Several years ago, the GIS Department at Fuss & O'Neill, was approached by Rich Blodgett, Manager of Environmental Resources for the Providence Water Supply Board with a request. The Environmental Resources Department of Providence Water, as the Providence Water Supply Board is commonly known, has a long history of using GIS in its watershed management activities. Mr. Blodgett felt that a GIS tool capable of facilitating the tracing of waterborne pollutants to their source, be it a point source or non-point source, could save many man-hours of investigation, not to mention possibly averting a serious emergency. The challenge was accepted, and the task, after a bit of shuffling, was handed to me. The applications available to Providence Water, which were to be used in building the tool were ArcGIS ModelBuilder, an out-of-the-box component of ArcGIS, and Arc Hydro, a free hydraulic analysis application created at the University of Texas. Construction of the tool consisted of creating a hydraulic model of the Scituate Watershed using Arc Hydro, and an ArcGIS Modelbuilder model to analyze the impervious. It required many hours of false starts and dead ends before, through trial and error, the correct sequence of process steps and variable parameters was developed. The tool is built so that a GIS user with minimal training can place a point on the desired location on a stream and get an outline of the entire land surface that drains to that particular point. Having done so, the user activates the analysis function, which outputs a single feature containing the designated area with all the impervious surface features contained therein, as well as a table giving an analysis of the amounts and relative percentages of the various classes of impervious surface in the area. The project was initially completed and delivered over two years ago using ArcGIS version 9.2. . With the introduction of ArcGIS 10, and its technological alterations it has been necessary to rebuild the tool to function in the new processing environment. This paper will explain the rationale and techniques used in the construction of the tool, with a focus on the more recent, ArcGIS 10 version.