For both environmental and economic reasons, it is important to map the distribution of submerged aquatic vegetation. Acoustic technologies seem to be the more efficient and cost effective methods for that. Many papers have been published on underwater vegetation detection using an acoustic echo-sounder. However, few general studies have been performed to quantitatively understand the acoustic process of scattering from underwater vegetation. First, the effect of fields of vegetation on the temporal signal of the echo-sounder is studied. Real echo-sounder data obtained from different sites and different kinds of submerged aquatic vegetation was used. In most cases, the vegetation signal has a relatively strong backscatter signal before the bottom detection time and a less abrupt leading edge. However, different species-dependant behaviors were found. In order to explain this phenomenon and to characterize underwater vegetation, a model, based on the sonar equation, is developed to highlight the interaction between the acoustic wave and the environment composed of underwater vegetation. The result is a submerged aquatic vegetation apparent scattering index which can be used to help to characterize underwater vegetation.