# NHD Plus

User Guide

September 1, 2010NHD Plus User Guide September 1, 2010 

### THIS PAGE LEFT BLANK INTENTIONALLYNHD Plus User Guide September 1, 2010 ii This guide is intended for use with the following N HDPlus schema versions.

NHDPlus Component Schema Version
Catchment Grid 01 Catchment Shape 01
Catchment Flowline Attributes 01
Elevation Grid 01

Flow Accumulation & Direction Grids 01

NHD 01 Stream Gage Events 01NHD Plus User Guide September 1, 2010 iii 

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### Table of Contents

Acknowledgments....................................

................................................... ............................. viii Chapter 1 Introduction to NHDPlus ...........................

................................................... ............ 1 Data Package Content and Directory Structure ......

................................................... ................. 3
NHDPlus Schema ....................................

................................................... ................................ 5 Projection Information ............................

................................................... ................................. 7 NHDPlus Versioning System .........................

................................................... ......................... 9 Chapter 2 Feature Class Descriptions ........................

................................................... ........... 13 Chapter 3 Attribute Table Descriptions ......................

................................................... .......... 24 Chapter 4 Geographic Coverage of Data Package ...............

................................................... 39 Chapter 5 Understanding and Using NHDPlus ...................

................................................... . 41 NHDPlus and Divergences ...........................

................................................... ......................... 41 Generating Less Dense Networks from NHDFlowline ...

................................................... ...... 44 Understanding NHDPlus Slope .......................

................................................... ...................... 44 Finding the Upstream Inflows to an NHDPlus Dataset

................................................... ......... 45 Finding all the Tributaries to a Stretch of River .

................................................... ................... 45 Building an NHDPlus Attribute Accumulator .........

................................................... .............. 46
Flowlines with "known flow" vs. Flowlines with “unk nown flow” ........................................ . 47
Main flowline network vs. isolated networks .......

................................................... ................. 48 Disagreements Between the Flow Accumulation Grid an d NHD isolated Networks .............. 50 Why does the Identify tool in ArcMap say that the v alue and ComID from the cat grid are different from the Grid_code and ComID shown in the Catchment shapefile? ....................... 51 Why would drainage areas computed using upstream na vigation differ from the cumulative drainage areas pre-computed in the NHDPlus? .......

................................................... .............. 52 What is the situation with names assigned to artifi cial paths inside lake/pond waterbodies? .. 52 Difficulties using NHDPlus Attribute .dbfs in Micro soft Access ....................................... ...... 52 Using the NHDPlus Value Added Attributes (VAAs) for Non-Navigation Tasks .................. 53 Appendix A – Process Descriptions for NHDPlus .....

................................................... ........... 65 Appendix B – Data Dictionary ......................

................................................... ...................... 101 Appendix C.1 – NLCD Land Cover Classification Syste m Key ............................................ 109 Appendix C.2 – NOAA C-CAP Land Cover Classification System Key .............................. 112
References ........................................

................................................... .................................... 113

Footnotes .........................................

................................................... ..................................... 115NHD Plus User Guide September 1, 2010 v 

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### List of Figures

Figure 1-1 – NHDPlus Schema........................

................................................... ............................ 5 Figure 4-1 – NHDPlus Hydrologic Regions and Product ion Unit Map ...................................... . 39 Figure 5-1 – NHDPlus Complex Hydrography ..........

................................................... ............... 41 Figure 5-2 – Main and Minor Paths in NHDPlus ......

................................................... ................ 42 Figure 5-3 – “Local” Divergences ..................

................................................... ........................... 42 Figure 5-4 – More Complex Divergent Junction Exampl e ................................................. .......... 43 Figure 5-5 – Cumulative attributes routed down the main path ......................................... .......... 44 Figure 5-6 – NHD Flowlines With Known and Unknown F low Direction .................................. 48 Figure 5-7 – Non-contributing Isolated Networks Tea l lines are the terminal segments of these isolated networks. The straight lines are USGS 7.5- minute quadrangle boundaries .................... 4 8 Figure 5-8 – Map error The cross-hairs are the edge s of USGS quad maps. .............................. . 50 Figure 5-9 – Uncommon problem between the NHDPlus n etwork and the flow accumulation grid

................................................... ................................................... ................................................. 5 1 Figure 5-10 – ThinnerCod = 0 ......................

................................................... ............................. 54 Figure 5-11 – ThinnerCod = 1 ......................

................................................... ............................. 55 Figure 5-12 – ThinnerCod = 1 or 2 .................

................................................... ........................... 56 Figure 5-13 – Streams Greater than or Equal to 100 Kilometers in Length .............................. ... 57 Figure 5-14 – The Mainstem of the Potomac River By Selecting LevelPathi=9169600001 ....... 58 Figure 5-15 – The Potomac River Basin By Selecting TerminalPA = 9169600001 .................... 59 Figure 5-16 – Profile Plot of the Mainstem Potomac River “MinElevSmo” values .................... 60 Figure 5-17 – Stream Orders in a Part of the Midatl antic Region ...................................... .......... 61 Figure 5-18 – Stream Orders in a Part of the Midatl antic Region with Order 1 Streams Removed

................................................... ................................................... ................................................. 6 2 Figure 5-19 – StreamLevel Values at a flowline Junc tion .............................................. .............. 63 Figure A-1 – (a) Differences in drainage between th e NHD and flow paths of a NED-derived stream, (b) Resultant NHD catchment delineations us ing unmodified NED DEM data, and (c) Resultant NHD catchment delineations using AGREE-mo dified NED data. .............................. 73 Figure A-2 – Schematic of AGREE process............

................................................... .................. 74 Figure A-3 – 3-D Perspective view of modified DEM w ith walling of existing Watershed boundaries and burning of NHD streams .............

................................................... ...................... 75 Figure A-4 – Illustration showing multiple polygon features defining a catchment area for a
flowline ..........................................

................................................... ............................................ 77 Figure A-5 – Illustration comparing the horizontal displacement between the NHD streams of an isolated network (in blue) to the synthetic streams of the “filled” HydroDEM ......................... .. 79 Figure A-6 – Assigning Minimum and Maximum Stream C hannel Elevations to NHD Flowlines

................................................... ................................................... ................................................. 8 1 Figure A-7 – Example of an international catchment on the Canadian border ............................ 82 Figure A-8 – Stream Gages in Region 21 ............

................................................... ...................... 82 Figure A-9 – Average Unit Runoff on Puerto Rico Mai nland.............................................. ........ 82 Figure A-10 – Digitized Mean Annual Runoff Values O n Puerto Rico Mainland ...................... 82 Figure A-11 – Results of using TopoGrid to Develop an Interpolated Raster Grid of Mean
Annual Runoff .....................................

................................................... ...................................... 92 Figure A-12 – Downstream Elevation Smoothing ......

................................................... ............... 99 Figure A-13 – Upstream Elevation Smoothing ........

................................................... ................. 99NHD Plus User Guide September 1, 2010 vii Figure A-14 – Hybrid Elevation Smoothing ..........

................................................... .................. 100NHD Plus User Guide September 1, 2010 viii 

### Acknowledgments

NHDPlus is the result of considerable efforts by ma ny organizations and individuals over time. The initial goal of estimating flow volume and velo city for the National Hydrography Dataset (NHD) led us to integrating the NHD with the Nation al Elevation Dataset and the Watershed Boundary Dataset to determine stream segment catchm ents and their associated drainage areas. This integration made possible many additional geos patial data products found in NHDPlus. We expect that the resulting suite of geospatial produ cts will become a valued data source for the water resources community. A special thank you goes to the primary contributors to the NHDPlus development effort who are listed below.

Tommy Dewald, Project Manager, EPA Office of Water

The NHD Plus

Team

Tim Bondelid – Independent Consultant (under contra ct to EPA) Craig Johnston – U.S. Geological Survey (USGS) Cindy McKay – Horizon Systems Corporation (under co ntract to EPA) Rich Moore – U.S. Geological Survey (USGS) Alan Rea – U.S. Geological Survey (USGS)

Other Participants in the Initial NHD Catchment Dev elopment Evaluation

Jon Goodall – University of Texas at Austin (UT) Sue Greenlee – U.S. Geological Survey (USGS) David Maidment – University of Texas at Austin (UT) Greg Schwarz – U.S. Geological Survey (USGS) Pete Steeves – U.S. Geological Survey (USGS) Kris Verdin – Hughes STX (under contract to USGS) Bruce Worstell – Hughes STX (under contract to USGS )NHD Plus User Guide September 1, 2010 ix 

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## Chapter 1 Introduction to NHDPlus

NHDPlus is an integrated suite of application-ready geospatial data products, incorporating many of the best features of the National Hydrography Da taset (NHD), the National Elevation Dataset (NED), and the National Watershed Boundary Dataset (WBD). NHDPlus includes a stream network based on the medium resolution NHD (1:100,0 00 scale), improved networking, feature naming, and “value-added attributes” (VAA). NHDPlus also includes elevation-derived catchments produced using a drainage enforcement te chnique first broadly applied in New England, and thus dubbed “The New-England Method.” This technique involves enforcing the 1:100,000-scale NHD drainage network by modifying t he NED elevations to fit with the network via trenching and using the WBD, where certified WB D is available, to enforce hydrologic divides. The resulting modified digital elevation m odel (DEM) was used to produce hydrologic derivatives that closely agree with the NHD and WBD . An interdisciplinary team from the U.S. Geological Survey (USGS) and the U.S. Environmental Protection Agency (EPA) found this method to produce the best-quality agreement among the ingredient datasets among the various
methods tested.

The VAAs include greatly enhanced capabilities for upstream and downstream navigation, analysis, and modeling. Examples include rapidly re trieving all flowlines and catchments upstream of a given flowline, using structured quer ies rather than slower traditional flowline-byflowline navigation; subsetting a stream level path sorted in hydrologic order for stream profile mapping, analysis, and plotting; and calculating cu mulative catchment attributes using streamlined VAA hydrologic sequencing routing attri butes. VAA-based routing techniques were used to produce the NHDPlus cumulative drainage are as and land cover, temperature, and precipitation distributions. These cumulative attri butes are used to estimate mean annual flow and velocity.

NHDPlus contains a static copy of the 1:100,000 sca le NHD that has been extensively improved. While these updates will eventually make their way back to the central USGS NHD repository, this will not happen prior to distribution of NHDPl us because the update process for the central NHD database at USGS was under development at the t ime these improvements were being made. Consequently, NHDPlus will contain some tempo rary database keys. As a result, NHDPlus users may not make additional updates to th e NHD portions of NHDPlus with the intent of sending these updates back to the central NHD database. Once the NHDPlus updates have been posted to the central NHD database, a fre sh copy of the improved data can be pulled from the NHD Web site (http://nhdgeo.usgs.gov) and that copy will be usable for data
maintenance.

The geospatial datasets included in NHDPlus already have been used in an application to develop estimates of mean annual streamflow and vel ocity for each NHD Flowline in the
conterminous United States. The results of these an alyses are included with the NHDPlus data.

EPA and USGS have already linked numerous water qua lity databases to the underlying NHD by assigning NHD stream (Reach) addresses to these ent ities, which include gaging stations, water quality monitoring sites, and impaired waters, enab ling them to be queried and analyzed inNHD Plus User Guide September 1, 2010 2 up/downstream order. The USGS gaging stations and associated data that were used to validate the NHDPlus flow volume and velocity estimates are included with the NHDPlus data. Figure
1-1 illustrates the NHDPlus Schema.

Additional information, tools, exercises, training opportunities, news, and the latest NHDPlus
data can be found at http://www.epa.gov/waters

.NHD Plus User Guide September 1, 2010 3 

### Data Package Content and Directory Structure

ProductionUnits (shapefile)

\NHDPlusRR, where RR is the Hydrologic Region numbe r \drainage

Cat (grid) catchment (shapefile)

\info (info tables for cat grid) \hydrography nhdflowline (shapefile) nhdwaterbody (shapefile) nhdpoint (shapefile) nhdline (shapefile) nhdarea (shapefile) \hydrologicunits Basin (empty placeholder 1 shapefile) Region (empty placeholder shapefile) Subbasin (shapefile) Subregion (empty placeholder shapefile) Subwatershed (empty placeholder shapefile) Watershed (empty placeholder shapefile) \elev_unit_a (elevation grids for production unit a) elev_cm (grid) \info (info tables for grids in unit_a)

(elevation grids for additional production units)

\fac_fdr_unit_a (fac and fdr grids for production unit a) fac (grid) fdr (grid) \info (info tables for grids in unit_a)

(fac and fdr grids for additional production units)

catchmentattributesNLCD (dBase file) catchmentattributesTempPrecip (dBase file) flowlineattributesFlow (dBase file) flowlineattributesNLCD (dBase file) flowlineattributesTempPrecip (dBase file) headwaternodearea (dBase file) NHDFcode (dBase file) NHDFeatureToMetadata (dBase file) NHDFlow (dBase file) NHDFlowlineVAA (dBase file)NHD Plus User Guide September 1, 2010 4 NHDHydroLineEvent (empty placeholder dBase file) NHDHydroPointEvent (empty placeholder dBase file) NHDMetadata (dBase file) NHDProcessingParameters (empty placeholder dBase fi le) NHDReachCode_Comid (empty placeholder dBase file) NHDReachCrossReference (dBase file) NHDSourceCitation (dBase file) NHDStatus (empty placeholder dBase File) NHDVerticalRelationship (dBase file) openme.txt (text) StreamGageEvent (shapefile)

Figure 1-1 contains a schema diagram of the NHDPlus . The feature classes and tables contained in the grey area are the standard NHD content. The feature classes and tables outside of the grey area are the components added during the building o f NHDPlus.NHDPlusUser GuideSeptember 1, 20105NHDPlus SchemaFigure 1-1 – NHDPlus SchemaNHDPlusUser GuideSeptember 1, 20106THIS PAGE LEFT BLANK INTENTIONALLYNHD Plus User Guide September 1, 2010 7 

### Projection Information

All geospatial datasets are stored in one of three projections/coordinate systems.

All vector data in shapefile format use the followi ng projection/coordinate system:

Projection GEOGRAPHIC
Datum NAD83
Zunits NO
Units DD Spheroid GRS1980
Xshift 0.0000000000
Yshift 0.0000000000

All grid datasets (cat, fac, fdr, elev_cm) for the conterminous U.S. are stored in an Albers EqualArea projection:

Projection ALBERS
Datum NAD83
Zunits NO 2

Units METERS Spheroid GRS1980
Xshift 0.0000000000
Yshift 0.0000000000

Parameters 29 30 0.000 /* 1st standard parallel 45 30 0.000 /* 2nd standard parallel
-96 0 0.000 /* central meridian 23 0 0.000 /* latitude of projection’s origin 0.00000 /* false easting (meters) 0.00000 /* false northing (meters)

Grid datasets (cat, fac, fdr, elev_cm) for Hawaii ( hydrologic region 20) are stored in the Universal Transverse Mercator (UTM) projection:

Projection UTM
Zone 4
Datum NAD83
Zunits NO 2

Units Meters Spheroid GRS1980
Xshift 0.0000000000
Yshift 0.0000000000NHD Plus User Guide September 1, 2010 8 Grid datasets (cat, fac, fdr, elev_cm) for Puerto R ico and the US Virgin Islands (hydrologic region 21) are stored in the Lambert projection:

Projection LAMBERT
Datum NAD83
Zunits NO 2

Units Meters Spheroid GRS1980
Xshift 0.0000000000
Yshift 0.0000000000NHD Plus User Guide September 1, 2010 9 

### NHDPlus Versioning System

NHDPlus has a dual versioning system; both the data model schema and the data content are
versioned. NHDPlus download .ZIP files each contai n the version information in the filename in the form of "Vss_cc", where “ss” is the schema vers ion and “cc” is the data content version. Each component can be versioned and distributed wit hout the need to re-release all components. For example, at any given time, both NHDPlus10LV02_ 03_CatShape.zip and NHDPlus10LV01_02_NHD.zip might be available for dow nload. In this example, the Catchment shape file has schema version 02 and data content version 03 while the NHD component has schema version 01 and data content ve rsion 02.

The NHDPlus download site will contain the most rec ent version for each component. When a change in NHDPlus affects more than one component, the new version of all affected components will be made available at the same time. Therefore, users can be assured that all components on the download site, regardless of thei r indicated versions, are compatible with
each other.

In order to be able to determine which version has been uncompressed onto your computer, the
.ZIP files contain a .TXT file with the same name a s the .ZIP file. For example, NHDPlus08V01_02_NHD.zip will contain a file called NHDPlus08V01_02_NHD.txt. The file you are reading is one such file.

When NHDPlus .ZIP files are uncompressed, the versi on text files are stored in the upper level directory of the NHDPlus data. The existence of th ese .TXT files on your computer means that you, at one time, installed the indicated versions of the indicated NHDPlus components. For example, the presence of the file named NHDPlus08V0 1_02_NHD.txt means that you have installed Version V01_02 of the NHD component for h ydrologic region 08.

If you retain these .TXT files and uncompress updat es into the same location, these files will not be overwritten and will serve as a history of what NHDPlus components and versions you have
installed.

When you download a current version of an NHDPlus c omponent and wish to replace a previously installed version, you must first delete the existing version of the component from your computer. This is best done with ArcCatalog, but may also be accomplished using Windows Explorer. Once you have deleted the compon ent, use the instructions in the NHDPlus User Guide for unpacking the new version of the com ponent.

Instructions for deleting with ArcCatalog:

Component Delete CatGrid \NHDPlusrr\Drainage\cat CatShape \NHDPlusrr\Drainage\catchment Cat_Flowline_Attr \NHDPlusrr\catchmentattribute snlcd \NHDPlusrr\catchmentattributestempprec ipNHD Plus User Guide September 1, 2010 10 \NHDPlusrr\flowlineattribute sflow \NHDPlusrr\flowlineattributes nlcd \NHDPlusrr\flowlineattribute stempprecip \NHDPlusrr\headwaternodearea Elev_Unit_x \NHDPlusrr\elev_unit_x FAC_FDR_Unit_x \NHDPlusrr\fac_fdr_unit_x NHD \NHDPlusrr\Hydrography

\NHDPlusrr\HydrologicUnits \NHDPlusrr\NHDFcode \NHDPlusrr\NHDFeatureToMetad ata \NHDPlusrr\NHDFlow \NHDPlusrr\NHDFlowlineVAA \NHDPlusrr\NHDHydroLineEvent \NHDPlusrr\NHDHydroPointEven t \NHDPlusrr\NHDMetadata \NHDPlusrr\NHDProcessingPara meters \NHDPlusrr\NHDReachCode_Comi d \NHDPlusrr\NHDReachCrossRefer ence \NHDPlusrr\NHDStatus \NHDPlusrr\NHDVerticalRelati onship \NHDPlusrr\TNavwork \NHDPlusrr\TNavigation_Event s_table \NHDPlusrr\TNavigation_Event s ProductionUnits \NHDPlus\ProductionUnits

Instructions for deleting with Windows Explorer:

Component Delete CatGrid \NHDPlusrr\Drainage\cat \NHDPlusrr\Drainage\info \NHDPlusrr\Drainage\cat.* CatShape \NHDPlusrr\Drainage\catchment. * Cat_Flowline_Attr \NHDPlusrr\catchmentattribute snlcd.* \NHDPlusrr\catchmentattribut estempprecip.* \NHDPlusrr\flowlineattribute sflow.* \NHDPlusrr\flowlineattribute snlcd.* \NHDPlusrr\flowlineattribute stempprecip.* \NHDPlusrr\headwaternodearea .* Elev_Unit_x \NHDPlusrr\elev_unit_x FAC_FDR_Unit_x \NHDPlusrr\fac_fdr_unit_x NHD \NHDPlusrr\Hydrography \NHDPlusrr\HydrologicUnits \NHDPlusrr\NHDFcode.* \NHDPlusrr\NHDFeatureToMetad ata.* \NHDPlusrr\NHDFlow.* \NHDPlusrr\NHDFlowlineVAA.*NHD Plus User Guide September 1, 2010 11 \NHDPlusrr\NHDHydroLineEvent .* \NHDPlusrr\NHDHydroPointEvent .* \NHDPlusrr\NHDMetadata.* \NHDPlusrr\NHDProcessingPara meters.* \NHDPlusrr\NHDReachCode_Comi d.* \NHDPlusrr\NHDReachCrossRefe rence.* \NHDPlusrr\NHDStatus.* \NHDPlusrr\NHDVerticalRelati onship.* \NHDPlusrr\TNav*.* ProductionUnits \NHDPlus\ProductionUnits

NHDPlus documentation is versioned by the schema ve rsion only (see first page of guide). When you download a component containing a new sche ma version, you should also download
new documentation.NHD Plus User Guide September 1, 2010 12 

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## Chapter 2 Feature Class Descriptions

cat (catchment grid)

Description: An Environmental Systems Research Institute (ESRI) integer grid dataset that
associates each cell with a catchment.

Field Name Description Value The value stored in grid cells A unique identification number for each catchment (compressed numbering system) Also known as Grid_code in related tables Count Number of cells with a particular value in th e Value field The count equals the number of 30x30 meter grid cel ls in each
catchment
ComID Common identifier of an NHD Flowline Prod_unit Production unit identifier

Catchment (shapefile)

Description: Contains a catchment polygon for each NHD Flowline that received a catchment.

Note: Some polygons may be multipart polygons.

