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National Hydrography Dataset The National Hydrography Dataset (NHD) is the surface water component of the National Map. It is a comprehensive set of digital, spatial data that contains information about surface water features such as lakes, ponds, streams, rivers, springs and wells. The United States Geological Survey (USGS) is the central repository and a key agency for the NHD. Within the NHD, surface water features are combined to form "reaches," which provide the framework for linking water-related data to the NHD surface water drainage network. These linkages enable the analysis and display of these water-related data in upstream and downstream order. The NHD is based upon the content of USGS Digital Line Graph (DLG) hydrography data integrated with reach-related information from the EPA Reach File Version 3 (RF3). While initially based on 1:100,000-scale data, the NHD is now complete at the 1:24,000-scale nationwide. The dataset is also designed to incorporate and encourage the development of higher resolution data required by many users. How NHD is Organized NHD data are available at scales of 1:24,000 (high resolution) and 1:100,000 (medium resolution) nationwide. The higher scale 1:24,000 data is the primary dataset to which most improvements and updates are being made. NHD data are not organized by political units like county maps or grids like the 7.5-minute quad maps. Rather, the data are organized by watersheds because water flow and distribution is affected by terrain and not by boundaries. A watershed can be defined to be the region drained upstream of a point selected along a stream or river. Since the watershed drainage basin size can vary so widely, we need a set of defined watershed boundaries which can contain hydrographic data, like in NHD. The U.S. Geological Survey, the Natural Resources Conservation Services, and the Environmental Protection Agency have developed a structure for naming and designating watersheds of various sizes. The table below provides information on six different levels of watersheds.  NHD data are made available by subbasin and will have an eight digit
reference number. For California, more detailed subdivisions of the
subbasins are currently being developed. In the above example, the San
Diego River subbasin would be divided into smaller basins representing
the drainage areas of tributaries of the San Diego River. The graphic below shows
the distribution of the six subbasins across San Diego County. Where Is the NHD DataThe official source for NHD is the USGS' National Hydrogrphy Dataset Site. You can get the most current official data by visiting http://nhd.usgs.gov/data.htmlNHD StewardshipMuch of the data for the NHD comes increasingly from local sources, necessitating a management and oversight at an intermediary level. These intermediaries, or ‘data stewards’, are required throughout the country on a statewide or county basis. In some cases, multiple stewards may be required in a given state. SanGIS is the NHD Steward for San Diego County. NHD Stewards coordinate and assume responsibility for the NHD for their region, and perform a number of responsibilities. For more information on these responsibilities or the program, visit the USGS NHD Stewardship website, or contact SanGIS. Procedurally, SanGIS staff will maintain the NHD within their jurisdiction and provide the updated dataset to USGS for incorporation into the larger dataset. How the NHD is UpdatedThere are six NHD subbasins covering different portions of San Diego County. These subbasins have been downloaded from the NHD website and are available here at SanGIS. Updates to NHD data are made by making changes to feature geometry or attribution using GIS software and then applying custom NHD tools to integrate the changed data back into the national database. The NHD tools, however, are complex and require training in their application. Thus, the tools will only be used by NHD stewards who have received training, like SanGIS, and USGS. How Everyone Can HelpThe success of the NHD stewardship program depends on input from NHD users across the county. NHD users can provide information on new hydrographic features and fixes to existing features.Examples of new hydrographic information may include:
We're asking that NHD users help identify places where the data need updating. Your knowledge of local hydrography can reveal instances like those listed above where updates need to be made. SanGIS has instituted a process whereby those changes can be made easily. Submitting NHD Updates to SanGISThe most important part in any updates, however, is simply knowing what needs to be changed. And this is where you can help. We are making all the San Diego NHD available as ESRI shapefiles. Basic changes to feature geometry and attribution are easily made using ESRI and other GIS software packages. In addition to applying your knowledge about the local hydrography, recent imagery can be compared to NHD to find changes.
Difference Between the NHD and Flood Plain DataThe NHD is the surface water component of the USGS National Map. It is intended to represent surface water features such as streams, lakes, reservoirs, etc. The NHD does not identify flood plain boundaries or statistical flood extents (e.g. the 100 year flood plain). FEMA Flood Insurance Rate Maps are the official source for flood plain extents and are not part of NHD. The NHD is a modeling and cartographic product that does not attempt to show flood boundaries or statistical extents. NHD and Related LinksNational Hydrography Dataset Standards
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