The U.S. Geological Survey has divided the country into various-sized cataloguing units based on surface hydrological features. The numbering system is called a hydrologic unit code or HUC, and is hierarchial in that each set of digits represents a certain level in the system. Under this cataloguing system a six-digit code is a basin, eight digits represent a subbasin, 10 digits is a watershed and 12 digits is a subwatershed. In this system only a 10 digit hydrologic unit is a watershed. These watersheds are generally in the range of 40,000 to 250,000 acres. There are 109 10-digit watersheds in New Hampshire. All of these levels, from basin to subwatershed and even to much smaller areas are watersheds under the general definition of a watershed used in this web site. The HUC system has recently undergone revisions in the numbering protocol. For the latest information and a more detailed explanation of the system visit http://www.nhq.nrcs.usda.gov/hu/ni170304.html or http://water.usgs.gov/GIS/huc.html.