Field Name Description
ComID Common Identifier of an NHD Flowline Grid_code The value stored in grid cells A unique identification number for each catchment (compressed numbering system) Grid_count Number of cells with a particular value in the value field The count equals the number of 30x30 meter grid cel ls in each
catchment Catchment area can be computed from this field Prod_unit Production unit identifier AreaSqKm Feature area in square kilometersNHD Plus User Guide September 1, 2010 14 elev cm (elevation grid)

Description: An ESRI integer grid dataset that gives the elevat ion in centimeters (from the North American Vertical Datum of 1988). The elevat ion grid does not contain an associated value attribute table (VAT). The grid was derived from the 30-meter resolution NED and was projected into the national Albers projection.

fac (flow accumulation grid)

Description: An ESRI integer grid dataset that counts the numbe r of cells draining to each cell in the grid. The flow accumulation grid does not c ontain an associated VAT.

fdr (flow direction grid)

Description: The values in this grid are codes indicating the d irection water would flow from each grid cell.

Field Name Description Value The value stored in grid cells. A grid cell can be assigned one of eight possible values:
1 – Flow is to the East
2 – Flow is to the Southeast
4 – Flow is to the South
8 – Flow is to the Southwest
16 – Flow is to the West
32 – Flow is to the Northwest
64 – Flow is to the North
128 – Flow is to the Northeast Count Number of cells with a particular value in th e Value fieldNHD Plus User Guide September 1, 2010 15 NHDFlowline (shapefile) 3

Description: NHD linear features of types: stream/river, canal/ ditch, pipeline, artificial path, coastline, and connector.

Note: The edits made to these features during the buildi ng of NHDPlus will be loaded into the
National NHD database at USGS sometime after NHDPlu s is released. New flowlines, split flowlines, and merged flowlines have temporary ComI Ds of the format 9nnnnnnnn. Permanent ComIDs will be assigned when the edits are processe d by USGS.

Field Name Description
ComID Common identifier of the NHD feature FDate Feature Currency Date Resolution Always “Medium” (i.e., 1:100K scale) GNIS_ID Geographic Names Information System ID for the value in GNIS_Name GNIS_Name Feature Name from the Geographic Names In formation System LengthKM Feature length in kilometers ReachCode Reach Code assigned to feature Flowdir Flow direction is “WithDigitized” or “Unini tialized” WBAreaComI ComID of an NHD polygonal water feature through which an
NHD “Artificial Path” flowline flows FType NHD Feature Type
FCode Numeric codes for various feature attributes in the NHDFCode lookup table Shape_Leng Feature length in decimal degrees Enabled Always “True”NHD Plus User Guide September 1, 2010 16 NHDWaterbody (shapefile) 3

Description: NHD polygonal features of types: Playa, Ice Mass, LakePond, Reservoir, SwampMarsh, and Estuary.

Note: Some waterbody features in the National NHD databa se at USGS have been merged across quadline boundaries. Therefore, some waterbo dy features distributed with NHDPlus are slightly different from those in the National NHD d atabase. However, after combining waterbody features by Reach Code in either the Nati onal NHD database or in NHDPlus, the waterbody Reaches derived should be the same.

Field Name Description
ComID Common identifier of the NHD feature FDate Feature Currency Date Resolution Always “Medium” (i.e., 1:100K scale) GNIS_ID Geographic Names Information System ID for the value in GNIS_Name GNIS_Name Feature Name from the Geographic Names I nformation System AreaSqKm Feature area in square kilometers
Elevation Feature elevation in feet ReachCode Reach Code assigned to feature FType NHD Feature Type FCode Numeric code for various feature attributes; definitions for codes found in the NHDFCode lookup table Shape_Leng Feature length in decimal degrees Shape_Area Feature area in square decimal degreesNHD Plus User Guide September 1, 2010 17 NHDPoint (shapefile) 3

Description: NHD point features of types: Gate, Lock Chamber, R apids, Reservoir, Rock, SinkRise, SpringSeep, Water IntakeOutflow, Waterfal l, and Well. The National NHD database contains a small sample of gaging station features in NHDPoint. These have been deleted from NHDPlus and replaced with a separate shapefile of a ll active stream gages (see StreamGageEvent shapefile below).

Field Name Description
ComID Common identifier of the NHD feature FDate Feature Currency Date Resolution Always “Medium” (i.e., 1:100K scale) GNIS_ID Geographic Names Information System ID for the value in GNIS_Name GNIS_Name Feature Name from the Geographic Names I nformation System ReachCode Reach Code assigned to feature FType NHD Feature Type FCode Numeric code for various feature attributes; definitions for codes found in the NHDFCode lookup table

NHDLine (shapefile) 3

Description: NHD linear features of types: Bridge, DamWeir, Flu me, Gate, Lock Chamber, Nonearthen Shore, Rapids, Reef, SinkRise, Tunnel, W all, Waterfall, Sounding Datum Line, and Special Use Zone Limit.

Field Name Description
ComID Common identifier of the NHD feature FDate Feature Currency Date Resolution Always “Medium” (i.e., 1:100K scale) GNIS_ID Geographic Names Information System ID for the value in GNIS_Name GNIS_Name Feature Name from the Geographic Names I nformation System LengthKM Feature length in kilometers FType NHD Feature Type
FCode Numeric codes for various feature attributes ; definitions for codes found in the NHDFCode lookup table Shape_Leng Feature length in decimal degreesNHD Plus User Guide September 1, 2010 18 NHDArea (shapefile) 3

Description: NHD polygonal features of types: Area to be Submer ged, BayInlet, Bridge, CanalDitch, DamWeir, Flume, Foreshore, Hazard Zone, Lock Chamber, Inundation Area, Rapids, SeaOcean, Special Use Zone, Spillway, Strea mRiver, Submerged Stream, Wash, Water IntakeOutflow, and Area of Complex Channels.

Note: Some of these features in the National NHD databas e at USGS have been merged across quadline boundaries and split at Subbasin boundarie s. Therefore, some area features distributed with NHDPlus are slightly different from those in t he National NHD database.

Field Name Description
ComID Common identifier of the NHD feature FDate Feature Currency Date Resolution Always “Medium” (i.e., 1:100K scale) GNIS_ID Geographic Names Information System ID for the value in GNIS_Name GNIS_Name Feature Name from the Geographic Names I nformation System AreaSqKm Feature area in square kilometers
Elevation Feature elevation in feet FType NHD Feature Type
FCode Numeric codes for various feature attributes ; definitions for codes found in the NHDFCode lookup table Shape_Leng Feature length in decimal degrees Shape_Area Feature area in square decimal degreesNHD Plus User Guide September 1, 2010 19 StreamGageEvent (shapefile)

Description: The StreamGageEvent shape file contains the physic al locations of the USGS stream gages as well as their location on NHDFlowli ne features through events linked to reach
codes and measures. More information can be found at: http://water.usgs.gov/GIS/metadata/usgswrd/XML/stre amgages.xml and http://www.usbr.gov/pmts/hydraulics_lab/twahl/bfi/i ndex.html

.

Field Name Description ComID Not populated
EventDate Date event was created Reachcode Reachcode on which Stream Gage is located ReachSMDat Reach Version Date – Not populated Reachresol Reach Resolution, always “Medium” (i.e., 1:100K scale)
FeatureCom Reserved for future use
FeatureCla Reserved for future use Source_ori Originator of Event Source_Dat Description of point entity Source_fea Gage Identifier/USGS Site Number Featuredet URL where detailed gage data can be foun d (NWISWEB) Measure Measure along reach where Stream Gage is lo cated in percent
from downstream end Offset Always zero EventType “StreamGage” Agency_cd Gov. Agency responsible for the Stream Ga ge Station_nm Station name State_cd 2 digit state FIPS code of the WSC maintai ning the gage. Puerto
Rico is listed as a state. State 2 character state postal abbreviation of the WSC maintaining the gage. Puerto Rico is listed as a state. Sitestatus Active (A) or Inactive (I) where active Stream Gage has streamflow data in water year(s) 2003 and/or 2004 DA_SQ_Mile Reported drainage area in square miles. Stations with drainage area -999999 means there is no reported drainage ar ea in the National Water Information System. Lon_site Longitude of the Stream Gage (site) locati on - gage house in decimal degrees, NAD83 Lat_site Latitude of the Stream Gage (site) locatio n - gage house in decimal degrees, NAD83 Lon_NHD Longitude of the NHD location in decimal de grees, NAD83 Lat_NHD Latitude of the NHD location in decimal deg rees, NAD83 NHD2Gage_d Distance between Stream Gage and NHD Rea ch Reviewed Flag to indicate location review status - Y or N BFIyrs Number of years used in the base-flow index computation BFI_Ave Average annual base-flow index value (fract ion)NHD Plus User Guide September 1, 2010 20 Field Name Description BFI_Stdev Standard deviation of annual base-flow in dex (fraction) GotBFI Flag indicating BFI data (1) or no BFI data (2) Day1 First date of flow data (yyyymmdd) DayN Last date of flow data (yyyymmdd) NDays Number of days of flow data NDaysGT0 Number of days of non-zero flow MIN_ Minimum daily flow for the period of record (c ubic feet per second) P1 1st percentile of daily flow for the period of r ecord. Negative values indicate reverse flow; tidal or backwater. ( cubic feet per second) P5 5th percentile of daily flow for the period of r ecord. Negative values indicate reverse flow; tidal or backwater. ( cubic feet per second)

P10 10 th percentile of daily flow for the period of record. Negative values indicate reverse flow; tidal or backwater. ( cubic feet per second)

P20 20 th percentile of daily flow for the period of record. Negative values indicate reverse flow; tidal or backwater. ( cubic feet per second)

P25 25 th percentile of daily flow for the period of record. Negative values indicate reverse flow; tidal or backwater. ( cubic feet per second)

P30 30 th percentile of daily flow for the period of record. Negative values indicate reverse flow; tidal or backwater. ( cubic feet per second)

P40 40 th percentile of daily flow for the period of record. Negative values indicate reverse flow; tidal or backwater. ( cubic feet per second)

P50 50 th percentile of daily flow for the period of record. Negative values indicate reverse flow; tidal or backwater. ( cubic feet per second)

P60 60 th percentile of daily flow for the period of record. Negative values indicate reverse flow; tidal or backwater. ( cubic feet per second)

P70 70 th percentile of daily flow for the period of record. Negative values indicate reverse flow; tidal or backwater. ( cubic feet per second)

P75 75 th percentile of daily flow for the period of record. Negative values indicate reverse flow; tidal or backwater. ( cubic feet per second)

P80 80 th percentile of daily flow for the period of record. Negative values indicate reverse flow; tidal or backwater. ( cubic feet per second)

P90 90 th percentile of daily flow for the period of record. NegativeNHD Plus User Guide September 1, 2010 21 Field Name Description values indicate reverse flow; tidal or backwater. ( cubic feet per second)
P95 95 th percentile of daily flow for the period of record. Negative values indicate reverse flow; tidal or backwater. ( cubic feet per second)

P99 99 th percentile of daily flow for the period of record. Negative values indicate reverse flow; tidal or backwater. ( cubic feet per second) Max_ Maximum daily flow for the period of record. ( cubic feet per second) Ave Average daily flow for the period of record. (c ubic feet per second) Stdev Standard deviation of daily flow for the peri od of record. (cubic feet per second) GOTQ Flag indicating flow data (1) or no flow data (0) HUC Hydrologic Cataloging Unit (8-digit HUC) of the Stream Gage HUC_Reg Hydrologic Region (2-digit HUC) of the Stre am Gage Subregion Hydrologic sub-region (4-digit HUC) of th e Stream Gage Accounting Hydrologic accounting unit (6-digit HUC) of the Stream Gage

Subbasin (shapefile) 3

Description: Boundaries of 8-digit Hydrologic Units (HUC8) used to assign NHD 1:100,000-
scale Reach Codes. These HUC8 boundaries are a der ivative of the 1:250,000-scale boundaries published by USGS in 1970. These boundaries are no t compatible with the more recent Watershed Boundary Dataset.

Field Name Description ObjectID Internal identifier Source Source of boundary: 0 – Watershed Boundary Dataset
1 – NHD
2 – Other HUC_8 8-digit Hydrologic Unit Code, also known as Subbasin code (formerly known as Cataloging Unit code) HU_8_Name Text name of Subbasin Shape_Leng Feature length in decimal degrees Shape_Area Feature area in square decimal degrees

Region (shapefile) 3

Description: This table is always empty in NHDPlus and is incl uded for completeness of the
NHD data model.NHD Plus User Guide September 1, 2010 22 SubRegion (shapefile) 3

Description: This table is always empty in NHDPlus and is incl uded for completeness of the
NHD data model.

Basin (shapefile) 3

Description: This table is always empty in NHDPlus and is incl uded for completeness of the
NHD data model.

ProductionUnit (shapefile)

Description: The sub-divisions used to process the elevation da ta into flow direction and accumulation grids. The production units are distr ibuted as a national shapefile.

Field Name Description Prod_Unit Production unit identifier Area Area of the production unit Perimeter Perimeter of the production unitNHD Plus User Guide September 1, 2010 23 

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## Chapter 3 Attribute Table Descriptions

Some fields in the attribute tables contain special coded values as follows:

The value “-9998” signifies that the applicable val ue for the field is missing or
undetermined. The value “-9999” signifies that there is no applic able value and one will never be assigned.

CatchmentAttributesNLCD for All Hydrologic Regions Except 20 (Hawaii) (dBase file)

Description: National Land Cover Dataset 1992 (NLCD) attributes derived for each flowline
catchment.

Note: The NLCD was available only for the conterminous U nited States. Percentages of the NLCD categories pertain only to the portion of a ca tchment within the United States but are computed as a percentage of the total area. Thus, any non-zero percentages in fields PCT_CN or PCT_MX must be added to the percentages in the NLCD categories to account for the entire
catchment area.

Field Name Description
ComID Common identifier of an NHD Flowline Grid_code Value field from Catchment Grid NLCD_11 % of catchment area classified as Open Wat er in NLCD NLCD_12 % of catchment area classified as Perennia l Ice/Snow in NLCD NLCD_21 % of catchment area classified as Low Inte nsity Residential in
NLCD NLCD_22 % of catchment area classified as High Int ensity Residential in
NLCD NLCD_23 % of catchment area classified as Commerci al/Industrial/ Transportation in NLCD NLCD_31 % of catchment area classified as Bare Roc k/Sand/Clay in
NLCD NLCD_32 % of catchment area classified as Quarries /Strip Mines/Gravel
Pits in NLCD NLCD_33 % of catchment area classified as Transiti onal in NLCD NLCD_41 % of catchment area classified as Deciduou s Forest in NLCD NLCD_42 % of catchment area classified as Evergree n Forest in NLCD NLCD_43 % of catchment area classified as Mixed Fo rest in NLCD NLCD_51 % of catchment area classified as Shrublan d in NLCD NLCD_61 % of catchment area classified as Orchards /Vineyards/Other in
NLCD NLCD_71 % of catchment area classified as Grasslan ds/Herbaceous inNHD Plus User Guide September 1, 2010 25 Field Name Description

NLCD NLCD_81 % of catchment area classified as Pasture/ Hay in NLCD NLCD_82 % of catchment area classified as Row Crop s in NLCD NLCD_83 % of catchment area classified as Small Gr ains in NLCD NLCD_84 % of catchment area classified as Fallow i n NLCD NLCD_85 % of catchment area classified as Urban/Re creational Grasses in
NLCD NLCD_91 % of catchment area classified as Woody We tlands in NLCD NLCD_92 % of catchment area classified as Emergent Herbaceous
Wetlands in NLCD PCT_CN % of catchment area in Canada and not classi fied in NLCD PCT_MX % of catchment area in Mexico and not classi fied in NLCD SUM_PCT Sum of the % catchment areas

CatchmentAttributesNLCD for Hydrologic Region 20 (H awaii) (dBase file)

Description: The NOAA Coastal Change Analysis Program (C-CAP) attributes derived for
each catchment.

Field Name Description
ComID Common identifier of an NHD Flowline Grid_code Value field from Catchment Grid HILC_1 % of catchment area classified as Unclassif ied in National Oceanic and Atmospheric Administration (NOAA) C-CAP HILC_2 % of catchment area classified as High Inte nsity Developed in
NOAA C-CAP
HILC 3 % of catchment area classified as Low Inten sity Developed in
NOAA C-CAP
HILC 4 % of catchment area classified as Cultivate d Land in NOAA CCAP
HILC 5 % of catchment area classified as Grassland in NOAA C-CAP
HILC 6 % of catchment area classified as Deciduous Forest in NOAA CCAP HILC_7 % of catchment area classified as Evergreen Forest in NOAA CCAP HILC_8 % of catchment area classified as Mixed For est in NOAA CCAP HILC_9 % of catchment area classified as Scrub/Shr ub in NOAA C-CAP
HILC 10 % of catchment area classified as Palustri ne Forested Wetland in
NOAA C-CAP HILC_11 % of catchment area classified as Palustri ne Scrub/Shrub
Wetland in NOAA C-CAPNHD Plus User Guide September 1, 2010 26 Field Name Description HILC_12 % of catchment area classified as Palustri ne Emergent Wetland
in NOAA C-CAP
HILC 13 % of catchment area classified as Estuarin e Forested Wetland in
NOAA C-CAP HILC_14 % of catchment area classified as Estuarin e Scrub/Shrub
Wetland in NOAA C-CAP HILC_15 % of catchment area classified as Estuarin e Emergent Wetland
in NOAA C-CAP
HILC 16 % of catchment area classified as Unconsol idated Shore in
NOAA C-CAP
HILC 17 % of catchment area classified as Bare Lan d in NOAA C-CAP HILC_18 % of catchment area classified as Water in NOAA C-CAP HILC_19 % of catchment area classified as Palustri ne Aquatic Bed in
NOAA C-CAP HILC_20 % of catchment area classified as Estuarin e Aquatic Bed in
NOAA C-CAP
HILC 21 % of catchment area classified as Tundra i n NOAA C-CAP HILC_22 % of catchment area classified as Snow/Ice in NOAA C-CAP PCT_CN % of catchment area in Canada and not classi fied in NOAA CCAP PCT_MX % of catchment area in Mexico and not classi fied in NOAA CCAP SUM_PCT Sum of the % catchment areas

CatchmentAttributesTempPrecip (dBase file)

Description: NHDPlus attributes derived for each flowline catch ment.

Note: Precipitation and temperature values are the averag e values over the catchment from the PRISM 1961-90, 2.5-minute (approximately 4 km) reso lution data by Daly and Taylor (1998). If a catchment extends into Canada or Mexico, the valu e will be the average over only the U.S. portion of the catchment. TEMP values are not prov ided for Hydrologic Region 20 (Hawaii) because TEMP is not needed for the regression-based flow estimates in that region (refer to Step 6 in Appendix A).

Note: In Hydrologic Region 21 (Puerto Rico and the Virgin Islands) Temp values are not provided and Precip values are provided for Puerto Rico only.

Field Name Description
ComID Common identifier of an NHD Flowline Grid_code Value field from Catchment Grid Precip Mean annual precipitation in mm Temp Mean annual temperature in degrees centigrade * 10NHD Plus User Guide September 1, 2010 27 FlowlineAttributesFlow for All Hydrologic Regions E xcept 21 (dBase file)

Description: NHDPlus attributes derived for NHD Flowlines.

Field Name Description
ComID Common identifier of an NHD Flowline Grid_code Value field from Catchment Grid CumDrainag Cumulative drainage area in square kilo meters(sq km) at bottom
of flowline MAFlowU Mean Annual Flow in cubic feet per second (cfs) at bottom of flowline as computed by Unit Runoff Method MAFlowV Mean Annual Flow (cfs) at bottom of flowli ne as computed by Vogel Method. In Hydrologic Region 20 (Hawaii), thi s value is the median annual flow (cfs) as computed using the method of Fontaine, et. al. (1992). MAVelU Mean Annual Velocity (fps) at bottom of flo wline as computed by Jobson Method (1996) using the flow in MAFlowU. MAVelV Mean Annual Velocity (fps) at bottom of flow line as computed by Jobson Method (1996) using the flow in MAFlowV. IncrFlowU Incremental Flow (cfs) for Flowline as c omputed by the Unit
Runoff Method MaxElevRaw Maximum elevation (unsmoothed) in meters , populated only for headwater Flowlines, -9998 for all non-headwater Fl owlines MinElevRaw Minimum elevation (unsmoothed) in meters MaxElevSmo Maximum elevation (smoothed) in meters MinElevSmo Minimum elevation (smoothed) in meters Slope Slope of flowline (m/m)NHD Plus User Guide September 1, 2010 28 FlowlineAttributesFlow For Region 21(dBase file)

Description: NHDPlus attributes derived for NHD Flowlines.

Field Name Description
ComID Common identifier of an NHD Flowline Grid_code Value field from Catchment Grid CumDrainag Cumulative drainage area in square kilo meters(sq km) at bottom
of flowline MAFlowU Mean Annual Flow in cubic feet per second (cfs) at bottom of flowline as computed by Unit Runoff Method MAVelU Mean Annual Velocity (fps) at bottom of flo wline as computed by Jobson Method (1996) using the flow in MAFlowU. IncrFlowU Incremental Flow (cfs) for Flowline as c omputed by the Unit
Runoff Method MaxElevRaw Maximum elevation (unsmoothed) in meters MinElevRaw Minimum elevation (unsmoothed) in meters , populated only for headwater Flowlines, -9998 for all non-headwater Fl owlines MaxElevSmo Maximum elevation (smoothed) in meters MinElevSmo Minimum elevation (smoothed) in meters Slope Slope of flowline (m/m)

FlowlineAttributesNLCD for All Hydrologic Regions E xcept 20 (Hawaii) (dBase file)

Description: NHDPlus cumulative attributes derived for NHD Flow lines.

Note: The NLCD 1992 was available only for the contermin ous United States. Percentages of the NLCD categories pertain only to the portion of a drainage area within the United States but are computed as a percentage of the total drainage area. Thus, any non-zero percentages in fields PCT_CN or PCT_MX must be added to the percentages i n the NLCD categories to account for the entire drainage area. The cumulative values re present values at the bottom of the flowline.

