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These are some of the more frequent questions that have been asked about the development or structure of the 100K-scale PNW River Reach Files. As new questions are identified, this file will be periodically updated. Please submit questions to StreamNetGIS@psmfc.org
REACH CODES
What constitutes a river reach number?
Reach numbers (codes) are comprised of three items HUC + SEG + RMI.
These items are found in the reach files' Arc Attribute Table.
HUC is the USGS eight-digit hydrologic unit code
SEG is a unique 4-digit segment number.
RMI is a 5,n,2 river mile marker
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RRN (River Reach Number) is a redefinment of HUC,SEG,RMI into a 17 character string.
In the orginal EPA RF2.5 data set there was one SEG per reach.
In the PNW Reach File there are multiple reaches with the same SEG. To meet the requirement
that each reach has a unique reach number the RMI field was populated. In most cases RMI is
the sum of the lengths of all downstream reaches with the same SEG value. RMI values for
reaches with a SEG greater than 499 should be 00.00
There shouldn't be duplicate reach numbers. One exception: where the length of a reach is
less than16 meters, (less than 1/100 of a mile), the value in RMI may be the same as that
of the previous reach.
Are the Reach numbers hydrologically ordered in the reach files?
Not really. When the EPA constructed their simplly structured RF1 files back in the '80's
they did attempt to assign SEG numbers in some hydrologic sequence. Even numbers for reaches
on the left side of the basin, odd numbers on the right. As the reach files evolved and
became more complex, this concept was abandoned.
How were these numbers moved to the 100K hydrography?
These numbers were transferred to the 100K-scalehydrography using a conflation algorithm
developed by the USGS. After intensively editing the EPA RF2.5 Trace Files to align their
beginningand ending nodes on top of the corresponding beginning and ending stream nodes
in the 100K-scalehydrography. A routing algorithm then transferred (conflated) the EPA
Trace-IDs to the corresponding 100K reaches. With the Trace -ID's now physically in the
100K-scale hydrography, JOINITEM commands were used to bring across the remaining
Trace attributes. A computer program then generated SEG numbers for the remaining
un-coded reaches. These provisional SEG numbers begin with 500.
How can I identify which reaches in a file have a SEG number from the EPA reach files?
EPA SEG numbers are less than 500. By reselecting for SEG lt 500 you can seperate these out.
HYDROLOGIC UNITS
What is a Hydrologic Unit?
In 1974, a new set of maps depicting approved boundaries and codes of river basins of the
United States was produced by the USGS and the U.S. Water Resources Council. These maps
present information on drainage, culture, hydrography and hydrologic boundaries. There
were four levels of classification:
Region
Subregion
Accounting Units
Catalog Units
These fourth level subdivisions are 'hydrologic units' and have been identified by a unique
code consisting of from 2 to eight digits.
For example: 17040101 is comprised of -
17 Region - (Columbia Basin)
04 Subregion - (Upper Snake River Basin)
01 Accounting Unit - (Headwaters of the Snake River)
01 Cataloging Unit - (Snake Headwaters, Wyoming)
CU is generally considered to be the descriptive name and number: HUC the digital file for that CU.
Is a HUC boundary the same as a watershed boundary?
No, the HUC boundary was determined by a number of factors, only one of which was basin
shape. Some HUC boundaries span wide rivers or cross over rivers at dams.
LLID NUMBERS
What are LLID numbers based on and how were they assigned?
After reviewing several existing stream numbering schemes, Streamnet participants in conjuctions with IRICC
agreed that the best method for uniquely identifying streams would be the use of an ID consisting of the
longitude and latitude of the mouth of the stream (Longitude/Latitude Identification System LLIDS).
All reaches that comprise a given stream would be assigned this LLID (see figure 1). Longitude would
precede latitude to conform with standard x,y ordering. The code would be 13 characters long, with 7 for
decimal degrees of longitude and 6 for decimal degrees of latitude, with implied decimal points.
This ID would be used in combination with the EPA River Reach system, the accepted standard for
1:100,000-scale efforts.
Streamnet GIS team members in the state departments of fish and game then took the 100k hydrography and
assigned arcs grouped into a logical stream or river an LLID unique to the route. This type of
non-artificial key is not intended to be used for georeferencing, but rather to facilitate assignment
of new id's without consulting a central data base. A facility for checking the uniqueness and vailidity
will be added to this page in the near future.
CENTERLINES
What was the purpose of inserting centerlines through water bodies and wide rivers?
Centerlines were added o provide single-line transport paths thru water body features.
All but just a few PNW reach files contained areal features such as lakes, ponds,
reservoirs, or open bodies of water at the mouths of large rivers or around the Puget Sound,
or wide rivers with left and right banks. These features were extracted out of each reach
file and put in a seperate coverage we named Banks-pnw.
We replaced the areal features with centerlines and connector segments from the centerline
to any tributary.
By modifying the reachfile in such a manner it created not only a hydrologically ordered
stream network but also allowed tabular users to migrate the network in both directions
and not be concerned that they may have traversed one side of a waterbody going one way
and then jumped to the opposite side going the other. If such a situation did occur it
would be impossible to tell in a tabular file if and where tributaries entered the
waterbody.
Centerlining was important for non-GIS users and data modelers
Centerlines were given a SEG number taken from one of the shorelines, connector arcs
assumed the EPA SEG number belonging to its tributary or they received provisionary
SEG numbers.
Last modified: Fri Sep 26 13:52:18 1997