December 2007

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MassGIS has produced a statewide digital datalayer of the approximately 2300 sub-basins as defined and used by the USGS Water Resources Division and the Mass Water Resources Commission and as modified by Executive Office of Environmental Affairs (EOEA) agencies. These sub-basins were aggregated together to make the 28 basins of the Major Basins Datalayer . Cape Cod and the Islands do not have much lateral 'surface' drainage because the soils are so porous. The sub-basin line shown for Cape Cod is the approximate groundwater divide between Cape Cod Bay, Vineyard Sound, and the Atlantic Ocean, taken from sub-surface groundwater contours. The state coastline and boundaries are included in the sub-basin coverages and are differentiated by arc attributes.



A set of 1:24,000 USGS paper quad sheets was delineated into approximately 2200 minor or drainage sub-basins. This work was produced over the past 20 years mainly by the USGS-WRD. Generally, the contours on the quads are the primary guide to basin boundaries. Often the 'mouth' of a basin is marked at the site of a stream gaging station, which can be different from the strict geographic location of the mouth.


All sub-basins on the manuscripts were digitized. Due to good manuscript quality, including the visual edgematching of the 189 sheets, digitizing and edgematching was straightforward. The manuscript author was consulted on the minor errors and ambiguities that were discovered.

Drainage boundaries added to Cape Cod were created by the Massachusetts Bays Program (the boundary separating drainage between Nantucket Sound/Cape Code Bay/Atlantic Ocean) and the Buzzards Bay Project (the boundary separating drainage between Buzzards Bay and Nantucket Sound/Cape Code Bay). The subdivisions were created by delineating groundwater divides using the 1:48,000 scale USGS Hydrologic Atlas Series maps. The delineations were reviewed for consistency by the USGS Water Resources Division, but they are not considered official basins of the USGS or the MA Water Resources Commission.

In the fall of 1992 the MA Department of Environmental Protection GIS Program added the state boundary and 1:100,000 coastline and extended or clipped the sub-basins to meet them. Additional drainage basins were delineated at the intake points of public water supplies.

Please note that in many cases the USGS maps used for delineating the subbasin boundaries are of an older vintage than those used in the Scanned USGS Topographic Quadrangle Images layer. As a result, discrepancies regarding the correct positioning of the subbasins may be apparent when compared against hydrographic and/or elevation contour features shown on the images.


Each drainage sub-basin has a unique 5-digit SUB_ID number that was derived from the numbering system on the manuscripts. The numbers are roughly hierarchically ordered based on the sub-basin's position within the major basin. The ID-numbers ascend as the water descends. The first two numbers in the 5-digit code identify the 28 major basins as numbered in Massachusetts (see listing in the Major Basins Datalayer). Offshore islands are given a SUB_ID of ##999 where ## is the nearest onshore major basin ID.

The SUB_NAME field stores the name of the major basin to which the sub-basin belongs.

The arcs are coded with the two digit attribute LINE_ID, which identifies the type of boundary the line represents:

0 Sub-Basin Boundary
1 Major Basin Boundary
2 Coastline or State Boundary


The entire datalayer was plotted at 1:100,000, selected areas at 1:25,000. Edgematching was done. The manuscript often had more than one ID per sub-basin. One was chosen by MassGIS. The water supply designation was plotted and checked by DEP regional staff.


This datalayer is maintained by MassGIS and the DEP GIS Group. Tile 6 was modified by the DEP GIS Group in September 2001. Tiles 1 and 5 required linework updates in February 2002 as a result of updates to the DEP Surface Water Supply Protection Area (Zone A, B, C) layers. In May 2002 the DEP GIS Group made Linework/Poly edits to existing sub-basin boundaries for tiles 3 and 5 and Polygon attribute updates to WSP field for tiles 1, 6, and 9. In July 2002 DEP GIS made similar updates to tiles 1, 5, 6, and 8. In September 2002 DEP GIS similarly updated tile 6. In November 2002 tiles 1, 5, 6, 7 and 9 had polygon attribute updates to WSP. In December 2002 two sub-basins were added (19301 and 13067) as a result of modifications to DEP SWPA ZoneC for Public Water Supplies 3040000-02S and 3128000-01S. In March 2003 tiles 1 and 3 had linework updates and tiles 5,6,7,8 and 9 had attribute (WSP) updates. In August 2003 tile 13 was updated. In November 2003 linework for tiles 6, 9, and 13 were updated in conjunction with the SWPA updates. In December 2005 DEP GIS made some updates to the WSP attribute in the polygon layer.

In June 2007 the attributes for Public Water Supplies (WSP) were removed, please refer to the datalayers Surface Water Supply Watersheds and Surface Water Supply Protection Areas for the extents of public water supply watersheds.

Last Updated 1/8/2008

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