Data Characteristics

The features in the NHD are organized into polygons, lines and points. The polygons typically portray waterbodies such as lakes; the lines typically portray streams. The stream lines are broken up into shorter segments stretching from confluence-to-confluence. The segments are then linked together to make it possible to trace the flow of water across the landscape. Artificial lines are added inside lakes to help represent the flow of water through a lake.

The primary features making up the nation’s surface water are labeled with nationally unique and permanent identifiers known as the reach code. This gives features an identity for inventory and analysis. Data can be linked to features such as the water chemistry for a particular lake. Many features also are labeled with the name of the feature, such as the Ohio River.

The network of lines contains linear measurements, making it possible to locate the position of a streamgage or dam along a stream or beside a lake. This is known as linear referencing and is similar to the address of a house on a street. By recording the measurements upstream on a reach code it is possible to uniquely identify any position along the Nation’s waterways. This makes it easier to perform calculations in a GIS such as identifying dams upstream from a streamgage, and then determining the distance to those streamgages. The system of linear referencing also makes it easy for any agency to link data to the NHD without having to customize the NHD.