Field Name Description
ComID Common identifier of an NHD Flowline Grid_Code Value field from Catchment Grid CumNLCD_11 % of cumulative drainage area classifie d as Open Water in
NLCD CumNLCD_12 % of cumulative drainage area classifie d as Perennial Ice/Snow
in NLCD CumNLCD_21 % of cumulative drainage area classifie d as Low Intensity
Residential in NLCD CumNLCD_22 % of cumulative drainage area classifie d as High Intensity
Residential in NLCDNHD Plus User Guide September 1, 2010 29 Field Name Description CumNLCD_23 % of cumulative drainage area classifie d as Commercial/ Industrial/Transportation in NLCD CumNLCD_31 % of cumulative drainage area classified as Bare Rock/Sand/ Clay in NLCD CumNLCD_32 % of cumulative drainage area classifie d as Quarries/Strip
Mines/Gravel Pits in NLCD CumNLCD_33 % of cumulative drainage area classifie d as Transitional in
NLCD CumNLCD_41 % of cumulative drainage area classifie d as Deciduous Forest in
NLCDD CumNLCD_42 % of cumulative drainage area classifie d as Evergreen Forest in
NLCD CumNLCD_43 % of cumulative drainage area classifie d as Mixed Forest in
NLCD CumNLCD_51 % of cumulative drainage area classifie d as Shrubland in NLCD CumNLCD_61 % of cumulative drainage area classifie d as Orchards/Vineyards/
Other in NLCD CumNLCD_71 % of cumulative drainage area classifie d as Grasslands/
Herbaceous in NLCD CumNLCD_81 % of cumulative drainage area classifie d as Pasture/Hay in
NLCD CumNLCD_82 % of cumulative drainage area classifie d as Row Crops in
NLCD CumNLCD_83 % of cumulative drainage area classifie d as Small Grains in
NLCD CumNLCD_84 % of cumulative drainage area classifie d as Fallow in NLCD CumNLCD_85 % of cumulative drainage area classifie d as Urban/Recreational
Grasses in NLCD CumNLCD_91 % of cumulative drainage area classifie d as Woody Wetlands in
NLCD CumNLCD_92 % of cumulative drainage area classifie d as Emergent
Herbaceous Wetlands in NLCD Cumpct_CN % of cumulative drainage area in Canada and not classified in
NLCD Cumpct_MX % of cumulative drainage area in Mexico and not classified in
NLCD CUMSUM_PCT Sum of the % cumulative drainage areasNHD Plus User Guide September 1, 2010 30 FlowlineAttributesNLCD for Hydrologic Region 20 (Ha waii) (dBase file)

Field Name Description
ComID Common identifier of an NHD Flowline Grid_code Value field from Catchment Grid CUMHILC_1 % of cumulative drainage area classified as Unclassified in
NOAA C-CAP CUMHILC_2 % of cumulative drainage area classified as High Intensity Developed in NOAA C-CAP CUMHILC_3 % of cumulative drainage area classified as Low Intensity Developed in NOAA C-CAP CUMHILC 4 % of cumulative drainage area classified as Cultivated Land in
NOAA C-CAP CUMHILC 5 % of cumulative drainage area classified as Grassland in NOAA
C-CAP CUMHILC_6 % of cumulative drainage area classified as Deciduous Forest in
NOAA C-CAP CUMHILC 7 % of cumulative drainage area classified as Evergreen Forest in
NOAA C-CAP CUMHILC 8 % of cumulative drainage area classified as Mixed Forest in
NOAA C-CAP CUMHILC 9 % of cumulative drainage area classified as Scrub/Shrub in
NOAA C-CAP CUMHILC 10 % of cumulative drainage area classifie d as Palustrine Forested
Wetland in NOAA C-CAP CUMHILC_11 % of cumulative drainage area classifie d as Palustrine
Scrub/Shrub Wetland in NOAA C-CAP CUMHILC 12 % of cumulative drainage area classifie d as Palustrine Emergent
Wetland in NOAA C-CAP CUMHILC 13 % of cumulative drainage area classifie d as Estuarine Forested
Wetland in NOAA C-CAP CUMHILC 14 % of cumulative drainage area classifie d as Estuarine
Scrub/Shrub Wetland in NOAA C-CAP CUMHILC 15 % of cumulative drainage area classifie d as Estuarine Emergent
Wetland in NOAA C-CAP CUMHILC 16 % of cumulative drainage area classifie d as Unconsolidated
Shore in NOAA C-CAP CUMHILC 17 % of cumulative drainage area classifie d as Bare Land in NOAA
C-CAP CUMHILC 18 % of cumulative drainage area classifie d as Water in NOAA CCAP CUMHILC 19 % of cumulative drainage area classifie d as Palustrine Aquatic
Bed in NOAA C-CAP CUMHILC 20 % of cumulative drainage area classifie d as Estuarine Aquatic
Bed in NOAA C-CAP CUMHILC_21 % of cumulative drainage area classifie d as Tundra in NOAA C-NHD Plus User Guide September 1, 2010 31 Field Name Description

CAP CUMHILC_22 % of cumulative drainage area classified as Snow/Ice in NOAA
C-CAP CUMPCT_CN % of cumulative drainage area in Canada a nd not classified in
NOAA C-CAP CUMPCT_MX % of cumulative drainage area in Mexico a nd not classified in
NOAA C-CAP CUMSUM_PCT Sum of the % cumulative drainage areas

FlowlineAttributesTempPrecip (dBase file)

Description: NHDPlus cumulative attributes derived for NHD Flow lines.

Note: AreaWtMAT values are not provided for Hydrologic R egion 20 (Hawaii) because Temperature is not needed for the regression-based flow estimates in this region; refer to Step 6 in Appendix A.

Note: In Hydrologic Region 21 (Puerto Rico and the Virgin Islands) AreaWtMAT values are not provided and AreaWtMAP values are provided for Puerto Rico only.

Field Name Description
ComID Common identifier of an NHD Flowline Grid_Code Value field from Catchment Grid AreaWtMAP Area Weighted Mean Annual Precipitation at bottom of flowline
in mm AreaWtMAT Area Weighted Mean Annual Temperature at bottom of flowline in degree C * 10

HeadWaterNodeArea (dBase file)

Description: For each headwater node in the surface water net work, the HeadWaterNodeArea
table contains the size of the land area that drain s to the node at the upstream end of the flowline.

Field Name Description
ComID Common identifier of an NHD Flowline Grid_Code Value field from Catchment Grid HwNodesqkm Catchment area in square kilometers that drains to the headwater node of the flowline indicated by ComIDNHD Plus User Guide September 1, 2010 32 NHDFCode (dBase file) 3

Description: The FCode table describes attribute codes used in the FCode fields of feature
tables.

Field Name Description FCode A numeric code that represents the feature t ype plus its encoded
attribute values Descriptio Text description of feature type and th e encoded attributes CanalDitch Canal Ditch Type (aqueduct, unspecified ) Constructi Construction material (earthen, noneart hen, unspecified) Hydrograph Intermittent or perennial Inundation Inundation Area Type (debris basin, dew atering area, duck pond, general case, percolation basin, retarding basin) Operationa Operational Status (abandoned, operatio nal, under construction) PipelineTy Pipeline type (aqueduct, general case, penstock, siphon) Positional Positional accuracy (approximate, defin ite, indefinite, not applicable) Relationsh Relationship to surface (abovewater, at or near, elevated, underground, underwater, unspecified) ReservoirT Reservoir type (aquaculture, decorative pool, disposal-tailings pond, disposal-unspecified, evaporator, swimming po ol, treatment-cooling pond, treatment-filtration pond, treatmentsettling pond, treatment-sewage treatment pond, uns pecified water storage) Stage Elevation stage (Normal Pool, Flood Elevatio n, Average Water Elevation, Date of Photography, High Water Elevatio n, Spillway Elevation) SpecialUse Special use category (dump site, spoil area)

NHDFeaturetometadata (dBase file) 3

Description: Links metadata to features.

Field Name Description
ComID Common identifier for an NHD feature

DUUID Metadata identifierNHD Plus User Guide September 1, 2010 33 NHDFlow (dBase file) 3

Description: A geometry-independent flow table that is QA/QC’ed for internal consistency and external agreement with flowline geometry. The flow table contains entries for: (1) every pair of flowlines that exchange water, (2) the beginning of each network, and (3) each network terminus where water flows into the ground. At the shorelin e, network flowline features have a “nonflowing” connection to coastline features. Coastlin e features also have “non-flowing”
connections to each other.

Field Name Description
FromComID Common identifier for an NHD Flowline
ToComID Common identifier for an NHD Flowline
DeltaLevel Numerical difference between stream lev el for FromComID and
stream level for ToComID Direction 714 – coastal connection (FromComID flow line may be a coastline and ToComID flowline is always a coastlin e) 709 – flowing connection 712 – network start (ToComID flowline is a headwate r) 713 – network end (FromComID flowline is a network end)
ToComIDMea Not valuedNHD Plus User Guide September 1, 2010 34 NHDFlowlineVAA (dBase file) 3

Description: Value Added Attributes (VAAs) for each flowline th at appears in the NHDPlus
Flow table. The NHDFlowlineVAA table is an offici al part of the NHD schema, however, with the exception of stream level, the VAAs not populat ed in the NHD data. It’s the NHDPlus building process that populates the VAAs. For addi tional descriptive information about the VAAs, see Appendix A, “Step 1”. “Understanding and Using NHDPlus” contains many examples of how to accomplish tasks using the VAAs.

Field Name Description
ComID Common identifier of NHD Flowline
StreamLeve Stream level StreamOrde Strahler stream order. Always 0. FromNode From node number (top of flowline) ToNode To node number (bottom of flowline) Hydroseq Hydrologic sequence number LevelPathi Hydrologic sequence number of most down stream flowline in level path PathLength Distance to terminal flowline downstrea m along the mainpath (kilometers) TerminalPa Hydrologic sequence number of terminal flowline ArbolateSu An estimate of miles of stream upstream of a flowline. Always 0. (square
kilometers Divergence 0 – not part of a divergence

1 – main path of a divergence 2 – minor path of a divergence StartFlag 0 – not a headwater flowline

1 – a headwater flowline
TerminalFl 0 – not a terminal flowline

1 – a terminal flowline
DnLevel Streamlevel of mainstem downstream flowlin e ThinnerCod Ordinal value used to display various n etwork densities UpLevelPat Upstream mainstem level path identifier UpHydroSeq Upstream mainstem hydrologic sequence n umber UpMinHydro Upstream minimum hydrologic sequence nu mber DnLevelPat Downstream mainstem level path identifi er DnMinHydro Downstream minor path hydrologic sequen ce number DnDrainCou Number of flowlines immediately downstr eam

NHDHydroLineEvent (dBase file) 3

Description: This table is always empty in NHDPlus and is inclu ded for completeness of the
NHD data model.NHD Plus User Guide September 1, 2010 35 NHDHydroPointEvent (dBase file) 3

Description: This table is always empty in NHDPlus and is inclu ded for completeness of the
NHD data model.

NHDMetadata (dBase file) 3

Description: Holds NHD metadata records that can be linked to i ndividual features through the
NHDFeaturetoMetadata table.

Field Name Description
DUUID Metadata identifier ProcessDes Process Description

ProssDat Process Date AttributeA Attribute Accuracy Report LogicalCon Logical Consistency Report Completene Completeness Report HorizPosit Horizontal Positional Accuracy Report VertPositi Vertical Positional Accuracy Report
MetadataSt Metadata Standard Name Metadata_1 Metadata Standard Version

MetadataDa Metadata Date
DataSetCre Dataset Credit ContactOrg Contact Organization AddressTyp Address Type

Address Address City City
StateorPro State or Province
PostalCode Postal Code ContactVoi Contact Voice Telephone
ContactIns Contact Instructions

NHDProcessingParameters (dBase file) 3

Description: This table is always empty in NHDPlus and is inclu ded for completeness of the
NHD data model.

NHDReachcode_ComID (dBase file) 3

Description: This table is always empty in NHDPlus and is inclu ded for completeness of the
NHD data model.NHD Plus User Guide September 1, 2010 36 NHDReachCrossReference (dBase file) 3

Description: This table tracks the creation, change, and deleti on of Reach Codes.

Field Name Description
OldReachCo Old Reach Code
OldReachDa Old Reach date

NewReachCo New Reach Code
NewReachDa New Reach date OldUpMI Not used
NewUPMI Not used ChangeCode Change code “A”dd, “D”elete, “11” = 1-1, “P1” or “1M” = Partial -1, “1P” or “M1” = 1-Partial,
PP = Partial-Partial

Process Process code
ReachFileV Reach file version OldHUCode Substr(OldReachCode,1,8) NewHUCode Substr(NewReachCode,1,8)

NHDSourceCitation (dBase file) 3

Description: This table contains NHD metadata source citation r ecords that can be linked to individual features through the NHDFeaturetoMetadat a table.

Field Name Description Title Title of data source used as input for creati ng or updating NHD
SourceCita Source Citation Abbreviation Originator Originator

Publicatio Publication Date BeginningD Beginning Date EndingDate Ending Date
SourceCont Source Contribution
SourceScal Source Scale Denominator TypeofSour Type of Source Media

CalendarDa Calendar Date
SourceCurr Source Currentness Reference DUUID Digital Update Unit IdentifierNHD Plus User Guide September 1, 2010 37 NHDStatus (dBase file) 3

Description: This table is always empty in NHDPlus and is inclu ded for completeness of the
NHD data model.

NHDVerticalRelationship (dBase file) 3

Description: This table describes locations where NHD Flowline features cross but do not
intersect.

Field Name Description
ComID Common identifier for the vertical relations hip
AboveComID Common identifier for an NHD Flowline

BelowComID Common identifier for an NHD Flowline

Openme.txt

Description: This text file provides a file handle for file bro wse dialogues in user applications. It also contains a small amount of general informat ion about the NHD data.NHD Plus User Guide September 1, 2010 38 

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## Chapter 4 Geographic Coverage of Data Package

All NHDPlus data (except grids) are distributed by hydrologic region, with two exceptions: Region 10 is split into upper and lower and sub-reg ion 0318 is distributed both inside Region 03 and as a standalone area. For NHDPlus grids, the h ydrologic regions are divided into production units and all grids are distributed by these units. Figure 4-1 shows the production units for NHDPlus components.

Figure 4-1 – NHDPlus Hydrologic Regions and Product ion Unit Map

The two-digit Hydrologic Region designations are ba sed on the Hydrologic Unit Coding system
in which the United States is divided and sub-divid ed into successively smaller hydrologic units which are classified into four levels: regions, sub -regions, accounting units, and cataloging units. Additional information about the coding system is d escribed at http://water.usgs.gov/GIS/huc.html and http://www.ncgc.nrcs.usda.gov/products/datasets/wat ershed/

.NHD Plus User Guide September 1, 2010 40 

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## Chapter 5 Understanding and Using NHDPlus

NHDPlus and Divergences

The NHDPlus network includes complex network compon ents, especially divergent and complex flow paths. These types of network compone nts are illustrated in Figure 5-1. Convergent junctions are the simplest types of junc tions and pose no problems in downstream routing and accumulating attributes, such as draina ge area. Divergent and other types of complex junctions, however, pose particular issues in compu ting cumulative values. These issues and how the NHDPlus FlowlineAttribute tables compute cumula tive values are described below.



# NHDPlus: Complex Hydrography

Convergent
Junction Divergent
Junction Complex
Junction

Figure 5-1 – NHDPlus Complex Hydrography

The DIVERGENCE field in the NHDFlowlineVAA Table de fines “main” and “minor” paths at divergences. A single path is designated as the ma in path and is given a DIVERGENCE field value of “1”. All other paths in the divergence ar e designated as minor paths and are given a DIVERGENCE field value of “2”, as illustrated in Fi gure 5-2.NHD Plus User Guide September 1, 2010 42 Main Path Divergence = 1 Minor Path Divergence = 2

Figure 5-2 – Main and Minor Paths in NHDPlus

In many cases, the divergences are “local”, for ins tance, as shown in Figure 5-3, the red flowlines represent the divergences. The blue lines represent flowlines not affected by these divergences because the divergent streams rejoin at a downstream point.

Figure 5-3 – “Local” DivergencesNHD Plus User Guide September 1, 2010 43 There are many situations in NHDPlus which are much more complex; for instance multiple divergences and divergences where the divergent flo wlines never re-join the network downstream. A somewhat more complicated case is sho wn in Figure 5-4 -. In routing and accumulating downstream, cumulative values will be affected by divergences on the red
flowlines while the blue flowlines are not affected by the divergences. As can be seen, the divergences can affect multiple flowlines. There ar e many divergent areas in NHDPlus which are much more complex; such as braided streams and in c oastal areas.

Figure 5-4 -– More Complex Divergent Junction Examp le

How Cumulative Values in the FlowlineAttributeFlow, FlowlineAttributesTempPrecip, and
FlowlineAttributesNLCD Tables are valued with Diver gences

Various options were considered, such as splitting the drainage areas, flows, etc. evenly between the main and minor paths or setting a default perce ntage split, such as 75% down the main path and 25% down the minor path. It is essential that the proportional split add up to 100%, or else there would eventually be double-counting of attrib utes when divergent paths re-entered the
network downstream.

The decision made for NHDPlus is to route and accum ulate all values down the main path (DIVERGENCE=1), and treat the minor path as essenti ally a “start/headwater” flowline. This isNHD Plus User Guide September 1, 2010 44 illustrated in Figure 5-5. The rationale behind thi s decision is that, given no other contextual information, there is no basis for selecting any pa rticular proportional split. The result of this decision is that, on minor divergent flowlines and the flowlines downstream of the minor divergence that have not re-joined the main path, t he values (drainage area, flow, and cumulative Temp, Precip, and NLCD) will not include the cumula tive drainage values upstream of the divergence.

Main Path Divergence = 1 Minor Path Divergence = 2 Cumulative Attributes Routed down the main path

Figure 5-5 – Cumulative attributes routed down the main path



### Generating Less Dense Networks from NHDFlowline

A connected network that forms the mainpaths of the 8-digit Hydrologic Unit Codes (i.e., Cataloging Units or Subbasins) may be generated by selecting NHDPlus flowlines that have NHDFlowlineVAA.Thinner = 1. Progressively denser n etworks may be generated by selecting
NHDPlus flowlines that have NHDFlowlineVAA.Thinner <= 2 then 3, 4, 5, and 6.



### Understanding NHDPlus Slope

NHDPlus slope is unit-less. It can be viewed as cm /cm or m/m or km/km. The units are the
same in both the numerator and denominator and ther efore cancel out. Cm/cm appears
elsewhere in this documentation to show that it is unit-less.

Minimum and maximum smoothed elevations for flowlin es, in the FlowlineAttributesFlow table, are expressed in meters and flowline length, in the NHDFlowline table, is in kilometers. Therefore when slope is calculated with these field s the result is slope in meters per kilometer (m/km):NHD Plus User Guide September 1, 2010 45 maxelevsmo(m) - minelevsmo(m) ____________________ = slope in m/km lengthkm(km)

To get the unit-less slope given in NHDPlus the uni ts must be converted as follows:

slope in m/km * 1 km/1000m = slope (unit-less) give n in NHDPlus. NHDPlus slopes are constrained to be greater than or equal to 0.00005.



### Finding the Upstream Inflows to an NHDPlus Dataset

All NHDPlus regional datasets are hydrologically co mplete drainage areas except the datasets that make up the Colorado River (NHDPlus14 and NHDP lus15) and the Mississippi River (NHDPlus05, NHDPlus06, NHDPlus07, NHDPlus08, NHDPlu s10L, NHDPlus10R, NHDPlus11, and NHDPlus0318). When navigating the s tream network in either of these areas, it is necessary to determine if the navigation shou ld be continued in an upstream or downstream
NHDPlus dataset.

It is possible to determine upstream inflows to any NHDPlus dataset by selecting
NHDflow.tocomids where the NHDflow.fromcomid is not in NHDFlowline. If those tocomids are in a particular upstream navigation result, the navigation may be continued in the upstream NHDPlus dataset by starting at the fromcomids.

It is possible to determine downstream outflows fro m any NHDPlus dataset by selecting
NHDflow.fromcomids where the NHDflow.tocomid is not in NHDFlowline. If those fromcomids are in a particular downstream navigatio n result, the navigation may be continued in the downstream NHDPlus dataset by starting at the t ocomids.



### Finding all the Tributaries to a Stretch of River

First, find the stretch of interest along the main river using one of these methods:

Method 1: Navigate upstream mainstem on the major r iver from the desired starting
flowline for the desired distance.

Method 2: Query all flowlines with the major river’ s levelpathid where (pathlength - StartingPathlength) <= desired distance. If the de sired stretch of river does not start at the mouth of the river, remove from the query any N HDFlowlineVAA.Hydroseq < the
downstream end of the desired stretch NHDFlowlineVA A.Hydroseq.

Then, find the tributaries to the stretch:
Join the NHDFlow.tocomid to the list of comids from Method1 or Method2. All the NHDflow.fromcomid's in the joined records a re the downstream flowlines of the
tributaries to the desired stretch of main river.NHD Plus User Guide September 1, 2010 46 

### Building an NHDPlus Attribute Accumulator

There are two requirements for attribute accumulati on: site specific or throughout the entire network. Site specific accumulation can be done ea sily with upstream navigation followed by aggregation, based on the navigation results, of an y attributes that have been assigned to
flowlines or their associated catchments. An entir e network accumulation is one where the
desired attributes are accumulated for each flowlin e and saved in an attribute table for future use. Entire network accumulations need a program or scri pt to accomplish the accumulation task. This section addresses the specifications for such a program.

