A watershed is an area of land that drains to a common outlet. The terms watershed and basin are often used interchangeably. Different sizes of watersheds nest within each other as larger ones are subdivided into smaller ones.

There are two types of watershed datasets on CT ECO. One dataset contains basins created specifically for Connecticut by CT DEEP (Dept of Energy and Environmental Protection) in 1981 (then called CT DEP) and the other is a national dataset created by the US Geological Survey (USGS), called hydrologic units, which are part of the watershed boundary dataset (WBD). Lumping and splitting watersheds can be done in different ways, and one is not necessarily better than another. The DEEP basins have been used in Connecticut for longer and may be what people are used to seeing. The HUCs have the advantage of adhering to standards that cross state lines.

An interesting activity is to visit the Advanced Viewer and look at the different watershed boundaries over the hillshade elevation.


CT DEEP Basins

A standardized mapping of natural drainage basins in Connecticut was completed in 1981 by the Connecticut Department of Environmental Protection (DEP). This drainage basin system divided Connecticut into 8 major basins, 45 regional basins, 337 subregional basins, 2,898 local basins, and 7,067 small drainage basin areas. The statewide mapping of natural drainage basins established a hierarchical system of basins based on drainage area size with large major basins subdivided into regional basins, regional basins subdivided into subregional basins, subregional basins subdivided into local basins, and local basins subdivided into smaller and more numerous drainage basin areas. It includes watersheds for Connecticut rivers, streams, brooks, lakes, reservoirs and ponds included on 1:24,000-scale 7.5 minute topographic quadrangle maps published by the U.S. Geological Survey (USGS) between 1969 and 1984. These basin units include smaller watersheds that drain into many of the small streams and ponds in Connecticut. These basin areas are the building blocks for the larger local, subregional, regional and major drainage basins defined by DEP.


Watershed Boundary Dataset Hydrologic Units

The Watershed Boundary Dataset (WBD) contains watersheds called hydrologic units. As USGS explains, the United States is divided and sub-divided into successively smaller hydrologic units. The hydrologic units are arranged or nested within each other, from the largest geographic area (few digits) to the smallest geographic area (more digits) where each hydrologic unit is identified by a unique hydrologic unit code (HUC) consisting of two or more digits. The WBD was originally developed by USGS and NRCS.

Regional, 2 digit HUCs are the largest.. Most of New England is in HUC 01 (light blue in the graphics).


2 digit HUC (blue)

4 digit HUCs (red)

8 digit HUCs (yellow) 

10 digit HUCs (purple)


12 digit HUCs (dark gray). The highlighted basin is the Blackledge River, HUC 010802050801. The 12 digits each represent the basins

  • 01 = first 2 digits (HUC 2) which is the New England region
  • 0108 = first 4 digits (HUC 4) is the Connecticut River basin
  • 01080205 = first 8 digits (HUC 8) is the Lower Connecticut River basin
  • 0108020508 = first 10 digits (HUC 10) is the Salmon River basin
  • 010802050801 = 12 digits (HUC 12) is the Blackledge River (highlighted)

Notice how the basins nest and the digits build.


HUC vs CT DEEP Basins

The HUC basins are not the same as the DEEP basins on CT ECO. Although the boundaries line up in many cases, there are differences.

The CT DEEP basins layer contains watersheds that are the smallest of them all. Next are local basins and following that, subregional basins, regional basins and major basins. The graphics below show the differences and similarities between HUC basins and DEEP basins. To interact with the different basins and compare them yourself, visit the Advanced Viewer.


CT DEEP Major Basins

HUC 8 & Major Basins

HUC 10

CT DEEP Regional Basins

HUC 10 & Regional Basins


HUC 12

CT DEEP Subregional Basins

HUC 12 & Subregional Basins