These two components, together with the generic programming framework, provide a basic database design and a set of tools that facilitate the analyses often performed in the water resources arena. Arc Hydro is intended to provide the initial functionality that can then be expanded by adding to it database structures and functions required by a specific task or application.The Arc Hydro tools operate in the ArcGIS environment. Some of the functions require the Spatial Analyst extension. The majority of the tools are accessed through the Arc Hydro Tools toolbar, where they are grouped by functionality into six menus and nine tools. Additional tools have been developed in the geoprocessing environment and are available in the Arc Hydro Tools toolbox that can be used both in ArcMap and in ArcCatalog.The Arc Hydro tools have two key purposes. The first purpose is to manipulate (assign) key attributes in the Arc Hydro data model. These attributes form the basis for further analyses. They include the key identifiers (such as HydroID, DrainID, NextDownID, etc.) and the measure attributes (such as LengthDown). The second purpose for the tools is to provide some core functionality often used in water resources applications.This includes DEM-based watershed delineation, network generation, and attribute-based tracing.