Local Drainage Basins

Example of Local Drainage Basins with Local Drainage Basin Direction arrows


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. Connecticut Drainage Basins is the most detailed delineation of natural drainage basins available on a statewide basis for Connecticut. 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 the DEP.


Connecticut Local Drainage Basins includes 2,898 drainage basin areas, the majority of which range from 0.5 to 8 square miles in size and make up, in order of increasing size the larger subregional, regional, and major drainage basin areas. The local basin number (LBAS_NO) uniquely identifies individual local basins and is 7 characters in length. There are 2,895 unique local basin numbers. Examples include 6000-00, 4300-00, and 6002-00. The first digit (column 1) designates the major basin, the first two digits (columns 1-2) designate the regional basin, the first 4 digits (columns 1-4) designate the subregional basin, and the first seven digits (columns 1-7) designate the local basin. Refer to the Connecticut Local Drainage Basin GIS metadata for more information about drainage basin numbers. Note, there are slightly more local basin areas delineated (2,898) than unique local basin numbers (2,895) primarily due to a few stream confluences that split the same local basin into two areas.


Accompanying the Local Drainage Basins are small arrows that depict the general direction of surface water flow within individual local drainage basins. Particular emphasis is given to directional arrows near the location of local drainage basin outlets where one basin drains into another. These and other arrows along stretches of local drainage basin main stem basins and in coastal basins that drain into Long Island Sound are intended to aid in visualizing the network of local drainage basins in Connecticut. These Local Drainage Basin Direction arrows are classified into three types - Outlet Direction, Main Stem Direction and Coastal Direction. Outlet Direction arrows represent general direction of a river or stream at the local drainage basin outlet. Main Stem Direction arrows represent the stream direction of main stem local basins. The local drainage basin numbers for Main Stem local basins end with "-00", such as 4302-00. Coastal Direction arrows indicate the very general direction of surface water within local coastal basins that drain into Long Island Sound.


A drainage basin is an area of land where water from rain or snow melt drains downhill into a body of water such as a river, lake, wetland or ocean. The drainage basin includes both the streams and rivers that convey the water as well as the land surface from which water drains into those channels. Used as the basis for DEP drainage basin mapping, a natural drainage basin is one in which the outlet point is a natural occurring feature. These outlet points occur at stream confluences, stream outfalls into waterbodies, and impoundments.

The hierarchical system of drainage basin delineation and numbering, as implemented by DEP, allow for the association of the physical resources to the naturally occuring drainage system that covers Connecticut. This information can be used to determine where rainfall naturally flows over the land and downstream to a particular watercourse. It can be used to identify the upstream contributing watershed for a point along a river, stream, reservoir, lake, or pond. And for cataloging purposes, a drainage basin identifier can be associated with any location on land or water in Connecticut.

Use Limitations

Drainage basin boundaries were manually delineated by interpreting the 10 foot contour lines and hydrography features shown on USGS 1:24,000-scale topographic quadrangle maps. The horizontal positional accuracy of this information is consistent with other 1:24,000-scale hydrography related data such as Connecticut Hydrography, Connecticut Waterbody, Connecticut Named Waterbody and Connecticut Surface Water Quality Classification. Only limited field checking was conducted to verify the location of these basin boundaries. Basin boundaries may not be accurate in areas that have been diked for flood control, upland wetland and reservoirs having outlets into two basins, and areas where topographic mapping is not up to date, is inaccurate, or is not detailed enough to adequately define local drainage. Residential and commercial development, highway construction, and other changes to the landscape may have resulted in local modifications to the natural drainage pattern since the time these basin boundaries were delineated.

Related Information

Dataset Information

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