Photo-mosaics generated automatically from as many as thousands of optical images have proved to be an effective technology to study the ecological patterns and dynamics of underwater ecosystems and benthic environments over spatial scales much larger than a single object or image. Unfortunately, optical systems, while useful in clear waters, are ineffective within environments with sources of turbidity and pollution, including lakes, marine sanctuaries, many ports and harbors. Two-dimensional high-resolution forward-scan imaging systems can serve as a suitable technology for constructing similar visual maps, provided that a range of complex imaging issues can be overcome. This paper investigates some of the complexities in analyzing dynamic events captured by a FS sonar imaging system when used in standard configuration to map the seafloor. Of special interest is the case of imaging targets at shorter ranges to maximize benthic object details. We give mathematical models that describe the dynamics associated with objects and shadows they cast on the seabed, and demonstrate some of these issues through examples from real data obtained in the lake on the University of Miami campus.