Hydrologic Unit Code (HUC) Hierarchy Table

Mystified by the often-confusing terminology associated with water drainage naming and numbering?

This table relates well to the official NHD/USGS system (source: Wikipedia 2015Feb05).

The GIS layer Hydrologic Units includes Level 3 (3rd-code/6-digit) through Level 6 (6th-code/12-digit) polygons.


These images should help clarify, beginning at the HUC3 level,

Lower Snake River Basin selected:

Lower Snake River Basin description


Zoom in to HUC4 with Imnaha River (Oregon) Subbasin selected:


Zoom in to HUC5 with Upper Imnaha River Watershed selected:


Zoom to HUC6, South Fork Imnaha River Subwatershed selected:


NHD_LLID Tools for Creating Whole Stream LLID Routes Based on NHD

The NHD LLID Tools are an ArcMap 10 Add-in programmed in ArcObjects VB.net.  The purpose is to transfer the LLID whole stream route identifiers from our existing 1:100,000 whole stream route system to a whole stream route system based on the 1:24,000 NHD Hydrography, and to identify and resolve stream name conflicts between the two systems.  

The tool has three main components, organized into tabs on the ArcMap Add-in dockable window.  The first tab contains a tool to initiate an algorithm which assigns LLIDs to NHD ComID segments based on matching stream names.  The second tab provides a set of tools for assigning LLIDs to ComID segments for individual streams.  This tab also provides a tool for creating new LLID routes. The third tab provides a tool for identifying the location of stream name conflicts, as well as the nature of the conflict, how the stream was routed, why the stream was routed in this way, and what further action is required.  This portion of the application can be used without the LLID assignment functionality.

Click the following link to download the application, instructions,sample data, and sample map documents.  https://fishgame.idaho.gov/ifwis/download/NHD_LLID/NHD_LLID_Routing.zip

Unzip the LLID_RoutingTools file to C:\ .  This will create the directory C:\ NHD_LLID_Routing which includes the geodatabase NHD_LLID_Routing.mdb.  This file must be in this location for the program to function.  If you need to put the directory in another location contact tim.williams@idfg.idaho.gov

The NHD_LLID_Routing.zip contains the following:

- NHD LLID Routing Tools Instructions.doc. This document outlines how to use the tools to assign LLIDs to lines in the NHDFlowline layer and how to create a measured route system from these lines.

- The NHD LLID Routing Tools ArcMap 10 Add-in.

- A sample map document with no LLIDs assigned and the NHDFlowline layer unsymbolized. Start with this mxd if you would like to try out all the features of the NHD_LLID Tools. (NHDSampleUnrouted.mxd)

- A sample map document with LLIDs assigned. The State LLID route layer and the NHDFlowline layer are symbolized by LLID. The document also contains example IssuePoints highlighting discrepancies between the layers.  A completed NHD_LLID Route layer is also present. (NHDSampleRouted.mxd)

- A sample map document with the State LLID route layer and the NHDFlowline layer symbolized by Name and GNIS_Name. The document also contains example IssuePoints highlighting discrepancies between the layers.  (NHDSampleByName.mxd)

- The NHD_LLID_Routing.mdb geodatabase that supports the application.

- A sample NHD geodatabase where no LLIDs have been assigned (NHD17010304Unrouted).

- A sample NHD geodatabase where LLIDs have been assigned and a route system has been created (NHD17010304Routed).