In order to properly manage a region�s water resources, it is important for water managers to know the time variability of flow in the streams of that region. Not only what are the highest flows, such as what would be available from a flood frequency study, but also how the flows vary day to day, season to season, and year to year. Studies such as water supply studies, hydropower studies and those involving sediment transport depend on this kind of long term variability data in order to develop the best management practices for a region�s water resources. The problem in Guam, as in most locations, is that stream flow information is not available for all possible sites where information is required. What is needed is a better means of estimating the variability of flow at ungaged locations that are likely to become candidate sites for water resources investigations. The project�s goal was to develop a means of predicting flow duration curves at ungaged sites in Guam. All of the major streams in Southern Guam were divided into stream reaches based on stream order and smaller stream segments based on similar average annual flow. These reaches and segments were identified on maps developed from the detailed Geographic Information System (GIS) map inventory of Guam available at the University of Guam, Water and Environmental Research Institute of the Western Pacific (WERI). Various statistical and analytical methods were applied to the existing streamflow data along with the physical characteristics of the reaches and segments in order to predict the streamflow variability in each stream reach and segment. An Excel application was also developed to perform a preliminary hydropower production and economic analysis for any new proposed site. Those wishing to explore the feasibility of hydropower at a particular site will be able to enter the average flow and available head (hydraulic drop) information into the simple spreadsheet application which is provided as part of the study. This application allows the user to explore various turbine sizing and economic considerations to determine the preliminary feasibility of developing a hydropower facility at a particular site. The GIS maps, and Excel application developed are available from the WERI web site: http://www.weriguam.org