National: In the 1970s the USGS (United States Geological Survey) and Water Resources Council created a mapping and classification system that partitioned the U.S. into four hierarchically nested watershed levels. These levels, called Hydrologic Units, divided and subdivided drainage areas down to 250,000 acres (390 sq mi) portions. Over time, however, more detailed hydrologic studies required even smaller subdivisions than the Level 4 (Subbasin) HUs. Recently the USGS, in cooperation with the Natural Resources Conservation Service (NRCS), further divided the HU system into six levels of hydrologic units (down to 10,000 acres or 15 sq mi at their smallest). This expanded delineation system is known as the national Watershed Boundary Dataset (WBD) .
Minnesota: Minnesota Statutes defined watersheds in terms of the State of Minnesota Watershed Boundaries - 1979 Mapping Project. This project, by the Department of Natural Resources, represented the first major effort to develop systematic and detailed height-of-land boundary maps for all watersheds of the state. The Watershed Mapping Project identified and delineated what has become known as the state's 81 Major Watersheds and (approximately) 5600 Minor Watersheds that comprise them.
Currently: The DNR Major Watersheds as mapped in 1979 were consistent with the federal watershed standards for HU Level 4 of that time. The current update of the watershed boundaries, now underway by the DNR as part of it's Lakeshed Mapping Project, retains the intent of the original DNR watershed mapping project, while making modifications to the delineation procedures which will make Minnesota watersheds more consistent with the new federal Watershed Boundary Dataset standard. The DNR mapping efforts are also delineating even smaller watersheds (called 'Hydrologic Units' - not to be confused with the WBD Hydrologic Units) that will be aggregated to create national Watershed Boundary Dataset Hydrologic Units.
The table below shows the HU System hierarchy as defined by the Watershed Boundary Dataset and the integrated DNR watershed hierarchy levels. (Grayed rows are the DNR levels added.)
|HU Level||HU Name||Area||Number of Digits||Example|
|1||Region||178,000 sq mi (avg)||2||07||Upper Mississippi||190,000 sq mi|
|2||Subregion||16,800 sq mi (avg)||4||0701||Mississippi Headwaters||20,100 sq mi|
|3||Basin||10,600 sq mi (avg)||6||070101||Mississippi Headwaters||11,600 sq mi|
|4||Subbasin (Major Watershed)*||250,000 acre minimum||8 (2)*||07010108 (14)*||Long Prairie||885 sq mi|
|5||Watershed||40,000 to 250,000 acres||10||0701010801||Lake Carlos||247 sq mi|
|6||Subwatershed||10,000 to 40,000 acres||12||070101080101||Fish Lake||33 sq mi|
|Minor Watershed||3000 acre minimum||5||14003||26 sq mi|
|Hydrologic Unit||100 acre minimum||7||1400301||Fish Lake|
* Major Watersheds of the DNR's Major/Minor Watershed system are equivalent to the Subbasins of the HU System except in their code number.
Standards are the "common ground" that technicians, hydrologists and other specialists can use to understand and communicate watershed delineation procedures and results.
FGDC Delineation Standards (PDF file - 7.19 MB) : View or download the Federal Geographic Data Committee (FGDC) standards for hydrologic unit delineation. These standards have been expanded upon by the DNR's Lakeshed Project.
Watershed Boundary Dataset (WBD) : This dataset is the federal government's effort to "establish a base-line drainage boundary framework, accounting for all land and surface areas."