Taiwan is a mountainous, steep-terrain island surrounded by oceans on all sides. Even though annual precipitation totals are typically relatively high, most of that water ultimately ends up in the sea, making storing water resources difficult. The question of how to keep water through artificial constructions is crucial. The reservoir is an essential construction for storing water resources in Taiwan. Most of them obtain water supply from rivers further upstream. Whenever there is a sudden downpour, rivers carry a large amount of mud and rocks into the reservoir, which can cause sedimentation at the reservoir’s bottom and a decrease in its capacity to store water, reduced. With the Shihmen Reservoir as an example, the underwater terrain of the reservoir is depicted using the multibeam echosounder and side-scan sonar system. In addition, the underwater terrain is given coordinates by combing the global positioning system (GPS). The point cloud data of the measured results above were substituted into the geographic information system to generate a 3D digital image of the dam’s underwater structure and siltation status. Through years of data collection, the gathered information can be utilized to compare the fluctuations of silt accumulation in Shimen Reservoir over time.