest. A model of the fish-counting transducer beam was plotted in the same coordinate system as the river bottom profile, Macros developed for Excel spreadsheets enabled us to rotate the beam and determine the optimal tilt angle for each river bottom profile.