This dataset depicts TMDL watershed boundaries. A TMDL watershed is the area covered by a TMDL equation. It consists of one or more impaired streams that have an assigned Cause Group Code (CGC). The CGC is the link between the TMDL and the Assessment Report. The TMDL equations are stored in a table in the TMDL database. Currently, the database contains about 650 equations, which are set across the entire state. Each equation includes the Waste Load Allocation (WLA), which is the portion of the TMDL that accounts for pollution from point sources (permits). The WLA cannot be exceeded during permit reissuance or its TMDL equation must be modified.

This data is the latest (Version 6) national hydrologic unit (HU) boundaries at levels 1-6 for Virginia developed and maintained by the Virginia Department of Conservation and Recreation. They have been created in compliance with the most recent published federal standards for delineation of hydrologic units and contain national WBD HU model changes made since that time up to the publication date. The Virginia NWBD schemas include variations from the national WBD schemas. This dataset covers the whole state and is seamless with the WBD products of surrounding states at the time of publication and was developed as part of a seamless hydrologic unit product for the nation at the sixth level to be used for more detailed watershed planning work in the state. This becomes the official statewide sixth level hydrologic unit delineation for Virginia.

Polygon feature class outlining the major river basin watershed areas within Virginia.