This is the Department's legacy master stream coverage. The last updates to this layer occurred in late 2004. This layer has been replaced by the High Resolution National Hydrography Dataset (NHD) and as such should only be used to aid in identifying legacy stream codes. This layer was digitized at a scale of 1/24,000. All streams are connected and have flow direction. Each stream is identified by a unique 5-digit stream code found in the WRDS (Water Resource Data System) field. The streams are further divided into segments. The first stream segment flows from the headwaters to the first tributary. Subsequent segments are tributary to tributary. The final segment flows from the last tributary to the mouth which for purposes of this data layer is defined as the point at which the stream crosses the Pennsylvania boundary. The unique segment identifier is constructed from three concatenated fields. The stream code, downstream river mile, and finally the upstream river mile of the segment boundaries (12345_8.000_ 12.000). The river miles are measured from the mouth, at the Pennsylvania boundary, upstream to the headwaters.