In multibeam echosounding, beamwidth and sidelobe levels constrain detection of echoes from near-bottom targets that are often masked by echoes from the bottom received in the mainlobe of individual beams or through their sidelobes. Acoustic backscatter data collected with multibeam echosounders from various manufacturers indicate that the customary half power point (−3 dB) metric for the angular width of individual beams underestimates the masking effects of bottom echoes. A better fit of the detected bottom echo trace is obtained with beamwidths estimated 10 dB below the maximum response of each beam. The corresponding beamwidths are roughly 66% larger at −10 dB than at −3 dB for a canonical sinc squared beam pattern. Therefore, using the manufacturers’ nominal −3 dB beamwidth specifications yields underestimates by the same percentage of the area ensonified by the pulse within a beam, and corresponding overestimates of the detected acoustic backscatter level used to infer target strength or bottom back...