What is a Reach File?
Any "Reach File", including NHD, needs to serve three simultaneous functions for surface waters:
- Provide a standard unique identifier for each surface water feature (with each feature called a Reach),
- Contain a tabular routing (navigation) network of these features, and
- Include a digital map representation of these features.
It is called a "Reach File" because the basic building blocks of the data set are called Reaches. Each Reach is a "significant" piece of surface water, especially a stretch of river between 2 stream confluences or lake entrance/exit points.
Why Use Reaches as the Basic Building Block?
Many of NHD's primary design considerations are tied to the goal of maintaining a nationally consistent numbering system and accommodating changes in scale in parts of the network. Stability in the definition of the basic building blocks is very important to the success of NHD. If these building blocks are too much of a "moving target" then it will be difficult to keep data indexed to NHD (the key to most applications) and users will have to spend more time just keeping up with the changes. A major part of the design of the NHD sets in place one set of Reach Numbers, which will then be unchanged (except in very rare cases) over time, even with change in scale.
Importance of a National Hydrography Dataset Standard
- It provides a single system for everyone to use. This allows for interagency compatibility between GIS projects. For example, a 305(b) database georeferenced to NHD could provided useful information for TMDL modeling.
- The maintenance of a national coverage will allow all users to benefit from the enhancements of others.
- A national coverage provides stability and ensures "Institutional Memory" for work performed using the national standard.
- Using the national standard ensures a readily accessible replacement for coverages that are inadvertently damaged during processing.