For example, assume that a new attribute has been d efined and that incremental values for the attribute have been assigned to each flowline/catch ment in the stream network. The objective of accumulation is to aggregate the incremental values such that, at any particular flowline/catchment in the network, the total value of the attribute upstream of the flowline/catchment is computed. Aggregation uses d ifferent mathematical operators depending on the attribute being aggregated. For example, dr ainage area is additive, while the percent of area in forest is computed using an area-weighted a verage.

There are two essential techniques for accumulating attributes throughout the network: downstream routing and upstream navigation.

The downstream routing method was employed to do th e NHDPlus cumulative attributes. This method starts at the top of the network and moves d ownstream aggregating together the incremental values and storing the result at each f lowline/catchment. The advantage of this method is that it is fast, taking only minutes to r un. The disadvantage of this method is that it must make decisions at flow splits (divergences) ab out which way to route the accumulated value. The value may not be routed down both paths of a divergence because, if those diverging paths come back together (which is frequently the c ase), the accumulation will begin to doublecount the attribute. In the NHDPlus accumulation, t he decision was made to route the entire accumulated value down the major path of each diver gence. (Refer to the “NHDPlus and Divergences” section for additional information.) It is easy to see that the downstream method is very sensitive to errors in divergence classificati ons. When the wrong path is designated as the major path, the accumulation goes down the wrong pa th. In addition, flowlines that are downstream of divergences that have not yet returne d to the major network path (i.e., unresolved divergences) will not receive the full accumulated value from upstream. This may be appropriate for some attributes but not for others.

The upstream approach is similar to the site specif ic method where the accumulation for each flowline aggregates all the incremental values from upstream. The advantages of this method are that it is not sensitive to errors in divergence cl assifications and always gives the full accumulated value regardless of the presence of ups tream unresolved divergences. The disadvantage of this method is that it takes some t ime to run – hours, not minutes.

Since the simplest approach to coding an accumulati on script is the downstream routing method, specifications for that method are provided here. A s an example, begin with a file that contains
flowline/catchment comid and the incremental value for a single attribute that has been assignedNHD Plus User Guide September 1, 2010 47 to each flowline or catchment. For this specificat ion, assume that the file is called
IncrementalAttributeFile and that it has the fields comid and AttrValue.

1. Add to IncrementalAttributeFile a field for the cum ulative value for the attribute. Let’s
call this CumAttrValue.
2. Join IncrementalAttributeFile with NHDFlowlineVAA u sing comid. For performance, we’ll delete from the joined file every record that has hydroseq = 0. These lines are not in the NHDPlus stream network and also do not get c atchments, therefore they are not
needed in an accumulation.
3. Sort the joined file by Hydroseq descending.
4. Accumulation procedure:
4.1. Get next record from the sorted file. If end of fi le, quit.
4.2. If Startflag = 1 (i.e., this is a headwater and the re is nothing upstream) or divergence = 2 (i.e., nothing is routed down the mi nor paths of the divergence), then set CumAttrValue = AttrValue and go to step 4. 1.
4.3. Find all the inflows to this current flowline: infl ows are records in the joined file
where the tonode = fromnode of the current flowline

. 4.4. Set CumAttrValue = AttrValue + the CumAttrValue for each inflow.
4.5. Go to step 4.1.

This procedure routes the accumulation down the mai npath at each divergence. Alternatively, the accumulation may be apportioned to the different pa ths in the divergence as long as 100% (neither less nor more) of the accumulation is rout ed.



### Flowlines with "known flow" vs. Flowlines with “unk nown flow”

There are over 2.9 million flowline features in NHD Plus. Of those, 2.6 million have a known
flow direction. This information is contained in an attribute called Flowdir in the NHDFlowline
feature class attribute table. Flowdir can have th e values “With Digitized” (known flow direction) or “Uninitialized” (unknown flow directi on). The flowlines having unknown flow direction are primarily of three types: isolated st ream segments, canal/ditches, and some channels
inside braided networks. The flowlines with known flow direction are the subset of the NHDFlowline feature class which makeup the NHDPlus surface water network. The “plus” part of NHDPlus is built just for the flowlines with kno wn flow direction. In other words, the catchment shapefile, CatchmentAttributesNLCD, Catch mentAttributesTempPrecip, FlowlineAttributesFlow, FlowlineAttributesNLCD, and FlowlineAttributesTempPrecip contain
records for flowlines with known flow direction. W hen using NHDPlus, it is useful to symbolize the NHDFlowline feature class using the Flowdir att ribute. This will help eliminate confusion
about what is considered in the NHDPlus surface wat er network and what is not. In Figure 5-6,
the dark blue lines are those NHDFlowlines with kno wn flow direction and, consequently, are included in the “plus” part of NHDPlus. The broad cyan lines are NHDFlowlines with unknown flow direction and, consequently, are not part of t he “plus” part of NHDPlus.NHD Plus User Guide September 1, 2010 48 Figure 5-6 – NHD Flowlines With Known and Unknown F low Direction



### Main flowline network vs. isolated networks

The majority of the NHDPlus surface water network f eatures drain to the Atlantic Ocean, Pacific Ocean, Gulf of Mexico, or to one of the Great Lakes . These features are the ones that compose the NHDPlus “main” flowline network. In addition, NHDPlus contains many isolated networks throughout the U.S. An isolated network is one tha t appears to terminate into the ground. Many isolated networks truly flow into the ground. Thes e are often called “non-contributing” networks and, while they can occur in any part of the countr y, they are found primarily found in hydrologic region 16 (The Great Basin) and in south western parts of hydrologic region 17 (Pacific Northwest). Some isolated networks are ma pping errors. These networks should be connected to the main NHDPlus network. Figure 5-7 and Figure 5-8 illustrate isolated networks that are non-contributing and mapping errors, respe ctively.NHD Plus User Guide September 1, 2010 49 Figure 5-7 – Non-contributing Isolated Networks Teal lines are the terminal segments of these isola ted networks. The straight lines are USGS 7.5-minut e quadrangle boundaries.NHD Plus User Guide September 1, 2010 50 Figure 5-8 – Map error The cross-hairs are the edges of USGS quad maps.

Isolated networks may be located in any NHDPlus dra inage area. To find isolated networks, join
the NHDflowlineVAA attribute table to the NHDFlowli ne feature class using the Comid field in
each. Then select all flowlines with NHDFlowlineVA A.Terminalfl = 1. The flowlines selected
will be those with known flow direction that are co nsidered by NHDPlus as terminal flowlines.



### Disagreements Between the Flow Accumulation Grid an d NHD
isolated Networks

Figure 5-9 illustrates an uncommon problem between the NHDPlus network and the flow accumulation grid (FAC). The dark blue lines are t he drainage channels defined by the FAC. The green lines are the mapped streams in the NHDFl owline feature class. According to the NHD, the teal stream is a terminal flowline of an i solated network. In the NHDFlowline feature class, all the features in this isolated network ar e flowing toward the teal stream. As shown by the solid blue area of the FAC, this terminal netwo rk should have flowed east toward the main
network. The flowlines to the southwest of the sol id blue area are pointed in the correct direction, while the flowlines north of the solid b lue area are pointing north and should be pointed south. A flowline is missing from the NHDP lus near the center of the solid blue area. It should carry flow from the isolated network and con nect to the network to the east, approximately following the path taken by the FAC. There is a dam in the area of the missingNHD Plus User Guide September 1, 2010 51 flowline. Because the flowline was missing, the bur ning process did not cut through the dam. The filling process filled up the flat area (reserv oir) upstream of the dam, resulting in the pattern
of flow accumulation that does not follow the flowl ines closely, however it did eventually overtop the dam and flow down the stream channel. T he smoothed elevations in the FlowAttributesFlow table may also exhibit some stra nge behavior in areas where the NHD network disagrees with the FAC.

Figure 5-9 – Uncommon problem between the NHDPlus n etwork and the flow accumulation grid



### Why does the Identify tool in ArcMap say that the v alue and ComID from the cat grid are different from the Grid_code and ComID shown in the Catchment shapefile?

In ArcMap version 9.1, the identify tool appears to round grid cell values to six significant digits. Thus a grid cell having the value “2769195” through “2769204” all are reported as having the value “2769200”, and the other grid attributes show n are those that go with cell value “2769200”. If you select the record in the cat grid attribute table, the grid cells having that v alue are highlighted on the map. You can use this method to verify that the grid values are in agreement with the Grid_code values in the Catchment shapefile. This issue has been reported
to ESRI and has been fixed in ARCGIS version 9.2..NHD Plus User Guide September 1, 2010 52 

### Why would drainage areas computed using upstream na vigation differ from the cumulative drainage areas pre-compu ted in the
NHDPlus?

The cumulative drainage areas pre-computed in NHDPl us are calculated using a downstream routing technique and at divergences all the draina ge area is routed down the major path (see section “How to Use NHDPlus” – “NHDPlus and Diverge nces”). Infrequently, NHDPlus chose the wrong path as the major path and, of those case s, sometimes that path does not return to the main network. In these cases, drainage areas compu ted using navigation results will be correct and will be greater than the pre-computed NHDPlus d rainage which are incorrect. This same
issue affects all NHDPlus cumulative attributes suc h as the NLCD land categories, temperature, and precipitation.



### What is the situation with names assigned to artifi cial paths inside lake/pond waterbodies?

When NHD was originally designed, flowlines inside waterbodies were combined into a single reach, often referred to as a branched reach. When a name was assigned to the reach inside the waterbody, all of the flowlines in the branched rea ch received that name, thus creating a branched name. When NHD was converted to ArcGIS ge odatabase in January 2004, a critical review of the NHD data model was performed and the decision was made to deconstruct the branched reaches into individual reaches; one for e ach flowline. This affected many thousands
of reaches across the U.S. Due to resource constra ints, the branched names were not corrected at that time. Therefore, as a named river path is fol lowed, the named path will be linear outside of lake/pond features and branched when it passes thro ugh lake/pond features.



### Difficulties using NHDPlus Attribute .dbfs in Micro soft Access

When trying to open an NHDPlus attribute table in n ative dBase format (dbf), you may get the following error message:

“The Microsoft Jet database engine could not find t he object ‘NHDtable_name.dbf’. Make sure the object exists and that you name its name and pa th name correctly.”

This error message occurs after trying to open the NHDPlus dbf file directly from the O pen dialogue box from the MS Access F ile pull-down menu. The reason for this error is t hat Microsoft Access 2003 and earlier versions only rec ognizes dbfs with the DOS 8.3 naming convention, consisting of base file names of 8 char acters or less followed by the .dbf file type extension. Many NHDPlus dbf files have much longer names and will not import into Access. To correct the problem, first copy the file and ren ame it to a name consisting of 8 characters or less, such as “myfile.dbf”. It is recommended tha t you do not rename the original copy of theNHD Plus User Guide September 1, 2010 53 file because tools built for NHDPlus may need the f ile and will look for the file using its original
file name.



### Using the NHDPlus Value Added Attributes (VAAs) for Non-Navigation
Tasks

The attributes in the NHDFlowLineVAA table provide several powerful capabilities for users.
Several of the exercises/tutorials available for th e NHDPlus show the power of the VAAs for navigation. The VAAs also provide many other non-na vigation capabilities that can be of great value to users of NHDPlus. The objective of this se ction is to demonstrate several of these nonnavigation capabilities and, in the process, these examples can help users better understand and use the VAA data content. All of the figures in the following examples use Hydrologic Region 2,
the Mid-Atlantic.

Details on the computation of the VAAs can be found in Appendix A, Step 2: Computation of Value Added Attributes (VAA)

.

Example 1: ThinnerCod

The ThinnerCod is an ordinal value that helps to di splay various network densities. The ThinnerCod values range from 0 to 6. ThinnerCod=0 w ill show all non-networked flowlines, including coastlines and isolated flowlines not inc luded in the network. Figure 5-10 shows an example of ThinnerCod=0 highlighted in red.NHD Plus User Guide September 1, 2010 54 Figure 5-10 – ThinnerCod = 0

A ThinnerCod value of 1 contains the major rivers a t a density appropriate for map displays at a small scale. The ThinnerCod = 1 is also very import ant because it has been built as the backbone
network that connects all of the HUC-8’s. This can be very useful when you want to extract a complete hydrologically networked set of HUC-8’s as a subset of NHDPlus. Figure 5-11 shows ThinnerCod = 1 highlighted in red.NHD Plus User Guide September 1, 2010 55 Figure 5-11 – ThinnerCod = 1NHD Plus User Guide September 1, 2010 56 Increasing ThinnerCod values provide increasing den sity of the stream network. For instance, Figure 5-12 shows ThinnerCod values of 1 and 2. Not e the increased density in relation to the previous figure. Selection of ThinnerCod values fro m 1 to 6 will incrementally increase the density until the entire set of networked flowlines is displayed.

Figure 5-12 – ThinnerCod = 1 or 2NHD Plus User Guide September 1, 2010 57 Example 2: Using LevelPathi to Build Your Own “Thin nerCod” based on Stream Length

The mainstem of each stream is assigned a unique id entifier named “LevelPathi”. LevelPathi is equal to the Hydroseq value of the most downstream flowline on that river. This feature of LevelPathi can be used in conjunction with the flow line Lengthkm attribute found in the NHDFlowline table. The following SQL statement will build a table of the total lengths for each mainstem of every networked stream/river:

SELECT NHDFlowlineVAA.LEVELPATHI, Sum(NHDFlowline!L ENGTHKM) AS strmlength INTO strmleng
FROM NHDFlowline INNER JOIN NHDFlowlineVAA ON Flowl ine.COMID =
NHDFlowlineVAA.COMID GROUP BY NHDFlowlineVAA.LEVELP ATHI HAVING (((NHDFlowlineVAA.LEVELPATHI)>0));

The above SQL statement creates a variable named st rmlength as the sum of the lengths of all flowlines by LevelPathi and puts the LevelPathi and strmlength variables into a table named “strmleng”. Figure 5-13 highlights all of the mai nstem rivers greater than or equal to 100 Km in length. Note that any length threshold criteria can be used as desired.

Figure 5-13 – Streams Greater than or Equal to 100 Kilometers in LengthNHD Plus User Guide September 1, 2010 58 Example 3: Selecting an Individual River or an Indi vidual Terminal River Basin

One use of the LevelPathi was shown in Example 2. That example used the information that every stream has Levelpathi as a unique identifier. In the following examples, uses for
LevelPathi and TerminalPa are shown. We will use t he Potomac River as our example river. The most downstream Hydroseq for the Potomac River is 9 169600001. This means that by the VAA conventions, the LevelPathi for the Potomac mainste m is assigned this value and every networked flowline in the Potomac Basin is assigned this value as the TerminalPA. Figure 5-14 shows the selection of LevelPathi = 9169600001, whi ch selects the mainstem of the Potomac River. Figure 5-15 shows the selection of Terminal PA = 9169600001 which selects the entire
Potomac River Basin.

Figure 5-14 – The Mainstem of the Potomac River By Selecting LevelPathi=9169600001NHD Plus User Guide September 1, 2010 59 Figure 5-15 – The Potomac River Basin By Selecting TerminalPA = 9169600001NHD Plus User Guide September 1, 2010 60 Example 4: Profile Plots

Plots of data along a river where the x-axis is the river mile (or river kilometer) are widely used for showing data and modeling results. The VAAs con tain the basic information to readily develop profile plots. Previous VAA examples showed how the LevelPathi can be used to identify every flowline on a river. Another VAA, Pa thLength, is the length from the bottom of
the flowline to the end of the network. For instanc e, every flowline in the Missouri Drainage Basin has a PathLength value that tells how far awa y it is from the mouth of the Mississippi
River.

The basic profile plotting procedure is to select t he LevelPathi for the river of interest and assign your data values to the comids of the flowlines of interest. Include the PathLength in the dataset, and then plot your data using PathLength as the x-a xis and your data as the y-axis. Of course, multiple types of data can be shown on one graph, s uch as data versus model results.

This example uses the mainstem of the Potomac River PathLength variable as the x-axis and the
“MinElevSmo” elevation values from the FlowLineAttr ibutesFlow table for the y-axis values. This particular plot is interesting because the ele vation change near PathLength 180 is dramatic; this is where the Potomac River changes from free-f lowing to estuarine. This plot goes from downstream to upstream but it is a simple matter to reverse the x-axis as desired.

Elevation Along the Potomac River Mainstem

0 100 200 300 400 500 600 700 800

0 100 200 300 400 500 600 700

Pathlength (kilometers)Flowline Elevation (meters)

Figure 5-16 – Profile Plot of the Mainstem Potomac River “MinElevSmo” valuesNHD Plus User Guide September 1, 2010 61 Example 5: Stream Order

The NHDPlus stream order is based on the Strahler M ethod. Stream order is a classic method for ranking streams according to size. Stream orde r computed for the NHDPlus flowlines is
distributed as an NHDPlus data extension in a table called SOSC.dbf

Mapping or classifying flowlines on the basis of st ream order can help to rank flowlines by size within the network, selecting out streams of only c ertain orders, or aggregating data by stream order. A recent example of this last capability, cl assifying data by stream order can be found in (Alexander, et. al., 2007)

Some examples of mapping by stream order are shown below. Figure 5-17 shows different
colors for each stream order for an area in the Mid atlantic Region. This figure illustrates how stream order helps rank streams by relative size. Figure 5-18 shows the same area but with
stream order 1 removed. This is one method to “thin ” the network based on hydrologic criteria. There are other excellent ways to rank or thin the NHDPlus network, especially by using mean annual flow as a criteria; this method is not illus trated here.

Figure 5-17 – Stream Orders in a Part of the Midatl antic RegionNHD Plus User Guide September 1, 2010 62 Figure 5-18 – Stream Orders in a Part of the Midatl antic Region with Order 1 Streams RemovedNHD Plus User Guide September 1, 2010 63 Example 6: Stream Level

The use of Stream Levels, the variable StreamLeve, is often misunderstood or misused by users of the VAAs. Quite often, users think of StreamLeve “as the opposite of Stream Order”. But, this is not true in any sense. Streamleve = 1 will apply to the Mississippi mainstem but also to every small or large stream that terminates on the coastline. Therefore, StreamLeve has nothing
to do with stream size.

The only valid use of StreamLeve for users is to id entify, at a particular junction, what is the mainstem and what is the tributary stream. The lowe r valued stream levels are the mainstem and the higher valued stream levels are the tributaries . Figure 5-19 illustrates this. The flowlines are labeled with the StreamLeve values. The flowlines g oing in the north-south direction are Level 2, and the flowline coming in from the West is Level 3 . Therefore, the North-South flowlines are the mainstem and the flowline coming in from the We st is the tributary.

There are additional aids in the VAAs, for instance using PathLength to determine flow direction. If you want to know which way the North- South stream is flowing, check the Pathlength values of both of these flowlines. In th is case, the Pathlength of the South flowline is 400.1 and the Pathlength of the northern flowline i s 397.5. Therefore, the northern flowline is closer to the terminus, so the flow is going from s outh to north. The Hydroseq can also be used to determine the flow direction at this junction. T he smaller Hydroseq value will be the
downstream flowline.

Figure 5-19 – StreamLevel Values at a flowline Junc tionNHD Plus User Guide September 1, 2010 64 

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### Appendix A – Process Descriptions for NHDPlus

NHDPlus is built in a series of seven steps:

Step 1: Improvements to NHD 1:100K Linear Network a nd Feature Attributes Step 2: Computation of Value Added Attributes Step 3: Development of Catchments, Max/Min Elevatio ns, Flow Accumulation, and Flow
Direction Grids Step 4: Development of International Catchment Area s Step 5: Development of Catchment Attributes Step 6: Development of Flow Volume and Velocity Est imates Step 7: Elevation Post-Processing, Smoothing, and S lope Calculations

Following these seven steps, the data is packaged a nd QA/QC’ed.

Each of these steps is described in detail in the f ollowing sections.NHD Plus User Guide September 1, 2010 66 

### THIS PAGE LEFT BLANK INTENTIONALLYNHD Plus User Guide September 1, 2010 67 

### Step 1: Improvements to NHD 4 

### 1:100K Linear Network and Feature
Attributes

During 2004 and 2005, the 1:100,000-scale NHD surfa ce water network was QA/QC’ed and corrected where necessary. An automatic process was used to identify:

 Reaches with invalid combination of flowline featur e types

 Reachable flowline feature that is not part of a li near Reach

 Circular Reaches

 Flowlines that touch internally or more than once

 Non-linear Reaches

 Non-linear flowlines

 Coordinate order that is not consistent with flow o r opposing flow, according to flow table

 Flowlines that flow and do not touch, according to flow table

 Flow table entries that are never navigated

 Non-Terminal flowline with no outgoing flow table e ntry

 Invalid flow table entries

 Non-Start flowline with no incoming flow table entr y

 Potential loop in flow table

 Possible outflowing inter-cu connection that needs to be validated

 Possible incoming inter-cu connection that needs to be validated

 Duplicate flow table entry

 Incomplete divergence in flow table

QA/QC and editing was performed at the Subbasin lev el. Both the flowline geometry and the
flow table were edited to correct the network. When a Subbasin was error free, it was navigated from its pour point(s) to find any flowlines that w ere not connected to the main network. These isolated flowlines were connected where it was poss ible to determine the proper connection points without reference to external information su ch as digital raster graphics (DRG) or digitalortho-photo quads (DOQ) 5

.

When all Subbasins in a Hydrologic Region were deem ed error free, the Subbasins were appended into a regional dataset, and a regional na vigation was performed. Large sets of isolated flowlines were examined with DRGs as a backdrop. If the proper connection points could be determined, the connections were made in both the N HD geometry and the flow table. It was not possible to connect all isolated flowlines.

