Hydrography represents stream centerlines and off centerlines with greater than 30 hectare flow accumulation for the Northeast region.

This dataset was developed as part of the Designing Sustainable Landscapes project led by Professor Kevin McGarigal of the University of Massachusetts and sponsored by the North Atlantic Landscape Conservation Cooperative; for more information about the entire project see: http://www.umass.edu/landeco/research/dsl/dsl.html

The purpose of this dataset is to improve the National Hydrography Dataset (NHD) for the Northeast region. The NHD is a national framework for assigning reach addresses to water-related entities, such as industrial discharges, drinking water supplies, fish habitat areas, wild and scenic rivers. Once linked by reach addresses, the upstream/downstream relationships of these water-related entities can be analyzed.
The data displayed on Science Base and Data Basin are raster for increased display efficiency. The data available for download are vector lines and are described below.
This dataset was derived from the following source:

The National Hydrography Dataset (NHD) high-resolution (1:24k) flowlines Steps taken to improve the NHD include:

• Editing to connect breaks in the network
• Remove salt marsh ditching where it caused flow direction problems
• Remove pipelines
• Fix flow reversals
• Extend flowlines to the bay/ocean.

NHD is a feature-based database that interconnects and uniquely identifies the stream segments or reaches that make up the nation’s surface water drainage system. The high-resolution NHD, generally developed at 1:24,000/1:12,000 scale, adds detail to the original 1:100,000-scale NHD. The NHD contains reach codes for networked features, flow direction, names, and centerline representations for areal water bodies. The NHD also incorporates the National Spatial Data Infrastructure framework criteria established by the Federal Geographic Data Committee. For more information on this dataset see: http://nhd.usgs.gov/index.html

For the Chesapeake Bay only, The Nature Conservancy’s (TNC) Chesapeake dendrite dataset was incorporated. The feature class is a single flowline dendrite also derived from the high resolution NHD flowlines. To create the Chesapeake dendrite the high-res NHD was edited by:

• Selecting and removing line segments which form loops or other downstream bifurcations.
• Several pre-existing datasets were used to facilitate this process including coverages in Maryland from Pete Steeves (USGS) and Pennsylvania from Scott Hoffman (USGS).
• These datasets were joined to the high-res NHD to eliminate approximately 80% of the unwanted segments (braids, loops, downstream birfurcations).
• Manual editing was used to eliminate the rest.
• Once a geometrically correct dendrite was produced, flow direction in the geometric network was set to digitized direction and edits made as needed to ensure proper flow direction.
• Catchments were calculated for each line segment using a 10m Digital Elevation Model (DEM) and Python
• The area of each segment was then summed for all upstream segments.

Questions regarding the Chesapeake dendrite can be directed to Erik Martin: emartin@tnc.org