Geoscience Reference
In-Depth Information
Figure 1:
Pomme de Terre Subbasin illustrating (a) artificial paths and (b) breaks within the
centerline feature
In addition to artificial paths, the NHD schema stores other attributes asso-
ciated with the flowline feature class including reaches. A reach is defined
as a confluence-to-confluence section of a stream in the NHDFlowline fea-
ture class with similar hydrologic characteristics (USGS 2000). Reaches
are broken into multiple segments when they cross a topographic mapsheet
tile, when a new tributary is added to the subbasin network, or at the
boundary of a Digital Update Unit (i.e., a region used for database mainte-
nance and field monitoring), and thus it is common to see multiple records
attributed with a single reach code (USGS 2007). A unique reach code
identifier is assigned to each reach in the NHDFlowline feature class of the
NHD. Reach codes are maintained as a means to permanently distinguish
stream tributaries for the entire United States across all resolutions of the
NHD database. Reach codes are assigned in chronological sequence as the
reach is incorporated into the NHD dataset, thus the ordering of reach
codes is temporal and not necessarily spatial. This aspect of the NHD
schema will figure prominently in subsequent discussion.