In addition to the network connectivity QA/QC, the flowline feature names were also QA/QC’ed. Data from the Geographic Names Informati on System (GNIS) were compared to the
names on the NHD Flowlines. Name corrections and ad ditions were made as needed.

Because much of the NHD editing was performed witho ut external reference material, it is possible that flow direction may not be 100 percent correct; however, during Step 7, the NHD network drainage areas are compared to National Wat er Information System (NWIS) flow gage drainage areas as a final QA/QC procedure.NHD Plus User Guide September 1, 2010 68 

### Step 2: Computation of Value Added Attributes (VAA)

The VAAs are designed to enhance NHD for display, n avigation, and analysis. The attributes are computed from existing NHD content rather than additional data collection. The attributes require a fully completed stream network from headw aters to mouth and are computed for large sections of the NHD as follows: Region 01; Region 0 2; Region 03 (except HUC4 0318); Regions 05, 06, 07, 08, 10, 11, and HUC4 0318; Region 12; R egion 13; Regions 14 and 15; Region 16; Region 17; Region18; and Region 20.

The attributes require three full passes of the dat a. The first pass is from upstream to downstream, followed by a pass from downstream to u pstream, with a final pass from upstream
to downstream.

There are 20 attributes, as follows:

From Node (FromNode) A nationally unique ID for the “from” node (i.e. th e upstream endpoint) of the flowline. These IDs are independent of the node IDs in an .NAT tabl e in a coverage format workspace. The From Node and To Node numbers provide a node-to-node tra versal method used by some modeling
software.

To Node (ToNode) A nationally unique ID for the “to” node (i.e. the downstream endpoint) of the flowline. These IDs are independent of node IDs in an .NAT table in a coverage format workspace. The From Node and To Node numbers provide a node-to-node tra versal method used by some modeling
software.

Hydrologic Sequence Number (Hydroseq) A nationally unique sequence number assigned to eac h flowline; this number places the flowlines into hydrologic sequence. If flowlines a re processed by hydrologic sequence number in descending order (i.e., starting at headwaters a nd proceeding downstream to the network termini), when a flowline is processed, all the flo wlines upstream have already been processed. If flowlines are processed by hydrologic sequence n umber in ascending order (i.e., starting at the network termini and proceeding upstream to the head waters), when any flowline is processed, all the downstream flowlines have been processed. Hydr ologic Sequence Number supports several new navigation methods including traversal through SQL queries.

For a given flowline, it is true that all flowlines upstream have higher hydrologic sequence numbers and all flowlines downstream have lower hyd rologic sequence numbers. However, for a given flowline, it is not true that all flowlines with higher hydrologic sequence numbers are upstream nor are all flowlines with lower hydrologi c sequence numbers downstream.

Start Flag (StartFlag) Indicates which flowlines are headwaters (i.e., net work starts) according to the flowline flow table. This flag is set to “1” if the flowline is a headwater flowline; otherwise, it is set to “0.”NHD Plus User Guide September 1, 2010 69 Terminal Flag (TerminalFl)
Indicates which flowlines are network ends accordin g to the flowline flow table. This flag is set to “1"” if the flowline is a terminal flowline; oth erwise, it is set to “0.” A terminal flowline stops at an ocean, the Great Lakes, Canada, Mexico, or it flows into the ground.

Terminal Path Identifier (TerminalPa) A unique identifier for all the flowlines which flo w to the same network terminus based on the flowline flow table. The Hydrologic Sequence Numbe r of the terminal flowline is used as the
Terminal Path Identifier. Terminal Path Identifier can be thought of as a “drainage area ID”. See also “Terminal Flag”.

Level Path Identifier (LevelPathI) A unique identifier for a stream which is assigned to all the flowlines from the stream’s mouth to the stream’s headwater. The Hydrologic Sequence Nu mber of the flowline at the mouth of the
stream is used for the value of the Level Path Iden tifier. This attribute is based on the flowline
flow table and stream level. Level Path Identifier can be thought of as a “stream ID” and can be used to support applications such as river mile ind exes.

Arbolate Sum (Arbolatsu) Based on the flowline flow table, this is the sum o f the lengths of all flowlines above the downstream end of a flowline. This field is empty i n the distributed NHDPlus data.

Path Length (PathLength)
Distance from a flowline’s downstream end to its te rminal flowline’s downstream end according to the flowline flow table. Path length is in kilometers.

Thinner Code (ThinnerCod) An ordinal value designed to allow selection of pro gressively denser networks. The least dense network is obtained by selecting thinner values of 1 which is the network of major streams and rivers that connect the 8-digit Hydrologic Units (i .e. subbasins or cataloging units). Each
successive value of Thinner adds more streams to th e network. At any Thinner level, the network is fully connected. Thinner values are des igned to be used for improving the performance of Web pages and other applications whe re the user zooms back and forth between large areas and small areas. There is no hydrologic basis, other than connectivity, for the thinner values and they should NOT be used for stream class ification or analysis.

Divergence Flag (Divergence) If the flow splits into more than one branch (i.e. a flow divergence), and the flowline is one branch of the flow split, the divergence flag is se t as follows: “1” if the flowline is the main
branch and “2” if the flowline is not a main branch . If the flowline is not a branch of a flow split, the divergence flag = 0.

Stream Level (StreamLeve) Stream levels are assigned beginning at network ter mini and working upstream. If the network terminus is at the ocean, the stream level begins w ith 1. If the network terminus is at the Great Lakes, the stream level begins with 2. If the netwo rk ends at the Canadian or Mexican border, theNHD Plus User Guide September 1, 2010 70 stream level begins with 3. If the network terminus flows into the ground, the stream level begins with 4. The stream level value is continued upstrea m, following the main path, until the headwater of the mainpath is reached. Tributaries t o the mainpath are assigned the next integral stream level beginning at their mouth and continuin g to their headwaters. This process is repeated for each additional level of tributaries u ntil all streams have been assigned a stream level. For example, the Mississippi River is level 1; tributaries to the Mississippi, like the Ohio River, are level 2; and tributaries to the Ohio Riv er, like the Tennessee River, are level 3. Stream level supports upstream mainstream navigation and i s NOT intended to be used for stream classification or analysis.

Downstream Level (DNLevel)
Downstream flowline level is the stream level of th e downstream mainstem flowline. This attribute supports downstream mainstem navigation.

Stream Order (StreamOrde) This attribute has been replaced by the Stream Orde r/Stream Calculator data distributed as an
NHDPlus data extension. The table is called SOSC.d bf. The field
NHDFlowlineVAA.StreamOrde has been set to 0 to enco urage the use of the better values found
in SOSC.dbf.

Upstream Level Path Identifier (UpLevelPat) This is the level path identifier of the mainstem f lowline which is immediately upstream. This attribute supports navigation traversals through SQ L queries.

Upstream Hydrologic Sequence Number (UpHydroSeq) Hydrologic Sequence Number of the immediately upstr eam mainstem flowline. This attribute supports navigation traversals through SQL queries.

Upstream Minimum Hydrologic Sequence Number (UpMinH ydro) Minimum Hydrologic Sequence Number of all immediate ly upstream flowlines. This attribute supports navigation traversals through SQL queries.

Downstream Level Path ID (DnLevelPat) This is the level path identifier of the downstream mainpath flowline. This attribute supports navigation traversals through SQL queries.

Downstream Flowline Count (DnDrainCou) This is the number of flowlines immediately downstr eam. This attribute supports navigation traversals through SQL queries.NHD Plus User Guide September 1, 2010 71 Downstream Minor Hydrologic Sequence Number (DNMinH ydro) At a divergence, the Hydrologic Sequence Number of the immediately downstream minor path flowline. If multiple minor paths exist, this is th e maximum hydrologic sequence number of all minor paths.NHD Plus User Guide September 1, 2010 72 

### Step 3: Development of Catchments, Max/Min Raw Elev ations, Flow

Accumulation and Flow Direction Grids, Max/Min Raw Stream Channel Elevations, and Headwater Node Areas

Catchments

Catchments were generated for each NHD Flowline tha t appears in the NHDFlow table. To create catchments, the flowlines were rasterized to serve as the source grid for the ARC/INFO GRID module command, WATERSHED (ESRI, 1999). Catchm ents were developed upslope from these stream-network grid cells. A flow direct ion grid derived from a Digital Elevation Model (DEM) was also required as input for the WATE RSHED command. The flow direction grid was produced using a hydrologically-conditione d DEM called HydroDEM. The source
DEM was obtained from the 30-meter resolution NED a nd was projected into the national Albers projection for each production unit. These projecte d DEMs (elevation grids), are provided in the NHDPlus product suite.

Modifications were applied to the DEMs to produce t he HydroDEM. These modifications were considered necessary because often the drainage pat h (flowpath) defined by the NED surface does not exactly match the 1:100,000-scale NHD. In many cases, the NHD streams and NEDderived streams are parallel or offset from each ot her. Figure A-1a illustrates a common example of the differences in the horizontal positions of N HD streams and NED-derived streams. If this offset distance is greater than one grid-cell width , then some cells may not be identified as being upslope from the NHD stream segment and therefore w ould be erroneously excluded from the delineated catchment (Figure A-1b).

To mitigate this mismatch of stream locations, the NHD vector drainage was integrated into the raster NED data, often referred to as “stream burni ng” (Saunders, 2000). This process uses computer algorithms in an Arc Macro Language (AML) program called AGREE, developed by Hellweger (1997). Figure A-1c illustrates how the A GREE program corrects for DEM flow path displacement errors when delineating catchments.NHD Plus User Guide September 1, 2010 73 Figure A-1 – (a) Differences in drainage between th e NHD and flow paths of a NED-derived stream, (b) Resultant NHD catchment delineations using unmodifi ed NED DEM data, and (c) Resultant NHD catchment delineations using AGREE-modified NED data.

AGREE “burns” a “canyon” into the NED-based DEM by subtracting a specified vertical
distance from the elevation beneath the NHD vector streamlines. The vertical exaggeration of the canyon is controlled by specifying a “Sharp Drop/Ra ise Distance.” For the HydroDEM, a negative “sharp” drop distance (10,000 meters) was applied to retain the new NHD stream flow path after subsequent depression filling processes needed for catchment delineation.

AGREE also “smoothes” the elevation adjacent to NHD stream cell locations in the DEM within a buffer distance specified by the AGREE program us er. Typically, the buffer distance is related to a common horizontal displacement error between N HD and NED-derived streams; this error is seldom exceeded. For HydroDEM production, the buffe r distance was set to 160 meters on each side of the NHD streamline. The smoothing process c hanges the DEM grid-cell elevations within the buffer area to create a downward sloping gradie nt towards the modeled canyon beneath the NHD streams. The steepness of the slope within the buffer is controlled by the AGREE “Smooth Drop/Raise Distance” option. For the HydroDEM, a sm ooth drop distance of 500 meters was specified, with acceptable results. Figure A-2 illu strates how AGREE changes the original DEM using all the specified parameters of AGREE.NHD Plus User Guide September 1, 2010 74 Figure A-2 – Schematic of AGREE process

The use of AGREE’s 160 meter smooth/drop buffer dis tance of the NHD streams may cause potential problems at headwater flowlines because t hey begin at or near drainage divides in the
DEM. The 160 meter buffer distance at these headwat er streams may extend across the DEM drainage divides and into the adjacent basin area, thereby including areas outside the true
catchment area.

To minimize the problem of extending headwater stre ams into adjacent watersheds, an AML program was created that selects only headwater str eams from the NHD stream network and
trims the NHD headwater vector streamlines back 90 meters from the starting point. The streamtrimming program is fully automated and is run prio r to the AGREE program.

The hydrologic-conditioning of the NED data also in cludes a process developed for the New England SPARROW water-quality models (Moore et al., 2004). This “New England Method” uses AGREE for drainage enforcement and establishes Watershed divides, if available, from the Watershed Boundary Dataset (WBD). The process of co nditioning DEM data to WBD Watersheds is called “walling” and uses an AML, wri tten by Moore and Johnston (2004), to vertically exaggerate DEM elevations corresponding to the location of WBD ridgelines. The vertical distance used to exaggerate the cells is a specified constant added to the elevation grid
cells above the WBD. Breaks in the walls were creat ed at locations where the stream network crosses the WBD to insure proper passage of water f rom one WBD SubWatershed unit to another. A graphic 3-D representation of a hydrolog ically-conditioned DEM with WBD walling and the NHD “burning” is shown in Figure A-3.NHD Plus User Guide September 1, 2010 75 Watershed Boundary Dataset (WBD) added to DEM surface in "walling" technique

NHD stream cells
subtracted from DEM surface in "burning" technique

Figure A-3 – 3-D Perspective view of modified DEM w ith walling of existing Watershed boundaries and burning of NHD streams

The New England Method for producing catchments was compared with other methods in an EPA-sponsored, NHD-NED Synchronization pilot study in cooperation with the USGS. The results of the study showed that the New England Me thod provides the most accurate catchment delineations, using nationally available data and a utomated methods.

The New England Method was further refined by the U SGS (Rea and Skinner, written commun., 2004) by imposing a “bathymetric gradient” within t wo-dimensional NHDWaterbody features (i.e., stream/river, lake/pond). The bathymetric g radient ensures that the catchments generated for artificial path flowlines within waterbodies ar e based on a gradient directed towards the artificial path flowlines. The gradient is applied prior to running AGREE. The bathymetric gradient process applies GRID algorithms, similar t o those used in AGREE, to create a broad sloping gradient with shorelines upslope from the a rtificial path flowline(s).

The last step in the HydroDEM creates a “filled” DE M surface grid with the ARC/INFO GRID command “FILL,” which removes depressions. This ins ures all elevation cells in the basin have a defined drainage direction. The required flow direc tion grid for the WATERSHED command is produced from the filled HydroDEM.NHD Plus User Guide September 1, 2010 76 Due to the resolution of the NED data, catchments w ere not generated for many flowlines approximately 42 meters or less in length (42 meter s is the diagonal distance across a 30 meter grid cell).

In rare circumstances, flowlines longer than 42 met ers may not have been assigned a catchment, depending on the spatial configuration of a flowlin e with other flowlines. In these cases, flowlines may run parallel with each other within a given cell and may continue for several cells over the entire length of one of the flowlines. In other rare cases, the trimming back of the NHD headwater stream segments did not allow for enough of the remaining flowline to be assigned a catchment and cells were assigned instead to a neig hboring flowline. Trimback errors are especially rare since the trim streams program is d esigned to limit these types of errors. For Region 17, with approximately 237,000 flowlines gre ater than 42 meters, three catchments were not generated as a result of spatially configured f lowlines running parallel to each other, and two catchments were not generated as a result of the tr imming back of headwater flowlines.

The catchments data are available in the native GRI D format and as vector polygon feature-class shapefiles. It is important to note that catchment features in a shapefile may be composed of one or more vector polygon features. Multiple polygon f eatures occur as a result of the source 30 meter grid-cell resolution and the grid-to-vector c onversion process. In these situations, one or more cells with directional flow traveling diagonal ly into an adjacent cell along a catchment boundary may create a separate polygon in the vecto r data model when these data are converted from a GRID (see Figure A-4).NHD Plus User Guide September 1, 2010 77 Figure A-4 – Illustration showing multiple polygon features defining a catchment area for a flowline

In creating the catchment shapefiles, the catchment data in GRID format were converted to a vector polygon ARC/INFO Coverage. This polygon cove rage of catchments was then combined using the ARC/INFO Regions data model to ensure mul tiple polygons with the same ComID were represented as one multi-part area feature. Th e Region feature class coverage was then used to create the catchment shapefile, which retains th e combined polygon features as one multi-part polygon. If the catchments are converted from shape file to Coverage format, it will be necessary to rebuild the Region class features to ensure that each catchment feature, for each ComID, is represented by a single unique area feature.



### Flow Direction and Accumulation GRIDs

The flow direction grid was created by using the ES RI ARC/INFO command FLOWDIRECTION. A flow direction grid contains integ ers representing the direction from each cell to its steepest downslope neighbor. The elevat ion grid used to compute flow direction was
created with the “FILL” command.

The flow accumulation grid was created by using the ESRI ARC/INFO command FLOWACCUMULATION. A flow accumulation grid contains integer values equal to the number of cells uphill from that cell. The flow dir ection grid was used as input in
FLOWACCUMULATION to determine the total number of c ells that flow into each downslope
cell.NHD Plus User Guide September 1, 2010 78 In this process, flow is ultimately directed to “si nks” created by placing “no-data cells” at certain locations, such as at the outlet to each processing unit. For Hydrologic Regions that drain directly into the ocean, the entire coastline is us ed as a sink. For networks that drain to Canada or Mexico, a sink is also applied at the network’s ter minus along the international border. Isolated drainage networks are generally eliminated by “fill ing” to create flow direction and accumulation grids. Sinks are only placed at the network ends of isolated networks if they are within 8-digit Hydrologic Units identified as closed basins (with the exception of Hydrologic Region 16, where sinks were applied region-wide).

A procedure for placing sinks in closed basins was applied to avoid: 1) placement in high precipitation areas (mean annual precipitation grea ter than 14 inches), 2) 1:100,000-scale map boundaries where network end may be erroneous due t o source quadrangle mapping inconsistencies, and 3) placement closer than 90 me ters to the main network to which the
isolated network either should be connected or ther e is apt to be highly permeable alluvium.

Figure A-5 shows how isolated networks are treated. Catchments are delineated for the isolated networks in the same manner as any other NHD Flowli ne. The flowlines are rasterized and converted to “NODATA” or “sinks” in the elevation g rid. The FILL process, followed by FLOWDIRECTION, directs flow to these NODATA cells. The rasterized NHD Flowlines are
then used as the seeds for catchment delineation us ing the WATERSHED function. This process produces the best representation of catchments for isolated networks.

The DEM is then processed with FILL a second time - this time to produce the final flowdirection grid. In this process, sinks are esta blished only at the outlet of the processing unit, unless there are Subbasins (8-digit HUCs) identifie d as closed basins. Therefore, the isolated networks fill up and spill over, producing a draina ge network similar to that which would be produced using a DEM without the stream burning pro cess.

The reason for generally letting the accumulated ce ll counts continue beyond the isolated networks is identification of likely flowlines to w hich the isolated network should (perhaps) be connected. Using this information, a user could cho ose to fix isolated networks that should be
connected to the main network or determine watershe ds that include the isolated network areas.

In Figure A-5, the synthetic hydrography (i.e., the hydrography derived from the DEM FLOWACCUMULATION function) is plotted in red. The N HD Flowlines are plotted in blue over the red. In most cases, the agreement is very good and the blue closely overplots the red. However, the isolated network in the top center of the figure shows a side effect of the AGREE process. For this isolated network, the synthetic s treams are horizontally displaced with the NHD
and even cross over from one stream to another. Sin ce the trench caused by the isolated network had to be filled back up until it could spill over, the surface became flat within the AGREE buffer area. There seems to have been very little o verall slope in this area; therefore, nearly the
entire isolated network was affected. Within the fl at area, the flow direction and flow
accumulation are not well controlled and suffer— to a limited extent— from the same problems occurring in DEMs processed without the benefit of stream burning. These effects are not common or widespread but may occur wherever there a re isolated networks.NHD Plus User Guide September 1, 2010 79 Figure A-5 – Illustration comparing the horizontal displacement between the NHD streams of an isolated network (in blue) to the synthetic streams of the “ filled” HydroDEM

The processes outlined above were applied to each “ production unit” (an area equal to or less than 73,000 square miles). The restricted size is d ue to GRID processing limits. The Production Unit map (Figure 4-1) shows the order of processing of the production units. Flow direction and flow accumulation grids are not merged for each Hyd rologic Region because the accumulation values only represent upstream cells within a defin ed area; they do not include cells in upstream production units. Catchment grids, on the other han d, are combined for final distribution. The catchment areas are not affected by excluding upstr eam accumulations from other processingNHD Plus User Guide September 1, 2010 80 units. The user should bear in mind, however, that the flow accumulation values (cell counts)
include cells accumulated from within a 10 kilomete r buffer.



### Assigning Minimum and Maximum Raw Stream Channel El evations (unsmoothed) to NHD Flowlines and Headwater Node Ar eas

NED was used as the source elevation dataset becaus e the elevations in the “burned” NHD stream paths in the HydroDEM are greatly modified. The maximum flowline elevation represents the elevation at the upstream end of the flowline and the minimum elevation is at the downstream end. Due to the varying degrees of accur acy of the NED, these “raw” elevations can produce negative slopes and, therefore, the elevati ons were later smoothed as described in Step

7.

A difficulty in estimating minimum and maximum stre am elevations is often caused by poor agreement between the location of the stream depict ed on the NHD and the flowpath generated by the elevation data alone. The locations of the N HD Flowlines typically are much more accurate. However, the minimum elevation value in t he entire catchment is generally a
reasonable estimate of the elevation of the stream as it leaves the catchment area. Using this value avoids the problems of horizontal displacemen t in the two stream networks (see NEDderived stream in red and NHD stream in blue in Fig ure A-6). The maximum elevation for each
flowline is the minimum elevation in the immediatel y upstream catchments (Figure A-6).

A different process was needed for the maximum elev ation on headwater flowlines. Temporary catchments were generated for the upstream node of each headwater flowline. The minimum elevation of that temporary headwater catchment is taken as the maximum elevation of the headwater flowline. Headwater node areas (in square kilometers) are included in the NHDPlus
HeadWaterNodeArea table.NHD Plus User Guide September 1, 2010 81 Figure A-6 – Assigning Minimum and Maximum Stream C hannel Elevations to NHD FlowlinesNHD Plus User Guide September 1, 2010 82 

### Step 4 - Development of International Catchment Are as

Hydrologic Regions with upstream areas in Canada or Mexico present special problems for computing streamflow or other hydrologic factors be cause many of the datasets are available only for the conterminous United States but not for Canada or Mexico.

