This dataset represents the Connecticut River Watershed Boundary and is from the Watershed Boundary Dataset (WBD). WBD provides a uniquely identified and uniform method of subdividing large drainage areas. The data is intended to be used as a tool for water-resource management and planning activities, particularly for site-specific and localized studies requiring a level of detail provided by large-scale map information.
The WBD defines the areal extent of surface water drainage to a point, accounting for all land and surface areas. Watershed Boundaries are determined solely upon science-based hydrologic principles, not favoring any administrative boundaries or special projects, nor particular program or agency. The intent of defining Hydrologic Units (HU) for the Watershed Boundary Dataset is to establish a baseline drainage boundary framework, accounting for all land and surface areas. At a minimum, the WBD is being delineated and georeferenced to the USGS 1:24,000 scale topographic base map meeting National Map Accuracy Standards (NMAS). Hydrologic units are given a Hydrologic Unit Code (HUC). For example, a hydrologic region has a 2-digit HUC. A HUC describes where the unit is in the country and the level of the unit. The document "Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)" can be found here: http://pubs.usgs.gov/tm/tm11a3/pdf/WBD-Ed3_052212.pdf
A hydrological unit is a drainage area delineated to nest in a multi-level, hierarchical drainage system. Its boundaries are defined by the hyrographic and topographic criteria that delinate an area of land upstream from a specific point on a river, stream, or similar surface waters. The hydrologic units are only synonymous with class watersheds when their boundaries include all the source area contributing surface water to a single defined outlet point.
Please see http://nhd.usgs.gov/wbd.html for more information