What is the difference between river areas and river segments?

In agency parlance used for planning purposes, river “areas” include the entire length of a study or designated river and its adjacent lands, an average of 320 acres per river mile (except on certain other rivers as specified by Congress and in Alaska, which is 640 acres for rivers located outside national parks). A river “segment” is a portion of the river area which has been delineated for evaluation and planning purposes. Its subsequent classification is dependent upon the level of development of the shoreline, watercourse and access at the time of designation. Significantly different levels of development within the river area help define appropriate termini for river segments. (Refer to the Interagency Guidelines.)

Interagency Wild & Scenic Rivers Council