Catchment areas for streams receiving flow from Can ada or Mexico were identified using generalized hydrography and watershed datasets. The extended catchment area for an NHD Flowline encompassed the entire non-U.S. upstream a rea for that stream; thus, these catchments are sometimes very large compared to most NHD Flowl ine catchments. Figure A-7 shows an example along the Canadian border. The light blue l ines show the generalized hydrography used for Canada, and darker blue lines show the NHD hydr ography. The boundary of the DEMderived catchment is shown in white, where the ligh t blue line enters the United States. This
catchment has been extended to include the entire a rea, shown in gold. Other catchments are shown in green. All catchments receiving major stre ams from Canada were expanded in a similar

manner.

Figure A-7 – Example of an international catchment on the Canadian borderNHD Plus User Guide September 1, 2010 83 Different datasets were used to develop the catchme nt areas for different regions. Details on the data sources used may be found in the Release Notes for each region having international
catchments.

Because of the previously described data issues, ca tchment areas and catchment characteristics, as well as cumulative drainage areas and characteri stics near an international border, are less accurate than in other areas. This inaccuracy is gr eatest in catchments touching the border. The greater the percentage of a catchment is in non-U.S . areas, the greater the data inaccuracy. The
effect diminishes farther downstream as the ratio o f non-U.S. to U.S. catchment areas becomes
smaller.NHD Plus User Guide September 1, 2010 84 

### Step 5: Development of Catchment Attributes

Catchment attributes were computed using ArcInfo Gr id, version 9.0 (ESRI, 2004). Mean Annual Precipitation and Mean Annual Temperature da ta for 1961-1990 were obtained from PRISM (Daly and Taylor, 1998). The PRISM datasets w ere projected from geographic coordinates to Albers Equal Area projection, and th e 2.5-arc-minute grid-cell resolution was transformed to approximately 4 km. The ArcInfo Grid function “ZONALSTATS” was used to compute the mean annual precipitation and temperatu re values for each catchment.

Catchments covering area in Canada or Mexico had mi ssing data in the mean annual temperature and mean annual precipitation grids underlying the non-U.S. portions of the catchments. The values computed for these catchments reflect the me an precipitation or temperature only for the catchment portion in the U.S.

The National Land Cover Dataset (NLCD 1992) data we re assembled for the Hydrologic Region being processed. The ArcInfo Grid function COMBINE was used to create a grid with a unique value for each unique combination of catchment numb er and NLCD category. The cell counts were converted to area percentage of each catchment . A custom program, named GROUP2ROW.AML, reformatted the resulting value attr ibute table to a table containing one record per catchment, with columns of area percenta ges for each NLCD category in that catchment. Empty columns were added for any NLCD ca tegories not present.

The NLCD grid was extended to cover the non-U.S. ar eas, specifying a code of 98 for Canada and 99 for Mexico. Subsequent processes operated on these catchments in the same way as with any other catchments. Attribute summaries for catch ments and flowlines may include percentages of catchment areas and cumulative drain age areas in Canada and Mexico. Total drainage areas are generated by combining the perce ntages of area in Canada or Mexico with the percentages of all NLCD categories.

The PCT_CN and PCT_MX fields in the CatchmentAttrib utesNLCD.dbf file and the Cumpct_CN and Cumpct_MX fields in the FlowlineAttri butesNLCD.dbf file contain the percentages of catchment and total drainage areas i n Canada and Mexico, respectively. Wherever these fields contain values other than zero, the us er needs to take into account the potential effects of the non-U.S. areas on the particular app lication.NHD Plus User Guide September 1, 2010 85 

### Step 6: Development of Flow Volume and Velocity Est imates

The Flow Volume and Velocity estimates are containe d in the FlowlineAttributes table, and
records are matched to the NHDPlusFlow table. The v alues in the FlowlineAttributes table are integrated from the NHDFlowlineVAA, the CatchmentAt tributes tables, and the flowline max/min elevations. The NHDFlowlineVAA provides the data needed to route and accumulate information from the CatchmentAttributes. The proce ss for calculating the smoothed elevation and slope values is described in Step 8.

The FlowlineAttributes table is cumulative for all flowlines and associated catchments upstream of, and including, any flowlines selected or of int erest. This accumulation is performed using the hydrologic routing information contained in the VAA s.

The cumulative drainage area at the downstream end of a flowline is calculated as:

CumArea = ∑ i = 1 to n (A i )

where n = number of catchments upstream of the flowline ( including that flowline), and

A i = the catchment area of the flowline.



### Divergences

A flow divergence occurs when two or more drains co nverge at a junction, and the flow downstream splits into two flowlines. One of the do wnstream flowlines is designated as the “major” flow path, and the other flowline is design ated as the “minor” flow path. At this stage of NHDPlus development, there is no information on the percentage of flow for either the major or minor paths. Therefore, all drainage areas, flows, etc. are routed and accumulated along the major flow path. The minor path is in effect treate d as a “start” flowline, with the cumulative values initialized to the values for that minor pat h flowline. Most flow divergences re-converge with the major flow path and, at the re-convergence point, the total cumulative values will represent the sums from the major path and the mino r path.



### Climate Data

Mean annual air temperature (MAT) and mean annual p recipitation (MAP) data are required for Vogel-based estimates of mean annual flow (Vogel et . al., 1999). These values are accumulated and averaged for each drain:NHD Plus User Guide September 1, 2010 86 AreaWtMAP – Area Weighted Precipitation at bottom o f flowline in mm

∑ i =1 to n (P i * A i )

= _______________________ ∑ i = 1 to n (A i )

where

P i and A i are the precipitation and catchment area for each flowline upstream.

AreaWtMAT – Area Weighted Temperature at bottom of flowline in degree C * 10

∑ i =1 to n (T i * A i )

= _______________________ ∑ i = 1 to n (A i )

where

T i and A i are the temperature and catchment area for each fl owline upstream.



### Land Cover Cumulative Characteristics

The cumulative NLCD % areas are computed by aggrega ting all of the individual NLCD catchment drainage areas:

CUMNLCD_xx = % of cumulative drainage area classifi ed as NLCD Class xx at the
bottom of the flowline:

∑ i =1 to n (NLCD_xx i * A i )

= _______________________ ∑ i = 1 to n (A i )

where NLCD_xx i = Percent of the catchment area in land cover clas s xx for flowline I

(for instance, “xx”= “11” is land cover class 11=Op en Water), and

A i = the catchment area for the i th flowline.



### Mean Annual Flow Estimates

Two methods are used for estimating mean annual flo w for each flowline. Values for the
attribute “MAFlowU” are based on the Unit Runoff Me thod (UROM), which was developed for
the National Water Pollution Control Assessment Mod el (NWPCAM) (Research Triangle Institute, 2001). Values in “MAFlowV” are based on methods from Vogel et al., 1999.

NHDPlus utilizes two flow estimation procedures, bo th developed by using the HydroClimatic Data Network (HCDN) of gages. These gages are usual ly not affected by human activities, such as major reservoirs, intakes, and irrigation withdr awals; thus, the mean annual flow estimates areNHD Plus User Guide September 1, 2010 87 most representative of “natural” flow conditions. T hese estimation methods used the HCDN gages because each method is developed for use at l arge scales; such as Hydrologic Regions. It was beyond the scope and capabilities of both metho ds to determine the human-induced effects at this scale. However, the QA comparisons of NHDPl us flow estimates are compared to the full set of gages to provide users with information on t he overall performance of each method.

UROM Flow Estimates

The UROM flow estimates use flowline catchment area estimates and unit runoff (cfs/km 2 ) data from associated watersheds (i.e., 8-digit Subbasin) . The USGS stream gages in HCDN were selected for developing the unit runoff values beca use those gages represent relatively natural hydrologic conditions and are not influenced by con trolled releases from reservoirs. Further, only gages with a drainage area smaller than the drainag e area of the 8-digit Subbasin where the gage is located were selected, so that the discharge dat a represents runoff for only that Subbasin. For instance, a gage on the Mississippi River at St. Lo uis is not representative of the unit runoff from that Subbasin. At the national level, a total of 1, 338 HCDN gages were identified for this dataset.

Based on the drainage area comparisons, the dataset of 1,338 HCDN gages and the dataset of associated stream discharge data for these gages we re used to derive mean annual and mean summer unit runoffs (ft 3 /sec/km 2 ) for each 8-digit Subbasin. The nearest HCDN gages were identified using a 200 mile maximum search radius f rom the centroid of an 8-digit Subbasin. In most Subbasins, five gages were selected, but some Subbasins had fewer than five within the 200
mile search radius. Mean annual and mean summer uni t runoffs for each 8-digit Subbasin were calculated using a weighted-average technique based on the square of the distance of the selected HCDN gages from the centroid of the Subbasin. The c omputations are defined as follows:

Q CU_MA = Σ (Q HCDN_MA

× 1/D 2 CU_HCDN )/ Σ (1/D 2 CU_HCDN )

where Q CU_MA = estimated mean annual unit discharge for t he 8-digit Subbasin of

interest, Q HCDN_MA

= mean annual unit discharge for the selected H CDN gage, and

D 2 CU_HCDN = square of distance from the selected HCDN gage to the centroid of the 8-digit Subbasin of interest.

IncrFlowU is the incremental flow at the bottom of the flowline, computed as:

IncrFlowU = A * CU_MA

where A = Drainage Area of the catchment (km 2 ), and CU_MA = Unit Runoff for the 8-digit Subbasin (cfs / km 2 ).

The UROM-based mean annual flow for each flowline ( MAFlowU) is computed as:

∑ i =1 to n (IncrFlowU i ).NHD Plus User Guide September 1, 2010 88 UROM “Tuning” Using Intermittent Flow Adjustment Fa ctors

The development of the UROM was done as part of the development of the National Water
Pollution Control Assessment Model (NWPCAM) (Research Triangle Institute, 2001). For Hydrologic Regions west of the Mississippi River, i nitial NWPCAM estimates of routed discharge generally were observed to be greater tha n the HCDN gage values. Consequently, for the Western Hydrologic Regions, a method was develo ped to better match (quasi-calibrate) discharge estimates to observed data. The discharge estimates were lowered by incorporating only a percentage of the stream segment-specific ru noff for intermittent segments. The method calculates discharge estimates assuming various con tributions (e.g., 100, 50, 25, 10, and 1 percent) of the unit runoff for intermittent flowli nes. This method proved successful in improving the match between gage flows and UROM-bas ed flow estimates. The method is employed in the Western Hydrologic Regions in NHDPl us. The best fit is selected based on graphical analyses that compare the UROM flow estim ates to the gaging data. As NHDPlus is produced, this tuning process will be completed in each Hydrologic Region west of the Mississippi (Regions 9, 10, 11, 12, 13, 14, 15, 16, 17, and 18). In Hydrologic Region 17, an intermittent unit runoff contribution of 10 percent appears to work best and is included as the
UROM estimates in the distribution datasets.

UROM Estimates in Areas with International Drainage

For a given flowline, the UROM bases each increment al flow contribution on unit runoff
estimates in the HUC8. There are no unit runoff est imates for areas outside of the U.S.; therefore, international drainage areas do not have equivalent values developed. A multi-step process is employed to compensate for the lack of equivalent v alues. To begin, the flow volume module is run, assuming a unit runoff in the international ar ea equals the unit runoff for the HUC8 where that drainage area comes into the U.S. Gages on flo wlines that enter the U.S. are then identified, and their mean annual flow is compared to the UROM mean annual flow estimate. Finally, unit runoff values are calculated, making the UROM flow equal the gage flow. In Hydrologic Region 17, where the Canadian contribution was relatively small (on the order of the area of one HUC8 or less), the default HUC8 unit runoff produced mea n annual flow estimates close to the observed gage flow. In the Columbia River, the unit runoff coefficient for the flowline receiving the Canadian drainage needed to be adjusted to matc h the gage flow. This adjustment re-sets the UROM flow estimate to be equal to the gage at the b order. The process is repeated at each international drainage location. The final run of t he flow estimation procedure uses these adjusted values for boundary flowlines.

“Vogel” Mean Annual Flow Estimates

The MAFlowV estimates are based on the work of Voge l et al., 1999. This method uses a log-log regression approach based on drainage area, precipi tation, and temperature data from HCDN gages. With this method, the mean annual flow for a drain is computed as:NHD Plus User Guide September 1, 2010 89 Convert AreaWtMAP from Deg. C * 10 to Deg. F * 10: V_AreaWtMAP = (AreaWtMAT / 10 * 9 / 5 + 32) * 10

MAFlowV = e a * CumAREA b * AreaWtMAP c * V_AreaWtMAT d * BCF * 35.31467

where a, b, c, and d are Hydrologic Region-specific regre ssion coefficients, and BCF = Bias Correction Factor by Hydrologic Region. 35.31467 = conversion factor from cubic meters per second to cubic feet per
second

The BCF is needed because the regression is log-log space. In addition, the Vogel flow estimates are valid only within the ranges of the original da ta used for computing the regressions. The regression coefficients, BCFs, and valid drainage a rea ranges for the Vogel Flow estimates are
shown in Table 1 6 . For cumulative drainage areas that fall outside o f these ranges, the Vogel flow and velocities are set to missing values in th e distributed data.

Hydrologic region a b c d BCF AreaMin (Sq. Km.) AreaMax

(Sq. Km.)

R01 -9.4301 1.01238 1.21308 -0.5118 1.004042 5.179976221 14672.28265
R02 -2.707 0.97938 1.6251 -2.051 1.007174 2.58998811 29940.26256
R03 -10.102 0.98445 2.2599 -1.607 1.014347 12.94994055 44547.7955
R04 -5.678 0.96519 2.2889 -2.3191 1.012719 72.51966709 5982.872535
R05 -4.891 0.99319 2.32521 -2.5093 1.007174 2.58998811 74164.30954
R06 -8.82 0.96418 1.3581 -0.7476 1.009752 12.94994055 6622.599598
R07 -11.861 1.00209 4.5596 -3.8984 1.009752 93.23957197 308208.5851
R08 0 0.98399 3.157 -4.1898 1.011187 132.0893936 7283.046566
R09 0 0.81629 6.4222 -7.6551 1.062826 51.79976221 16757.22307
R10 -10.927 0.89405 3.2 -2.4524 1.156028 10.35995244 53491.02444
R11 -18.627 0.96494 3.8152 -1.9665 1.044031 62.15971465 46236.46775
R12 0 0.84712 3.8336 -4.7145 1.146124 85.46960764 101032.8462
R13 0 0.77247 1.9636 -2.8284 1.122542 28.48986921 25200.58431
R14 -9.856 0.98744 2.469 -1.8771 1.08462 90.64958386 116160.9667
R15 0 0.8663 2.5065 -3.427 1.286895 23.30989299 68634.68492
R16 0 0.83708 2.1672 -3.0535 1.118034 18.12991677 12975.84043
R17 -10.18 1.00269 1.86412 -1.1579 1.059481 20.71990488 35094.3389
R18 -8.438 0.97398 1.99863 -1.5319 1.109234 15.53992866 8062.632987

Table 1: Vogel Flow Values by Hydrologic Region

The “Vogel” based flow and, therefore, velocity est imates depend upon estimates for mean annual precipitation and temperature, as well as dr ainage area. In cases where there is drainage entering from Canada or Mexico, the temperature and precipitation values for the U.S. portion of that international catchment are assigned to the en tire catchment. For small catchments, this probably does not result in large errors, but some international catchments are very large, with only a very small area in the U.S. Therefore, the V ogel-based flow estimates may be erroneous where there is a relatively large international dra inage contribution. In this NHDPlus release, ifNHD Plus User Guide September 1, 2010 90 the percentage of cumulative international drainage area for a flowline is greater than or equal to 25 percent, the Vogel-based flow and velocity estim ates are assigned missing values of -9998.

Median Annual Flow Estimates For Hydrologic Region 20 (Hawaii)

The regression-based flow estimates in Hawaii are b ased on the work of Fontaine, et. al. (1992).

In subbasins 20060000, 20050000, and 20010000 (Oahu , Molokai, and Hawaii, respectively) the median annual flow is computed as:

Q 50 = 4.25 * (DA) 1.04

In subbasins 20070000 and 20020000 (Kauai and Maui, respectively) the median annual flow is computed as:

Q 50 = 4.49 * (DA) 0.808 * (CE) -0.641 * (P) 0.985

Where: Q 50 = Median annual flow (cfs), DA = Drainage area (square miles), CE = mean altitude of the main stream channel (fee t), and P = mean annual precipitation (inches)

Mean Annual Flow Estimates For Hydrologic Region 21 (Puerto Rico)

Hydrologoc Region 21, Puerto Rice and the Virgin Is lands, has 3 primary issues:

1. A regression model, similar to the “Vogel” method c ould not be located. Therefore, there will not be a set of regression-based mean annual f low and velocity in the final Region
Flowlineattributesflow.dbf file.
2. No data for Mean annual Temperature (MAT), mean ann ual precipitation (MAP), or land cover (NLCD) were developed for Region 21, so the c atchment-level MAP, MAP, and NLCD and the cumulative MAP, MAT and NLCD tables ar e not available for Region 21.
Note that the MAP and MAT values are not needed to support a regression-based estimation method equivalent to the “Vogel” method.
3. There is no equivalent to the HCDN gages that repre sent “natural” flow that is used as the
basis for UROM flows in the continental U.S. The lo gical alternative to this method is presented below. The Puerto mainland is divided int o four HUC-8’s (21010002, 21010003, 21010004, 21010005 ) and the islands are assigned their own HUC-8
numbers.

A map of the stream gages is shown in Fig. A-8.NHD Plus User Guide September 1, 2010 91 Figure A-8. Stream Gages in Region 21

Developing a Unit Runoff Methodology for the Puerto Rico Mainland

To develop HCDN-equivalent runoff, figure A-9 below is used as the UROM source, which contains the citation for the figure.

Figure A-9. Average Unit Runoff on Puerto Rico Main landNHD Plus User Guide September 1, 2010 92 A digital format of the runoff isopleths was not av ailable, so the map was extracted, registered to the Puerto mainland and then the isopleth lines wer e digitized. Figure A-10 shows the original runoff map with the digitized lines. Note the very high variability in runoff in areas in the eastern portion of HUC 21020005. This shows why a single un it runoff value by HUC would not produce good results.

Figure A-10. Digitized Mean Annual Runoff Values On Puerto Rico Mainland

The next step is to develop a rasterized version of the digitized lines to interpolate between the runoff isopleths. The ArcGIS TOPO-TO-GRID function was used for this. The basic results of this process are shown in Fig. A-11. This map uses the same runoff category ranges as the original runoff isopleths, but it is important to n ote that the individual rasterized values vary based on the interpolation technique in the ArcGIS processing.

Figure A-11. Results of using TopoGrid to Develop a n Interpolated Raster Grid of Mean Annual Runoff

The rasterized data was then overlayed onto the cat chments, which assigned a UROM value (cfs/sq km) to each catchment.NHD Plus User Guide September 1, 2010 93 The mean annual runoff values for the islands were done based on the gages available in each of the HUC-8’s. Due to a lack of gages in 21010006, no UROM flows and velocities were developed. UROM flow and velocity estimates were de veloped for 21020001 and 21020002.



### Velocity Estimates

Velocities are estimated for mean annual flow condi tions using the work of Jobson (1996). This method uses regression analyses on hydraulic variab les for over 980 time-of-travel studies, which represent about 90 different rivers in the U. S. These rivers represent a range of sizes, slopes, and channel geometries. Four principal Flow line variables are used in the Jobson methods: drainage area, Flowline slope, mean annual discharge, and discharge at the time of the measurement. Based on his analyses, regression equa tions were developed to relate velocity (meters/second) to drainage area, a dimensionless d rainage area, Flowline slope, discharge, and a dimensionless relative discharge.

Because all Flowlines have slopes greater than or e qual to 0.00005, the “slope” regression method is used. The section “Understanding NHDPlus Slope“ in the “Understanding and Using NHDPlus” section of this User Guide provides more i nformation on NHDPlus Slope. Also, see Steps 3 and 7 for additional information on how slo pe calculations are done.

Caveats and Considerations in Using the NHDPlus Vel ocity Estimates

Care should be exercised in using these velocity es timates:
1. These velocities represent estimates for flowing st reams and rivers and are not applicable to lakes or estuaries. The velocity estimates are c omputed and reported for all networked Flowlines, but users should exercise care when usin g them in lakes or estuaries.
2. Artificial path Flowlines that are in lakes (natura l and impounded) can be determined as
follows: Select all Flowlines in the NHDFlowline ta ble with an FCODE = 55800 (Artificial path). Join these Flowlines to the NHDW aterBody Table using WBAreaComI in NHDFlowline to Comid in the NHDWaterBody table. All artificial path Flowlines with NHDWaterBody FCODE values in the 390xx series are lakes.
3. Users need to be aware that there are impounded wat erbodies not identified in the
medium-resolution NHD that is the basis for NHDPlus , so there is no way to ensure that all Flowlines in impoundments can be identified.
4. Estuarine Flowlines have not been identified in NHD Plus so performing estuarinespecific velocities is the user’s responsibility.

The equations for velocity estimates are described below:

The dimensionless relative discharge (Q’
a ) (Jobson, 1996) is expressed as

Q’
a = Q/Q
a

where Q = Flowline discharge at the time of measurement ( cubic meters/second), and Q
a = mean annual discharge.NHD Plus User Guide September 1, 2010 94 The dimensionless drainage area (D’
a ) (Jobson, 1996) is expressed as

D’
a = (D
a 1.25 * g 0.5 )/Q
a

where

D
a = drainage area (square meters), g = acceleration of gravity (9.8 meters/second/seco nd), and Q
a = mean annual discharge (cubic meters/second).

