Cross-MSDI Relationships

The Hydrologic Units database relates to the following other MSDI framework themes:

• Hydrography (high resolution NHD)—WBD is the container for hydrography.  These two data sets have been linked at the national level.

• Elevation—Accurate watershed boundaries depend on accurate elevation data.  The original watershed specifications normalized the watershed lines to 1:24,000 USGS topographic maps.  Watershed lines are now being refined with DEMs, LiDAR, and ground surveys, so the WBD continues to evolve.

• Climate— Climate induced catastrophic events, such as flooding and mudslides, can alter both hydrography and watershed boundaries.  Precipitation and the normal changing of seasons both contribute to erosion, which in turn affects hydrography.

• Geographic Names—Watersheds follow the GNIS whenever possible.

• Orthoimagery—While elevation data is only a model, orthoimagery provides a snapshot in time of what is actually occurring on the ground.  This can be extremely helpful in accurately delineating pour points, clarifying confusing areas, and capturing small features not captured by elevation models.