Rivers are valuable and important water resources for human beings, but sustainable development and usage of the river has become a complex issue in modem times. River usage includes both legal (agricultural water, recreation and tourism), and illegal (quarrying, and dumping of waste into the river) activities all of which need to be monitored. However, the area is large, including the many tributaries draining into large river basins, which, in Taiwan are comprised of complex tectonic terrain. It costs money and time for river patrols to monitor these areas. The objective of this study is to develop a Viewshed analysis method based on the Geographic Information Systems (GIS) that can be used for locating optimized observation points to facilitate carrying out river patrols. Viewshed analysis can be utilized to define both the visible and invisible range from each observation point by combining digital terrain model (DTM) and road vector data. The method is designed to first locate several observation points about 2 km along the closest road near the river, after which Viewshed analysis is run. If there are some invisible areas left after the first Viewshed analysis, new observation points can be added and the Viewshed analysis run again. This process is iterated until the whole river area is visible. This study attempts to define the optimal observation points for river patrolmen in order to save time and resources.