MarinMap’s GIS applications will now show new hydrology content from the local version of the National Hydrography Dataset (NHD) and Watershed Boundary Dataset. These feature classes were created from a three-dimensional digital elevation model derived from the USGS LiDAR that we receive in 2011. Because of the precision of the DEM, both flowlines and watersheds are much more precise and correctly located than any previous hydrology data (especially the old USGS “blue line” streams).

Both NHD flowlines and watersheds were created using a modeling process with ERSI ArcHydro software. With the DEM representing the earth’s surface, the software “drew” flowlines on the model as if water were flowing on it. In many instances, staff altered the model surface to account for storm water infrastructure that re-directed modeled flow. Using the upper reaches of the flowlines, the model generated catchment areas (watersheds). Small catchment areas were combined to create larger catchment areas according to USGS guidelines for setting the size of catchment areas at various map scales.

USGS has now standardized on the term “flowline” to denote the route that water takes on a “linear surface water drainage network”. One type of flowline is “Stream/River”. There are others as well.

Within the GIS applications, we have queried and displayed the types of flowlines in order to show the distinctions more clearly. Here is the list: Stream - Perennial (NHD) Stream - Intermittent (NHD) Stream - Ephemeral (NHD) Channel, Canal, Ditch (NHD) Artificial Path (NHD) Other hydrography features include Point Entities (NHD), Waterbody (NHD) and Area (NHD).

These will make sense when you view them. We strongly encourage you to learn about NHD at the USGS web site: URL = https://nhd.usgs.gov/

The watersheds have been subdivided by size of catchment area and denoted with USGS Hydrologic Unit Code (HUC). The two-digit code is a quick reference about the relative size of the catchment area and how they "nest" within each other. We have published four eastern watersheds built on a 1.5 meter resolution DEM: Eastern Watersheds (HUC16), Eastern Watersheds (HUC14), Eastern Watersheds (HUC12) and Eastern Watersheds (HUC10). Using USGS standards, the HUC 16 is the smallest catchment area and “nests” within the HUC 14 (in turn, HUC 14 nests within HUC 12 which nests within HUC 10). We published three western watersheds built on a 2 meter resolution DEM: Western Watersheds (HUC16), Western Watersheds (HUC14), and Western Watersheds (HUC12). These also nest.

We strongly encourage you to learn about watershed boundary dataset at the USGS web site: URL = https://nhd.usgs.gov/

NHD content is not a final product in the sense of being “complete and accurate”. There will be ongoing review with both geometry and attribute updates needed. (e.g. artificial path).

A subsequent email will describe how to document changes that should be made to improve the data.