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initiation must use an alternative method to the intersection approach
applied in this paper, since there are few artificial paths initially.
One solution being explored involves a prerequisite form of database en-
richment, wherein upstream drainage areas are estimated (Stanislawski
2009, Stanislawski et al. 2007) prior to centerline delineation. This paper
already discussed the use of upstream drainage area values to determine
which of several reaches is a better choice for tracing upwards through a
drainage network. Upstream drainage area also can be used to locate the
mouth (pour point) for the subbasin, which will be characterized as having
the highest upstream drainage area value for any reach in the subbasin.
From the pour point, one can trace upstream through the beginning and
end nodes of reaches to the penultimate confluence. As in the paper's ear-
lier discussion, where multiple reaches share a node, the reach is chosen
with a higher upstream drainage area estimate. Specifically, the use of up-
stream drainage area estimates might accelerate processing times to the
extent that it can reduce unnecessary scanning of the node lists; and this
forms an area for future research.
Other potential problems include identifying multiple pour points in a sub-
basin, as for example in processing a coastal hydrographic network. In dry
landscapes, the delineation must deal with centerline gaps which are
formed by subterranean flows. In these regions, it seems logical to process
master and candidate flowline lists using full network topologies, which
creates an extra processing step but will ensure that node lists can support
searching continuously through the subbasin.
A promising application which is also slated to be explored is the identifi-
cation of a centerline within a braided stream, as described by Buttenfield
et al. (2010). In braided hydrography, multiple channels will share a node,
and indeed, may carry equivalent values for upstream drainage area. De-
lineating a centerline automatically through a stream braid must resolve
these ambiguities; and at present, no clear solution is apparent based on the
existing NHD schema.
7- Summary
The delineation of a primary channel is important to topographic base
mapping for cartographic and analytic reasons. Centerline delineation
enriches the NHD database, making additional information available to the