The “Slope” method Flowline velocity (VelA_p) (Jobs on, 1996) is

VelA_p = 0.094 + (0.0143 * (D’
a 0.919 ) * (Q’
a -0.469 ) * (slope 0.159 ) * (Q/D
a )).

For purposes of NHDPlus, Q is assumed equal to Q
a , so that the dimensionless relative discharge (Q’
a ) has a value of 1. Flowline slopes are described i n Step 8.

Jobson has also specified a “noslope” velocity equa tion. However, the noslope equation is not
used in NHDPlus.

Because velocities are in part based on flow, veloc ity estimates are computed for both the UROM (MAVelU) and the Vogel (MAVelV) Methods.



### Quality Assurance (QA) For NHDPlus Cumulative Drain age Area and Flow

Overview of Procedures Used

The basic procedure is to compare NHDPlus Drainage Areas and Flows to values reported in the National Water Information System (NWIS) for USGS G aging Stations. The gaging station drainage areas and mean annual period-of-record flo ws from the NWIS database are considered as “truth” for the flowline to which the gaging sta tion is assigned. The gaging station values are plotted on the x-axis and the corresponding NHDPlus values are plotted on the y-axis. With both axes at the same scale, a 45 degree line of values indicates agreement between NHDPlus and NWIS. Values not on the line represent the amount t hat NHDPlus cumulative drainage areas or flows are valid “outliers”, a systematic issue, or NHDPlus cumulative values are inaccurate.

The gaging station location data was provided by Da ve Stewart, USGS. The gage locations were snapped to the nearest 1:100,000-scale NHD flowline and then reviewed to ensure the snapped location is representative of the position on the s tream network being measured by the gage. Summary flow statistics from the gage records were taken from Wolock, (2003).

Each Hydrologic Region release contains a QA spread sheet with a “QA_notes” worksheet noting specific issues found during the QA of that Hydrolo gic Region. The spreadsheet is contained in “NHDPlusrrVss_dd_QA/QC_SINKS.zip”.NHD Plus User Guide September 1, 2010 95 Systematic Issues

There are three types of systematic issues found in the QA results. The first two types of systematic issues usually appear as a sequence of g aging stations that are approximately parallel to the 45 degree line, either above or below. These gaging stations will be along a given river so that the “problem” at the most upstream gaging stat ion is carried downstream to subsequent gaging stations along that river. This situation us ually occurs for one of the first two reasons listed below. The third systematic issue can show u p at very small drainage areas, where NHDPlus appears to over-estimate the drainage area.

1. NHDPlus has unconnected isolated areas upstream of the gage. The parallel Line will in this case be below the 45 degree line which indicat es that NHDPlus is under-estimating the drainage area in relation to USGS gaging statio n data. This situation can be considered to be caused by a difference between wha t the USGS considers total drainage area for a gage and what NHDPlus considers total co ntributing area.

2. NHDPlus connects a large drainage area that is not considered to be part of the gaging station drainage area. The most obvious occurrence of this is in the Klamath River Basin in Hydrologic Region 18; where there is a diversion canal that joins the Lost River to the Klamath mainstem. The gaging stations do not consid er the Lost River to be part of the Klamath drainage area, so there is a series of Klam ath River gages above the 45 degree line, indicating that NHDPlus is over-estimating th e drainage area in relation to the gage
data.

3. The NHDPlus cumulative drainage areas include the e ntire catchment area for the given flowline. A gage may be located at any point along a flowline. Therefore, the gage may be measuring an area that is less than the full cat chment area. In these cases, the NHDPlus drainage area will be greater than the gage area. This is not an error, per se, in the NHDPlus drainage area, but it can show up as an overestimation by NHDPlus for these small drainage areas. This issue becomes unde tectable and of little consequence with more downstream flowlines having larger draina ge areas. Additional processing is possible using the NHDPlus flowdirection grids to s plit the catchments at the gages in order to eliminate this bias, but this processing w as beyond the scope of the QA process.

Outliers

Significant outliers occur primarily in situations where the gaging station is on the minor path of a divergence, the gage is in the wrong location, or the gage is on a tributary in a lake or reservoir. For more information, see the section in the User G uide on Divergences. For examples of these outliers, see the NHDPlus presentations from the Fe bruary 2006 NHDPlus Workshop titled:

“Tue_1455_CumulativeCatchmentAttributes_Bondelid” a nd “Tue_1455_FlowVolumeVelocityEstimates_Bondelid”.

These can be found on the NHDPlus website and FTPsi te.NHD Plus User Guide September 1, 2010 96 Structure of Excel QA file

The QA table is designed to develop graphs and subs equent diagnostic analyses for outliers and other deviations from the 45 degree line as describ ed above.

The data table is in a worksheet named “FVVQARnn”, where nn refers to the particular Hydrologic Region. There follows a series of QA gra phs, one set for cumulative drainage area and one for mean annual flows. The drainage area gr aphs start with the prefix “DA QA”. The flow graphs start with the prefix “Flow QA”. There is always more than one QA graph for DA and Flow. The basic graph shows the full range of d ata. The other graphs “zoom” to the lower left corner of the data by changing the x and y max imum scale values. These graphs are constructed because the full range of data used in the QA is quite large and the majority of x-y points tend to be clustered at the lower end of the graphs. There can be several graphs, each one “zooming” into the lower sets of data values.

The Excel data table worksheet contains the followi ng data elements:

SITE_NO: The site number of the USGS gaging station STATION_NM: the name of the USGS gaging station COM_ID: the comid of the NHDPlus flowline that the gage falls upon GAGEAREA: The NWIS reported drainage area of the ga ge in square kilometers. Stations need to have a drainage area value to be c onsidered in these analyses. CUMDRAINAG: The NHDPlus cumulative drainage area in square kilometers NDAYS: The number of days the gaging station has be en active. Stations need at least 10 years of data to be considered in these analyses. gageavgQ: The mean annual flow over the period of r ecord reported for that gage in cfs.
MAFLOWU: The UROM Mean annual flow in cfs. MAFLOWV: The “Vogel” based mean annual flow in cfs. Note that there will be numerous times this field is unvalued (-9999 or -99 98). See Step 6 in Appendix A
of the NHDPlus User Guide for more information on w hy this field can be
unvalued. BESTEVENTX: The longitude in decimal degrees that b est places the gage on an
NHDPlus flowline. BESTEVENTY: The latitude in decimal degrees that be st places the gage on an
NHDPlus flowline.

The table is sorted by descending GAGEAREA. All of the graphical QA is built using these data
elements.NHD Plus User Guide September 1, 2010 97 Step 7: Elevation Post-Processing, Smoothing, and S lope Calculations

The raw flowline elevations developed in Step 3 pro vide upstream elevations at all headwater
flowlines and downstream elevations for all flowlin es. These raw elevation values may result in negative slopes where elevations decrease as the fl owlines are traversed from downstream to upstream. To develop non-negative slope estimates f or all flowlines and consistent elevations at nodes, several post-processing steps are performed. The post-processing and elevation smoothing take full advantage of advanced NHDPlus network tra versal capabilities. The elevations go through the following processes:

First, the minimum elevations for flowlines that jo in at a downstream node are independently developed and may not be equal. The elevations at e ach node are made equal by taking the minimum elevation of the two (or more) flowlines th at are immediately upstream of the node. Figure A-6 in Step 3 illustrates this process.

Second, the node elevations are also assigned as th e maximum elevation for each flowline that is immediately downstream of the node.

The results of these two processes are consistent n ode elevations for flowlines that have catchments. When all of the flowlines immediately u pstream of a node are too short to generate catchments, the node will have a “missing” elevatio n.

Third, raw elevations are smoothing so that most fl owlines will have non-negative slopes. With raw elevations, flowlines can have minimum elevatio n greater than the maximum elevation which results in a negative flowline slope; essenti ally, the water flows “uphill.” This problem is not uncommon when using digital elevation models fo r estimating flowline slopes. The solution used in NHDPlus is to smooth the elevations along a flow path so that the negative slopes are removed. This process involves interpolating betwee n elevations upstream and downstream of flowlines that have negative slopes. The result of the smoothing is that all of the flowlines will have a positive (“downhill”) slope. NHDPlus slopes are constrained to be greater than or equal to 0.00005. Another important reason to perform the sm oothing is to ensure that all networked flowlines have elevations and slopes. For nodes wit h missing elevations, as described above, ; the smoothing process fills in these elevations and slopes based on the elevation values of the flowlines upstream and downstream.

The results of the elevation processing are stored in the FlowlineAttributes table.

One of the many powerful features of NHDPlus is the ability to extract all of the flowlines for a stream path and sort them in an upstream or downstr eam order. This capability permits smoothing to be done on a stream level path basis ( e.g., the Ohio River mainstem). The elevation smoothing is done sequentially, going from the main stem to tributaries.NHD Plus User Guide September 1, 2010 98 

### Smoothing Techniques

Three basic approaches to elevation smoothing have been evaluated. The first approach, referred to as “downstream smoothing,” interpolates progress ively downstream. This approach generates
a smoothed elevation set that forms the lower envel ope of the elevation profile. An example of this approach is shown in Figure A-8. The second ap proach, “upstream smoothing,” interpolates in the upstream direction and forms the upper envel ope of the elevation profile. An example is shown in Figure A-9. A third approach, a hybrid dow nstream-upstream approach, was also evaluated. The hybrid approach mainly uses the down stream elevation smoothing technique and then uses the upstream smoothing technique until a slope greater than zero is produced (Figure A-10).



### Smoothed Elevations and Slopes for NHDPlus

Of the three smoothing techniques evaluated as part of the development of NHDPlus, the upstream smoothing technique was selected. The down stream smoothing proved unsatisfactory
because of ramifications of the method used for ass igning downstream elevations of flowlines. The downstream elevations are determined using the lowest elevation in the catchment. While this method works in most cases, sometimes the mini mum elevation is in a “sink,” such as a rock quarry or mine, that is at a much lower elevation t han the true downstream elevation of the flowline. The downstream and hybrid smoothing techn iques will then set all flowline elevations downstream of an artificially low elevation equal t o the “sink” value, until a flowline elevation is
encountered that is lower than the “sink” value. Th e effect of this process results in many flowlines having artificially low elevations and ze ro slopes. The upstream smoothing technique avoids this problem.

In NHDPlus, the FlowlineAttributes table contains s moothed elevations by LevelPathid, using the upstream smoothing approach, with slopes constr ained to be greater than or equal to 0.00005

. A relatively small number of flowline connections e xist where the minimum elevation of the inflowing flowlines and the maximum elevation of th e out flowing flows are not all equal. These elevation inconsistencies occur only where some lev el paths meet. As a result of elevation smoothing, every flowline in the network receives a slope greater than, or equal to, 0.00005.NHD Plus User Guide September 1, 2010 99 Downstream Elevation Smoothing

0 10000 20000 30000

1000 1100 1200 1300 1400 1500

pa t hl e ngt h ( k m) Original

Smoot hed

Figure A-12 – Downstream Elevation Smoothing

Upstream Elevation Smoothing

0 10000 20000 30000

1000 1100 1200 1300 1400 1500

P a t h Le ngt h ( Km) Original

Smoot hed

Figure A-13 – Upstream Elevation SmoothingNHD Plus User Guide September 1, 2010 100 Hybrid Smoothing

8000 9000 10000

0 50 100 150 200

pa t hl e ngt h ( k m) Original

Smoothed

Figure A-14 – Hybrid Elevation SmoothingNHD Plus User Guide September 1, 2010 101 

### Appendix B – Data Dictionary

Accounting – Hydrologic accounting unit (6-digit HU C) of a stream gage
AboveComID – Common identifier of feature that is a bove another (see BelowComID)
Address – Metadata Address AddressTyp – Metadata Address Type Agency_Cd – Gov. Agency responsible for a stream ga ge ArbolateSu – An estimate of miles of stream upstrea m of a flowline. Not populated. AreaSqKm – Feature area in square kilometers AreaWtMAP – Area Weighted Mean Annual Precipitation at bottom of flowline in mm AreaWtMAT – Area Weighted Mean Annual Temperature a t bottom of flowline in degree C *
10 AttributeA – Metadata Attribute Accuracy Ave – Average daily flow for the period of record f or a stream gage BCF – Vogel flow estimate bias correction factor BeginningD – Metadata Beginning Date
BelowComID – Common identifier of feature that is b elow another (see AboveComID) BFIYrs – Number of years used in the base-flow inde x computation for a stream gage BFI_Ave – Average annual base-flow index value for a stream gage BFI_StDev – Standard deviation of annual base-flow index for a stream gage
CalendarDa – Metadata Calendar Date CanalDitch – Canal Ditch Type Cat_Area – Area of catchment in square kilometers City – Metadata City Name
ComID – Common identifier of an NHD feature or rela tionship Completene – Metadata Completeness
Constructi – Construction status
ContactIns – Metadata Contact Instructions ContactOrg – Metadata Contact Organization ContactVoi – Metadata Contact Voice Telephone Count – Number of cells with a particular value in the Value field of a grid CumDrainag – Cumulative drainage area in square kil ometers CUMHILC_1 – % of cumulative drainage area classifie d as Unclassified in NOAA C-CAP CUMHILC_2 – % of cumulative drainage area classifie d as High Intensity Developed in NOAA
C-CAP CUMHILC_3 – % of cumulative drainage area classifie d as Low Intensity Developed in NOAA
C-CAP CUMHILC_4 – % of cumulative drainage area classifie d as Cultivated Land in NOAA C-CAP CUMHILC_5 – % of cumulative drainage area classifie d as Grassland in NOAA C-CAP CUMHILC_6 – % of cumulative drainage area classifie d as Deciduous Forest in NOAA C-CAP CUMHILC_7 – % of cumulative drainage area classifie d as Evergreen Forest in NOAA C-CAP CUMHILC_8 – % of cumulative drainage area classifie d as Mixed Forest in NOAA C-CAP CUMHILC_9 – % of cumulative drainage area classifie d as Scrub/Shrub in NOAA C-CAP CUMHILC_10 – % of cumulative drainage area classifi ed as Palustrine Forested Wetland in
NOAA C-CAP CUMHILC_11 – % of cumulative drainage area classifi ed as Palustrine Scrub/Shrub Wetland in
NOAA C-CAPNHD Plus User Guide September 1, 2010 102 CUMHILC_12 – % of cumulative drainage area classifi ed as Palustrine Emergent Wetland in
NOAA C-CAP CUMHILC_13 – % of cumulative drainage area classifi ed as Estuarine Forested Wetland in
NOAA C-CAP CUMHILC_14 – % of cumulative drainage area classifi ed as Estuarine Scrub/Shrub Wetland in
NOAA C-CAP CUMHILC_15 – % of cumulative drainage area classifi ed as Estuarine Emergent Wetland in
NOAA C-CAP CUMHILC_16 – % of cumulative drainage area classifi ed as Unconsolidated Shore in NOAA CCAP CUMHILC_17 – % of cumulative drainage area classifi ed as Bare Land in NOAA C-CAP CUMHILC_18 – % of cumulative drainage area classifi ed as Water in NOAA C-CAP CUMHILC_19 – % of cumulative drainage area classifi ed as Palustrine Aquatic Bed in NOAA
C-CAP CUMHILC_20 – % of cumulative drainage area classifi ed as Estuarine Aquatic Bed in NOAA
C-CAP CUMHILC_21– % of cumulative drainage area classifie d as Tundra in NOAA C-CAP CUMHILC_22 – % of cumulative drainage area classifi ed as Snow/Ice in NOAA C-CAP CumNLCD_11 – % of cumulative drainage area classifi ed as Open Water in NLCD CumNLCD_12 – % of cumulative drainage area classifi ed as Perennial Ice/Snow in NLCD CumNLCD_21 – % of cumulative drainage area classifi ed as Low Intensity Residential in
NLCD CumNLCD_22 – % of cumulative drainage area classifi ed as High Intensity Residential in
NLCD CumNLCD_23 – % of cumulative drainage area classifi ed as Commercial/Industrial/Transportation in NLCD CumNLCD_31 – % of cumulative drainage area classifi ed as Bare Rock/Sand/Clay in NLCD CumNLCD_32 – % of cumulative drainage area classifi ed as Quarries/Strip Mines/Gravel Pits in
NLCD CumNLCD_33 – % of cumulative drainage area classifi ed as Transitional in NLCD CumNLCD_41 – % of cumulative drainage area classifi ed as Deciduous Forest in NLCD CumNLCD_42 – % of cumulative drainage area classifi ed as Evergreen Forest in NLCD CumNLCD_43 – % of cumulative drainage area classifi ed as Mixed Forest in NLCD CumNLCD_51 – % of cumulative drainage area classifi ed as Shrubland in NLCD CumNLCD_61 – % of cumulative drainage area classifi ed as Orchards/Vineyards/Other in
NLCD CumNLCD_71 – % of cumulative drainage area classifi ed as Grasslands/Herbaceous in NLCD CumNLCD_81 – % of cumulative drainage area classifi ed as Pasture/Hay in NLCD CumNLCD_82 – % of cumulative drainage area classifi ed as Row Crops in NLCD CumNLCD_83 – % of cumulative drainage area classifi ed as Small Grains in NLCD CumNLCD_84 – % of cumulative drainage area classifi ed as Fallow in NLCD CumNLCD_85 – % of cumulative drainage area classifi ed as Urban/Recreational Grasses in
NLCD CumNLCD_91 – % of cumulative drainage area classifi ed as Woody Wetlands in NLCD CumNLCD_92 – % of cumulative drainage area classifi ed as Emergent Herbaceous Wetlands in
NLCDNHD Plus User Guide September 1, 2010 103 Cumpct_CN – % of cumulative drainage area in Canada and not classified in NLCD Cumpct_MX – % of cumulative drainage area in Mexico and not classified in NLCD Cumsum_pct – Sum of the % cumulative drainage areas DA_Sq_Mile – Reported drainage area in square miles of a stream gage. Stations with drainage area of -999999 means there is no reported drainage area in National Water Information System.
DataSetCre – Metadata Dataset Credit Day1 – First date of flow data (yyyymmdd) for a str eam gage DayN – Last date of flow data (yyyymmdd) for a stre am gage DeltaLevel – Numerical difference between stream le vel for FromComID and stream level for
ToComID in the NHDFlow table Descriptio – Text description of feature type and t he encoded attributes Direction – 714 for coastal connection, 709 for flo wing connection, 712 for network starts, 713
for network ends Divergence – 0 if flowline is not part of a diverge nce, 1 if flowline is the main path of a divergence, 2 if flowline is a minor path of a dive rgence DnDrainCou – Number of flowlines immediately downst ream DnLevel – Stream level of mainstem downstream flowl ine DnLevelPat – Downstream mainstem level path identif ier DnMinHydro – Downstream minor path hydrologic seque nce number
DUUID – Metadata identifier
Elevation – Feature elevation in feet Enabled – Always “True” EndingDate – Metadata Ending Date
EventDate – Date an event was created EventType – Type of entity in an event FCode – Numeric code that contains the feature type and its attributes as found in the NHDFCode lookup table FDate – Feature Currency Date
FeatureCom – Reserved for future use
FeatureCla – Reserved for future use FeatureDet – URL where detailed event entity data c an be found Flowdir – Flow direction is “WithDigitized” or “Uni nitialized”
FromComID – The common identifier for a flowline in a flow relationship from which water flows (see ToComID) FromNode – A unique number assigned to the implied node at the upstream end of an NHD
Flowline FType – NHD feature type GNIS_ID – Geographic Names Information System ID fo r the value in GNIS_Name GNIS_Name – Feature Name as found in the Geographic Names Information System GOTBFI – Flag indicating BFI data (1) or no BFI dat a (2) for a stream gage GOTQ – Flag indicating flow data (1) or no flow dat a (0) for a stream gage Grid_code – Value stored in grid cells; a unique id entification number for each catchment (compressed numbering system) Grid_count – Number of cells with a particular valu e in the value field; equals the number of 30x30 meter grid cells in each catchment; catchment area can be computed from this fieldNHD Plus User Guide September 1, 2010 104 HILC_1 – % of catchment area classified as Unclassi fied in NOAA C-CAP HILC_2 – % of catchment area classified as High Int ensity Developed in NOAA C-CAP HILC_3 – % of catchment area classified as Low Inte nsity Developed in NOAA C-CAP HILC_4 – % of catchment area classified as Cultivat ed Land in NOAA C-CAP HILC_5 – % of catchment area classified as Grasslan d in NOAA C-CAP HILC_6 – % of catchment area classified as Deciduou s Forest in NOAA C-CAP HILC_7 – % of catchment area classified as Evergree n Forest in NOAA C-CAP HILC_8 – % of catchment area classified as Mixed Fo rest in NOAA C-CAP HILC_9 – % of catchment area classified as Scrub/Sh rub in NOAA C-CAP HILC_10 – % of catchment area classified as Palustr ine Forested Wetland in NOAA C-CAP HILC_11 – % of catchment area classified as Palustr ine Scrub/Shrub Wetland in NOAA C-CAP HILC_12 – % of catchment area classified as Palustr ine Emergent Wetland in NOAA C-CAP HILC_13 – % of catchment area classified as Estuari ne Forested Wetland in NOAA C-CAP HILC_14 – % of catchment area classified as Estuari ne Scrub/Shrub Wetland in NOAA C-CAP HILC_15 – % of catchment area classified as Estuari ne Emergent Wetland in NOAA C-CAP HILC_16 – % of catchment area classified as Unconso lidated Shore in NOAA C-CAP HILC_17 – % of catchment area classified as Bare La nd in NOAA C-CAP HILC_18 – % of catchment area classified as Water i n NOAA C-CAP HILC_19 – % of catchment area classified as Palustr ine Aquatic Bed in NOAA C-CAP HILC_20 – % of catchment area classified as Estuari ne Aquatic Bed in NOAA C-CAP HILC_21– % of catchment area classified as Tundra i n NOAA C-CAP HILC_22 – % of catchment area classified as Snow/Ic e in NOAA C-CAP
HorizPosit – Metadata Horizontal Positional Accurac y HU_8_Name –Text name of Subbasin HUC – Hydrologic Cataloging Unit (8-digit HUC) of t he gage HUC_8 – 8-digit Hydrologic Unit Code, also known as Subbasin code (formerly known as catalog unit code) HUC_REG – Hydrologic Region (2-digit HUC) of the ga ge HwNodesqkm –Catchment area in square kilometers tha t drains to the headwater node of the flowline indicated by ComID Hydrograph – hydrographic category, Intermittent or perennial Hydroseq – Hydrologic sequence number IncrFlowU – Incremental Flow (cfs) for Flowline as computed by the Unit Runoff Method Inundation – Inundation Area Type Lat_NHD – Latitude of the NHD location in decimal d egrees, NAD83 Lat_Site – Latitude of the streamgage (site) locati on - gage house in decimal degrees, NAD83 LengthKM – Feature Length in kilometers LevelPathi – Hydrologic sequence number of most dow nstream flowline in level path LogicalCon – Metadata Logical Consistency Lon_NHD – Longitude of the NHD location in decimal degrees, NAD83 Lon_Site – Longitude of the streamgage (site) locat ion - gage house in decimal degrees, NAD83 MAFlowU – Mean Annual Flow (cfs) at bottom of flowl ine as computed by Unit Runoff Method MAFlowV – Mean Annual Flow (cfs) at bottom of flowl ine as computed by Vogel Method MAVelU – Mean Annual Velocity (fps) at bottom of fl owline as computed by Unit Runoff
Method MAVelV – Mean Annual Velocity (fps) at bottom of fl owline as computed by Vogel MethodNHD Plus User Guide September 1, 2010 105 MaxElevRaw – Maximum elevation (unsmoothed) in mete rs MaxElevSmo – Maximum elevation (smoothed) in meters Measure – Measure along reach where a point event i s located, in percent from downstream end Metadata_1 – Metadata Standard Version Max_ – Maximum daily flow for the period of record for a stream gage
MetadataDa – Metadata Date
MetadataSt – Metadata Standard Name Min_ – Minimum daily flow for the period of record for a stream gage MinElevRaw – Minimum elevation (unsmoothed) in mete rs MinElevSmo – Minimum elevation (smoothed) in meters NewHUCode – Substr(NewReachCo,1,8)
NewReachCo –New Reach Code in a Reach cross referen ce entry NewReachDa –New Reach date in a Reach cross referen ce entry NewUPMI – not used NDays – Number of days of flow data for a stream ga ge NDaysGT0 – Number of days of non-zero flow for a st ream gage NHD2Gage_D – Distance between a Stream Gage and the NHD Reach to which it is linked NLCD_11 – % of catchment area classified as Open Wa ter in National Land Cover Dataset 1992 (NLCD) NLCD_12 – % of catchment area classified as Perenni al Ice/Snow in NLCD NLCD_21 – % of catchment area classified as Low Int ensity Residential in NLCD NLCD_22 – % of catchment area classified as High In tensity Residential in NLCD NLCD_23 – % of catchment area classified as Commerc ial/Industrial/Transportation in NLCD NLCD_31 – % of catchment area classified as Bare Ro ck/Sand/Clay in NLCD NLCD_32 – % of catchment area classified as Quarrie s/Strip Mines/Gravel Pits in NLCD NLCD_33 – % of catchment area classified as Transit ional in NLCD NLCD_41 – % of catchment area classified as Deciduo us Forest in NLCD NLCD_42 – % of catchment area classified as Evergre en Forest in NLCD NLCD_43 – % of catchment area classified as Mixed F orest in NLCD NLCD_51 – % of catchment area classified as Shrubla nd in NLCD NLCD_61 – % of catchment area classified as Orchard s/Vineyards/Other in NLCD NLCD_71 – % of catchment area classified as Grassla nds/Herbaceous in NLCD NLCD_81 – % of catchment area classified as Pasture /Hay in NLCD NLCD_82 – % of catchment area classified as Row Cro ps in NLCD NLCD_83 – % of catchment area classified as Small G rains in NLCD NLCD_84 – % of catchment area classified as Fallow in NLCD NLCD_85 – % of catchment area classified as Urban/R ecreational Grasses in NLCD NLCD_91 – % of catchment area classified as Woody W etlands in NLCD NLCD_92 – % of catchment area classified as Emergen t Herbaceous Wetlands in NLCD Offset – Display offset for an event
OID – Internal ESRI Table row ID OldHUCode – Substr(OldReachCo,1,8)
OldReachCo – Old Reach Code in a reach cross refere nce entry OldReachDa – Old Reach Date in a Reach cross refere nce entry OldUpMI –Not used Operationa – Operational statusNHD Plus User Guide September 1, 2010 106 Originator – Metadata Originator P1 – 1st percentile of daily flow for the period of record of a stream gage. Negative values indicate reverse flow; tidal or backwater P5 – 5th percentile of daily flow for the period of record of a stream gage. Negative values indicate reverse flow; tidal or backwater P10 – 10th percentile of daily flow for the period of record of a stream gage. Negative values indicate reverse flow; tidal or backwater P20 – 20th percentile of daily flow for the period of record of a stream gage. Negative values indicate reverse flow; tidal or backwater P25 – 25th percentile of daily flow for the period of record of a stream gage. Negative values indicate reverse flow; tidal or backwater P30 – 30th percentile of daily flow for the period of record of a stream gage. Negative values indicate reverse flow; tidal or backwater P40 – 40th percentile of daily flow for the period of record of a stream gage. Negative values indicate reverse flow; tidal or backwater P50 –50th percentile of daily flow for the period o f record of a stream gage. Negative values indicate reverse flow; tidal or backwater P60 – 60th percentile of daily flow for the period of record of a stream gage. Negative values indicate reverse flow; tidal or backwater P70 – 70th percentile of daily flow for the period of record of a stream gage. Negative values indicate reverse flow; tidal or backwater P75 – 75th percentile of daily flow for the period of record of a stream gage. Negative values indicate reverse flow; tidal or backwater P80 – 80th percentile of daily flow for the period of record of a stream gage. Negative values indicate reverse flow; tidal or backwater P90 – 90th percentile of daily flow for the period of record of a stream gage. Negative values indicate reverse flow; tidal or backwater P95 – 95th percentile of daily flow for the period of record of a stream gage. Negative values indicate reverse flow; tidal or backwater P99 – 99th percentile of daily flow for the period of record of a stream gage. Negative values indicate reverse flow; tidal or backwater PathLength – Distance to terminal flowline Pct_CN – % of catchment area in Canada and not clas sified in NLCD Pct_MX – % of catchment area in Mexico and not clas sified in NLCD PipelineTy – Pipeline type Positional – Metadata Positional accuracy
PostalCode – Metadata Postal Code Precip – Mean annual precipitation in mm Process –In a Reach cross reference entry, the name of the process that created the Reach Code change ProcessDes – Metadata Process Description Prod_unit – Production unit identifier
ProssDat – Metadata Process Date
Publicatio – Metadata Publication Date ReachCode – Reach Code assigned to feature or reach on which an event is located
ReachFileV – Reach file version in a Reach cross re ference entryNHD Plus User Guide September 1, 2010 107 ReachsmDat – Reach Version Date
ReachResol – Reach Resolution Relationsh – Relationship to surface ReservoirT – Reservoir type Resolution – Always “Medium” Reviewed – Flag to indicate review status - Y or N – of an event location. Shape_Area – ESRI feature area in square decimal de grees Shape_Leng – ESRI feature length in decimal degrees SiteStatus – Active (A) or Inactive (I) where activ e StreamGageEvents have streamflow data in water year(s) 2003 and/or 2004 Slope – Slope of flowline (cm/cm) Source – Source of boundary;, 0 = Watershed Boundar y Dataset, 1 = NHD, 2 = Other
SourceCita – Metadata Source Citation Abbreviation
SourceCont – Metadata Source Contribution
SourceCurr – Metadata Source Currentness Reference
SourceScal – Metadata Source Scale Denominator Source_Ori – Originator of an event Source_Dat – Description of event entity Source_Fea – Unique Identifier of event entity, lin k to external data about entity SpecialUse – Special use category Stage – Elevation stage StartFlag – 0 if a flowline is not a headwater flow line, 1 if a flowline is a headwater flowline State – 2 char. state postal abbreviation of the US GS Water Science Center maintaining the gage.
Puerto Rico is listed as a state. State_Cd – 2-digit state FIPS code of the WSC maint aining the gage. Puerto Rico is listed as a

