Horizon SystemsNHDPlus Version 1 (Archive)Data Extensions
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Table of Contents

Strahler Stream Order in the NHDPlus

The NHDPlus is distributed with a stream order attribute (NHDFlowlineVAA.StreamOrder) that was built using an algorithm that contained a simple correction for the effect of divergences. While these stream order values were fairly reasonable for low order streams (1,2, and 3), in large rivers and braided area, the values became unpleasingly large. Several folks in the EPA laboratory in Corvalis, Oregon designed a more complex algorithm which was implemented by the NHDPlus team. The data along with further documentation is available below.

NHDPlus in HUC8 (aka. Catalog Unit or Subbasin)

NHDPlus has been extracted into HUC8 datasets that mimic the native NHDPlus format (i.e. shapefiles, grids, and dbfs). The standard NHDPlus documentation and release notes are applicable to these datasets. The NHDPlus tools should work with all HUC8 datasets. Shortly, there will be an append tool available that will enable HUC8 datasets to be appended into larger areas of interest.

TNavWork.mdb for the NHDPlus

Many of the NHDPlus tools utilize an Access database (TNavwork.mdb) for various functions. On some operating systems, the tools are unable to create this database correctly. Most commonly, this occurs with Windows 2000. To solve this problem, download the zip file containing the TNavWork.mdb for the NHDPlus region of interest. Uncompress the zip file into the same place as you uncompressed the NHDPlus data. Be sure to preserve the directory structure that is in the zip file. The TNavWork.mdb should end up in the high level directory for the NHDPlus region (e.g., NHDPlus01\). No other changes are required. The TNavwork databases for each NHDPlus region are available below.

Smoothed Catchment Shapefiles for the NHDPlus

The NHDPlus catchments are built from 30 meter grids and, as a result, each catchment polygon contains many vertices. Shapes with a large number of vertices may cause long display and processing times. Also, when processing shapes with many vertices, some ArcGIS geoprocessing tools may fail, often with memory problems. These issues are compounded when catchment shapes are dissolved together into bigger polygons, for example in basin delineation. The resulting basins may have an extremely large number of vertices. To circumvent these difficulties, a smoothed version of NHDPlus catchment polygons have been created. Like the original catchments, these smoothed catchments completely cover the landscape without gaps. However, the original catchments and the smoothed catchments will yield slightly different results when used in spatial analyses. Users should carefully consider whether original or smoothed catchments are appropriate for their application. The Smoothed Catchments for each NHDPlus region are available below.

NLCD 2001 Catchment Allocation and Accumulation Attributes

The NLCD 2001 Land Cover and Impervious Cover data has been allocated and accumulated using the NHDPlus CA3T tool. The allocation attributes are the values for each catchment and the accumulation attributes are the total upstream values for the attributes. For NLCD land cover, the area and the area-weighted percents are provided for each NLCD land cover category. For NLCD impervious cover, the impervious area within the catchment and the total upstream impervious area are provided. Documentation of the process and file formats are provided in the metadata. Additional information about the NHDPlus CA3T tool can be found on the NHDPlus Tools page. The NLCD 2001 Catchment Allocation and Accumulation Attributes for each NHDPlus region are available below.

Linking NHDPlus Linear & Point Events to NHDPlus Catchments - Instructions and Data

Once information is linked to the NHDPlus NHDFlowline route system as linear or point events, these events can be associated with NHDPlus catchments. A procedure for performing this linkage is now documented and data to assist in this process is available.

The procedure document is available here.

The data needed for the procedure is available here.