A reach has both a reach code and a common identifier (com_id). What is the difference?
A reach code uniquely identifies each reach. This 14-digit code has 2 parts: the first 8 digits are the hydrologic unit code for the subbasin (formerly, known as cataloging unit) in which the reach exists; the last 6 digits are a sequence number assigned in arbitrary order to the reaches within that subbasin. Each reach code occurs only once throughout the Nation. Once assigned, a reach code is associated with its reach permanently. If a reach is deleted, its reach code is retired. Reach codes facilitate the geocoding or linking of observations, such as a water quality sampling sites, to reaches. They form the basis for a national linear referencing system which supports linking such observations to a point along a reach, an entire reach, or groups of reaches. Reach codes are stored in the data element named “RCH_CODE”. In addition to the reach code, the date on which the code was assigned in the NHD is also stored in the data element named “RCH_DATE”. The only link between NHD at diff