state. Station_NM – Station name
StateorPro – Metadata State or Province StDev – Standard deviation of daily flow for the pe riod of record StreamLeve –Stream level (dimensionless) StreamOrde – Strahler stream order. Not populated. See NHDPlus data extension table
SOSC.dbf. SubRegion – Hydrologic sub-region (4-digit HUC) Sum_pct – Sum of the % catchment areas Temp –Mean annual temperature in degrees centigrade * 10
TerminalFl – 0 if a flowline is not a network termi nus, 1 if a flowline is a network terminus TerminalPa – Hydrologic sequence number of terminal flowline ThinnerCod – Ordinal value used to display various network densities Title – Metadata title of source data used as input for creating or updating NHD
ToComID – Common identifier for a flowline which is receiving flow in a flow relationship (see FromComID)
ToComIDMea – Not valued ToNode – A unique number assigned to the implied no de at the downstream end of a flowline TypeofSour – Metadata type of Source Media UpHydroSeq – Upstream mainstem hydrologic sequence number UpLevelPat – Upstream mainstem level path identifie rNHD Plus User Guide September 1, 2010 108 UpMinHydro – Upstream minimum hydrologic sequence n umber Value – The value stored in grid cells; also known as Grid_code in related tables VertPositi – Metadata Vertical Positional Accuracy WBAreaComI – ComID of an NHD polygonal water featur e through which an NHD “Artificial
Path” flowline flowsNHD Plus User Guide September 1, 2010 109 

### Appendix C.1 – NLCD Land Cover Classification Syste m Key

Used for Continental US Only (Rev. July 20, 1999)

Water All areas of open water or permanent ice/snow cover

.

11. Open Water – All areas of open water; typically at least 25 p ercent of pixel or cell is

water.

12. Perennial Ice/Snow – All areas characterized by year-long cover of ic e and/or snow.

Developed Areas characterized by at least 30 percent of const ructed materials (e.g., asphalt, concrete, buildings, etc).

21. Low Intensity Residential – Areas with a mixture of constructed materials an d vegetation. Constructed materials account for 30-80 percent of the cover; the remaining percentage is vegetation. These areas mos t commonly include singlefamily housing units. Population densities are lowe r than in the high intensity
residential class.

22. High Intensity Residential – Highly developed areas where large numbers of pe ople reside, such as apartment complexes and row houses. Vegetation occupies less than 20 percent of the area, and constructed materials c over 80 to100 percent.

23. Commercial/Industrial/Transportation – Includes infrastructure (e.g., roads, railroads, etc.) and all highly developed areas not classified as High Intensity
Residential.

Barren Areas characterized by bare rock, gravel, sand, sil t, clay, or other earthen material, with little or no “green” vegetation present, regardless of any in herent ability to support life. Vegetation, if present, is sparse and scrubby compared to other ve getated categories, and lichen may be
extensive.

31. Bare Rock/Sand/Clay – Perennially barren areas of bedrock, desert pave ment, scarps, talus, slides, volcanic material, glacial d ebris, beaches, and other
accumulations of earthen material.

32. Quarries/Strip Mines/Gravel Pits – Areas of extractive mining activities with significant surface expression.NHD Plus User Guide September 1, 2010 110 33. Transitional – Areas of sparse vegetative cover (less than 25 p ercent of area) that are dynamically changing from one land cover to another , often because of human activities. Examples include forest clearcuts, tran sition between forest and agricultural land, temporary removal of vegetation, and changes due to natural causes (e.g., fire, flood, etc.).

Forested Upland – Areas characterized by tree cover (natural or se mi-natural woody vegetation,

generally greater than 6 meters tall). The tree can opy accounts for 25-100 percent of the land cover.

41. Deciduous Forest

– Areas dominated by trees where 75 percent or mor e of the tree species shed foliage simultaneously in response to seasonal change.

42. Evergreen Forest

– Areas dominated by trees where 75 percent or mor e of the tree species maintain their leaves all year. Canopy is n ever without green foliage.

43. Mixed Forest

– Areas dominated by trees where neither deciduous nor evergreen species represent more than 75 percent of the land cover.

Shrubland – Areas characterized by natural or semi-natural w oody vegetation that is generally less than 6 meters tall. Individual shrubs or clump s may be isolated or interlocking. Shrubland includes evergreen and deciduous species of true sh rubs, young trees, and small or stunted trees
or shrubs.

51. Shrubland – Areas dominated by shrubs, with shrub canopy cov ering 25-100 percent of the land. Shrub cover is generally greater than 25 percent when tree cover is less than 25 percent. Shrubs may cover less than 25 percent where no other vegetation type has a greater percentage to establi sh a distinct classification.

Non-Natural Woody – Areas dominated by non-natural woody vegetation occupying 25-100 percent of the area. Classifying non-natural woody land cover depends upon the availability of ancillary data to differentiate non-natural from na tural woody vegetation.

61. Orchards/Vineyards/Other – Orchards, vineyards, and other areas planted or maintained for the production of fruits, nuts, berr ies, or ornamentals.

Herbaceous Upland – Upland areas characterized by natural or semi-na tural herbaceous vegetation that accounts for 75-100 percent of the cover.

71. Grasslands/Herbaceous – Areas dominated by upland grasses and forbs. In rare cases, herbaceous cover is less than 25 percent but exceeds the combined cover of the woody species present. These areas are not subj ect to intensive management, although they are often grazed.NHD Plus User Guide September 1, 2010 111 Planted/Cultivated – Areas characterized by herbaceous vegetation tha t has been planted or is intensively managed for the production of food, fee d, or fiber or is maintained in developed settings for specific purposes. Herbaceous vegetati on accounts for 75-100 percent of the cover.

81. Pasture/Hay – Areas of grasses, legumes, or grass-legume mixtu res planted for livestock grazing or the production of seed or hay crops.

82. Row Crops – Areas used for the production of crops such as c orn, soybeans, vegetables, tobacco, and cotton.

83. Small Grains – Areas used for the production of graminoid crops such as wheat, barley, oats, and rice.

84. Fallow – Areas used for the production of crops that are temporarily barren or with sparse vegetative cover due to land management that alternates between cropping and tillage.

85. Urban/Recreational Grasses – Vegetation (primarily grasses) planted in develo ped settings for recreation, erosion control, or aesthe tic purposes. Examples include parks, lawns, golf courses, airport grasses, and in dustrial site grasses.

Wetlands – Areas where water covers or saturates the soil o r substrate periodically, as defined by Cowardin et al.

91. Woody Wetlands – Areas where forest or shrubland vegetation accou nts for 25-100 percent of the cover, and the soil or substrate is periodically saturated with, or covered with, water.

92. Emergent Herbaceous Wetlands – Areas where perennial herbaceous vegetation accounts for 75-100 percent of the cover, and the s oil or substrate is periodically saturated with, or covered with, water.NHD Plus User Guide September 1, 2010 112 

### Appendix C.2 – NOAA C-CAP Land Cover Classification System Key

Used for Hawaii Only

0 Background
1 Unclassified 2 High Intensity Developed 3 Low Intensity Developed
4 Cultivated Land
5 Grassland
6 Deciduous Forest 7 Evergreen Forest
8 Mixed Forest
9 Scrub/Shrub
10 Palustrine Forested Wetland
11 Palustrine Scrub/Shrub Wetland 12 Palustrine Emergent Wetland
13 Estuarine Forested Wetland
14 Estuarine Scrub/Shrub Wetland 15 Estuarine Emergent Wetland
16 Unconsolidated Shore
17 Bare Land
18 Water 19 Palustrine Aquatic Bed 20 Estuarine Aquatic Bed
21 Tundra
22 Snow/IceNHD Plus User Guide September 1, 2010 113 

### References

Alexander, Richard B., Elizabeth W. Boyer, Richard A. Smith, Gregory E. Schwarz, and Richard B. Moore, 2007. The Role of Headwater Streams in Do wnstream Water Quality. Journal of
the American Water Resources Association (JAWRA) 43(1):41-59. DOI: 10.1111/j.1752-
1688.2007.00005x

Daly, C. and Taylor, G., United States Average Monthly or Annual Mean Temper ature, 1961-90 (Corvallis, OR: Spatial Climate Analysis Service at Oregon State University, 1998).

Daly, C. and Taylor, G., United States Average Monthly or Annual Precipitati on, 1961-90 (Corvallis, OR: Spatial Climate Analysis Service at Oregon State University, 1998).

Environmental Systems Research Institute, Inc. (ESR I), “ARC/INFO Command Reference,” ARC/INFO Help, Version 9.0 (Redlands, CA.: ESRI, 2004).

Environmental Systems Research Institute, Inc. (ESR I), Using ARC GRID with ARC/INFO,
Version 2 (Redlands, CA: ESRI, 1999).

Fontaine, R.A., Wong, M.F., Matsuoka, I., Estimatio n of Median Streamflows At Perennial Stream Sites In Hawaii, U.S. Geological Survey Wate r-Resources Investigations Report 92-4099 (1992), U.S. Geological Survey.

Hellweger, F., Maidment, D., and the Center for Res earch in Water Resources, AGREE-DEM Surface Reconditioning System (Austin: University of Texas, 1998). http://www.crwr.utexas.edu/gis/gishyd98/quality/agr ee/agree.htm

Jobson, H. E., Prediction of Traveltime and Longitudinal Dispersio n in Rivers and Streams , U.S. Geological Survey Water Resources Investigations Re port 96-4013 (1996), U.S. Geological Survey.

Johnston, C.M., Dewald, T.G., Bondelid, T.R., Worst ell, B.B., McKay, L.D., Rea, Alan, Moore, R.B., and Goodall, J.L., 2009, Evaluation of catchm ent delineation methods for the mediumresolution National Hydrography Dataset: U.S. Geolo gical Survey Scientific Investigations Report 2009–5233, 88 p. (Also available at http://pubs.usgs.gov/sir/2009/5233/ .)

Moore, R.B., Johnston, C.M., Robinson, K.W., and De acon, J.R., Estimation of Total Nitrogen and Phosphorus in New England Streams Using Spatial ly Referenced Regression Models

, U.S. Geological Survey Scientific Investigations Re port 2004-5012, pp. 1-42 (2004), U.S. Geological Survey. http://pubs.water.usgs.gov/sir2004-5012

.

National Elevation Dataset (NED) Web page http://ned.usgs.gov/NHD Plus User Guide September 1, 2010 114 National Hydrography Dataset (NHD) Data Distributio n Web page http://nhdgeo.usgs.gov

National Hydrography Dataset (NHD) Information Web page http://nhd.usgs.gov

National Land Cover Dataset (NLCD) Web page http://landcover.usgs.gov/natllandcover.asp

National Oceanic and Atmospheric Administration (NO AA) Coastal Change Analysis Program (C-CAP) Web page http://www.csc.noaa.gov/crs/lca/ccap.html

National Watershed Boundary Dataset (WBD) Web page http://www.ncgc.nrcs.usda.gov/products/datasets/Wat ershed/

Research Triangle Institute, The National Water Pollution Control Assessment Mod el (NWPCAM) Version 2 Draft Report , prepared for the U.S. Environmental Protection Ag ency (Research Triangle Park, NC: Research Triangle Inst itute, 2001).

Saunders, W., “Preparation of DEMs for Use in Envir onmental Modeling Analysis,” Hydrologic and Hydraulic Modeling Support , Maidment, D. and Djokic, D. [eds.], pp. 29-51 (Re dlands, CA: Environmental Systems Research Institute, Inc., 2000).

Stewart, D.W., Rea, A., and Wolock, D.M., USGS Streamgages Linked to the Medium Resolution NHD, U.S. Geological Survey Data Series 195 (2006), U.S . Geological Survey.

Vogel, R.M., Wilson, I.W., and Daly, C., “Regional Regression Models of Annual Streamflow for the United States,” Journal of Irrigation and Drainage Engineering Vol. 125, No. 3, pp. 148-157, 1999).

Wolock, D., 2003, Flow characteristics at U.S. Geol ogical Survey streamgages in the conterminous United States, U.S. Geological Survey Open-File Report 03-146, downloaded
from http://water.usgs.gov/lookup/getspatial?qsitesdd

.NHD Plus User Guide September 1, 2010 1 15 

### Footnotes

1 All referenced placeholder shapefiles and dBase fi les are included to complete the official NHD data model and are empty in NHDPlus.

2 The elev_cm grids have “Zunits 100” instead of “Zu nits NO.”

3 Additional information about the NHD content and fo rmat can be found at the NHD Web site: http://nhd.u sgs.gov. The NHD Schema is described at http://nhd.usgs.gov/ NHD.pdf.

4 Additional information about the NHD content and f ormat are available on the NHD Web site at http://nhd.usgs.gov.

5 In Hydrologic Regions 01, 02, and 03, USGS WRD fie ld offices were able to specify NHD network updates that were based on local knowledge and/or the use of DRG s.
6 Table 1 contains published and unpublished materia l from Vogel (2005). The information in columns BCF

, AreaMin, and AreaMax is unpublished and was supplie d by Vogel through e-mail and telephone communicati on to Timothy R. Bondelid, Consulting Engineer.