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FROM NHDPLUSV2 User Guide:
Catchment The incremental drainage area for a linear hydrologic feature found in the National Hydrography

Dataset (NHD).

Flowline A mapped stream segment or a path through a waterbody in the surface-water network of the National

Hydrography Dataset. The basic unit of the NHD linear surface-water network.

Reach (also NHD Reach) A uniquely identified linear feature that consists of one or more flowlines.

Stream segment (also see flowline) Part of a stream, often extending between tributary confluences

 

Stream spatial network terminology used in discussion points (based on Figure below)

Blue Line = Stream reach, typically defined as a stream segment connecting 2 of the following;  stream origins, stream confluences, lake or reservoir inflows or outflows, and/or stream outflows
Yellow = 90 meter buffer of stream
Blue Polygon = Local Catchment, catchment draining to an individual stream reach

Green Polygon = Upstream Drainage Catchment, upstream drainage of a given reach NOT including the local catchment

Green Polygon + Blue Polygon = Network Catchment, entire upstream drainage of a given reach including the local catchment

 

Figure borrowed from NFHP’s 2015 Through A Fish’s Eye Report

 

 

 

Summarization Terminology

Summarization of information to local catchment is called Attribution (MSU and NAQWA) or Allocation (StreamCAT and NHDPlus).

Summarization of information to network catchments is called Aggregation (MSU and NAQWA) or Accumulation (StreamCAT and NHDPlus)