Watersheds depicted in eWRIMS are derived from the California Interagency Watershed Map of 1999 (aka "Calwater"), version 2.2.1 updated May 2004. Calwater, the state's official watershed boundary map, apportions the state's 101 million acres into ten Hydrologic Regions. Each region is progressively subdivided into six smaller, nested levels: Hydrologic Units ( major rivers), Hydrologic Areas (major tributaries), Hydrologic Sub-Areas, Super Planning Watersheds, and Planning Watershed. The Planning Watersheds (the most detailed level) range in size from approximately 3,000 to 10,000 acres.
Federal Hydrologic Unit Codes (4-digit HUC) Sub-Region codes and names are provided below as a cross reference for Calwater numbers. Note that some state designations map to more than one federal designation. More detailed HUC designations terminate with the 8-digit HUC, the Cataloging Unit (CU) (USGS1986, Water Supply Paper 2294). Calwater maps fairly neatly to the federal Hydrologic Unit Codes (HUCS). Generally Calwater watersheds fit within HUCs, though there are some exceptions.
More information can be found on the California Spatial Information Library's Calwater metadata page (http://gis.ca.gov/meta.epl?oid=22175).
| HR | SUBR | Federal HUC Hydrologic Sub-Region Name (Remarks) |
| 1 | 1801 | Klamath-Northern California Coastal (incl. upper Klamath, OR) |
| 1 | 1710 | Oregon-Washington Coastal (minor CA portion of OR drainages) |
| 2 | 1805 | San Francisco Bay |
| 3 | 1806 | Central California Coastal |
| 4 | 1807 | Southern California Coastal |
| 5 | 1802 | Sacramento |
| 6 | 1804 | San Joaquin |
| 7 | 1803 | Tulare-Buena Vista Lakes |
| 8 | 1808 | North Lahontan (incl. portions of NV basins draining into CA) |
| 8 | 1604 | Black Rock Desert-Humboldt (minor CA portion of NV basins) |
| 8 | 1605 | Central Lahontan (includes Tahoe basin and portions of NV) |
| 8 | 1712 | Oregon Closed Basins (minor CA portion near Goose Lake) |
| 9 | 1809 | Northern Mojave-Mono Lake (minor NV portions included) |
| 9 | 1606 | Central Nevada Desert Basins (minor CA portion) |
| 10 | 1810 | Southern Mojave-Salton Sea |
| 10 | 1503 | Lower Colorado (includes CA portion west of Colorado River) |