See full Data Guide here. Regional Drainage Basin Set:

Connecticut Regional Drainage Basins is 1:24,000-scale, polygon and line feature data that define Regional drainage basin areas in Connecticut. These large basins mostly range from 40 to 400 square miles in size and make up the even larger major drainage basin areas. Connecticut Regional Drainage Basins includes drainage areas for all Connecticut rivers, streams, brooks, lakes, reservoirs and ponds published on 1:24,000-scale 7.5 minute topographic quadrangle maps prepared by the USGS between 1969 and 1984. Data is compiled at 1:24,000 scale (1 inch = 2,000 feet). This information is not updated. Polygon and line features represent drainage basin areas and boundaries, respectively. Each basin area (polygon) feature is outlined by one or more major and regional basin boundary (line) feature. These data include 85 regional basin area (polygon) features and 529 regional basin boundary (line) features. Regional Basin area (polygon) attributes include major and regional basin number, and feature size in acres and square miles. The regional basin number (RBAS_NO) uniquely identifies individual basins and is 2 characters in length. There are 44 unique regional basin numbers. Examples include 43, 60 and 61. The first digit (column 1) designates the major basin and the first two digits (columns 1-2) designate the regional basin. Note, there are slightly more regional basin polygon features (85) than unique regional basin numbers (44) due to coastal regional basins defined by series of polygon features located along the Connecticut shoreline. Regional basin boundary (line) attributes include a drainage divide type attribute (DIVIDE) used to cartographically represent the hierarchical drainage basin system. This divide type attribute is used to assign different line symbology to individual major and regional drainage basin divides. For example, major basin drainage divides are more pronounced and shown with a wider line symbol than regional basin drainage divides. Connecticut Regional Drainage Basin polygon and line feature data are derived from the geometry and attributes of the Connecticut Drainage Basins data. Purpose: The polygon features define the contributing drainage area for individual reservoirs, lakes, ponds and river and stream reaches in Connecticut. These are hydrologic land units where precipitation is collected. Rain falling in a basin may take two courses. It may both run over the land and quickly enter surface watercourses, or it may soak into the ground moving through the earth until it surfaces at a wetland or stream. In an undisturbed natural drainage basin, the surface and ground water arrive as precipitation and leave either by evaporation or as surface runoff at the basin's outlet. A basin is a self-contained hydrologic system, with a clearly defined water budget and cycle. The amount of water that flows into the basins equals the amount that leaves. A drainage divide is the topographic barrier along a ridge or line of hilltops separating adjacent drainage basins. For example, rain or snow melt draining down one side of a hill generally will flow into a different basin and stream than water draining down the other side of the hill. These hillsides are separated by a drainage divided that follows nearby hilltops and ridge lines. Use these basin data to identify where rainfall flows over land and downstream to a particular watercourse. Use these data to categorize and tabulate information according to drainage basin by identifying the local basin number for individual reservoir, lake, pond, stream reach, or location of interest. Due to the hierarchical nature of the basin numbering system, a database that records the 2-digit regional basin number for individual geographic locations of interest will support tabulations by major and regional basin a