Technical background
The Surface Water Quality Monitoring (SWQM) program of the Division of Environmental Science developed the data necessary for implementation of Arc Hydro at the St. Johns River Water Management District (SJRWMD) Department of Water Resources with the assistance of staff from other departments at SJRWMD. The SWQM Arc Hydro project was developed to delineate the drainage areas, or watersheds, for the program’s 73 water-quality monitoring sites.
Arc Hydro represents advancement in the field of geographic information systems (GIS) specifically for water resources applications. GIS has been used at SJRWMD for many years, but the advancements offered by Arc Hydro implementation allowed SJRWMD staff to perform GIS functions that were previously not possible or that were much more difficult and time consuming to perform.
The map below contains selected features from the SJRWMD Arc Hydro geodatabase. The legend to the right of the map includes the Arc Hydro network, Arc Hydro Polygon Feature Classes and HydroPoints. The Arc Hydro network is composed of HydroEdges and HydroJunctions. The HydroEdges are a hybrid feature class, composed of two types of lines that represent surface water flow: those that represent streams and rivers, which are linear in nature, and those that represent flow through lakes and wetlands, not usually represented as linear features. Thus, the HydroEdges seen in lakes and wetland areas on a map are a shorthand notation of sorts, and they exist to establish the connectivity of features in the Arc Hydro network. The arrowheads on the HydroEdges indicate the primary direction of surface water flow, ignoring tidal influence along the coast. The HydroJunctions are points where significant features can be linked to the network. The network is essential to Arc Hydro, allowing for scientists to easily determine upstream and downstream relationships of surface water-related features. The Arc Hydro Polygon Feature Classes are SJRWMD Catchment and Waterbody. The catchments represent local drainage areas and the waterbodies are lakes. HydroPoints are hydrologic points of interest and include such features as water quality monitoring points, engineering structures, and springs.
Links to additional information about Arc Hydro at SJRWMD:
Explanation of
the how the Spatial Data Summaries were created for the fact pages 
AWRA GIS in Water Resources 2006 proceedings paper —
The NHD: QAQC, Editing
and Arc Hydro Application in Northeast Florida