Spearheaded by the US Environmental Protection Agency (EPA) and the US Geological Survey (USGS), the National Hydrography Dataset (NHD) is "a comprehensive set of digital spatial data that encodes information about naturally occurring and constructed bodies of water, paths through which water flows, and related entities." The dataset is useful in the creation of maps, in geocoding observations (i.e., linking data to water features), and in modeling the flow of water and water-borne materials. To make the data more accessible to potential users, the USGS has developed a series of National Hydrography Dataset Tutorials, describing database components such as Feature Classes, Reach Flow, and Event Tables. A series of internal links points users to a wealth of additional information.