Fairfax County, Virginia/Hydrography
No import of NHD or county data has been performed in the county. To date (2011-05-28) it appears all water features have been manually added by contributors. The quality of this data varies wildly, and it would certainly be advisable to compare existing data to other data sets like NHD or the county.
There are at least some smaller streams which aren't present in either of the two existing governmental hydrography datasets.
National Hydrography Dataset (NHD)
Fairfax County is part of the Potomac basin, (HUC_6 020700), and at least part of the county is in HUC_8 02070010 (at least Burke).
Fairfax County local data
The county does provide polygon and line Shapefiles on their website, but they are 3 years old and quality is unknown.