The NHD was created to assist scientists in modeling hydrologic features and it is also useful for cartographic mapping purposes. Development began in 1993 as a cooperative effort between the EPA and the USGS. Originally, it was a combination of USGS hydrologic digital line graph files (DLG) and EPA reach files (version 3.0, RF3). The USGS files were used for spatial accuracy and the EPA files were used for attribute information. The 1:100,000 scale NHD was completed in 2000. The 1:24,000-scale NHD was completed in 2007 and is based on the USGS 7.5-minute series topographic map.
The NHD is a series of GIS feature classes representing the drainage network with features such as rivers, streams, canals, lakes, and ponds housed in a comprehensive data model. This data model includes a geometric network for tracing along streams and rivers throughout the nation. As with all active data models, improvements are continually being made, and the current data model is version 2.2. The ongoing maintenance of the NHD helps improve these datasets to meet the ever-increasing demand for currency, additional detail, and more significant attribution.
In August 2007, IDWR was appointed as the Steward for Idaho's NHD.
Beginning in the 1970s, the US Geological Survey (USGS) developed Hydrologic Units (HUs) for the United States dividing the country in to 21 Regions, 222 Subregions, and 2,149 Accounting Units. During the late 1970s, the Natural Resources Conservation Service (NRCS) initiated a program to further divide the HUs into Watersheds and Subwatersheds. By the early 1980s, the advent of GIS made mapping of digital HU boundaries feasible and the NRCS started to delineate HUs to meet 1:24,000 National Map Accuracy Standards. The goal of this initiative was to provide a hydrologically-correct, seamless, and consistent national GIS watershed boundary database.
|Hydrologic Unit Level
||Size (in acres)
||250,000 - 450,000
||40,000 - 250,000
||5 to 15
||10,000 - 40,000
||5 to 15
The Idaho 4th, 5th, and 6th field level boundaries were created using the "USGS Interagency Guideline on Delineation of Watershed and Subwatershed Hydrologic Unit Boundaries" standards (version dated October 4, 2004). Each Subbasin, formerly known as a "HU" (hydrologic unit), was assigned a data steward at the 4th field (8-digit) level based on land
ownership, management, and interest for each area. Data stewards reviewed their areas and coordinated with other interested cooperators and surrounding states to create new draft 4th, 5th, and 6th field watershed lines. Statewide 24k Digital Raster Graphics (DRG) and hydrography data was used along with Shaded Relief and NAIP aerial photography
to help determine boundary lines. Polygons were attributed with new names and nesting watershed numbers pursuant to the watershed standards.
A WBD Technical Working Group (TWG) was formed and members of the TWG, in cooperation with various federal, state, and local agencies participated in monthly meetings to review and update subbasin delineations during each day-long session. After each session, edits were collected and sent for further review to all interested parties, which included coordination and edge-matching with surrounding states. As a result of this process, new 4th, 5th, and 6th field watershed boundaries were created at 1:24,000 using local expertise to more accurately depict drainage patterns throughout the state. After all subbasins were reviewed, a statewide dataset was assembled and sent for a final review before being submitted for certification consideration. Certification in Idaho was obtained on December 2, 2008.