How are river corridor boundaries determined?

Corridors may not exceed an average of 320 acres per river mile over the designated portion of the river (except on certain other rivers as specified by Congress and in Alaska, which is 640 acres for rivers located outside national parks). Agencies delineate boundaries based on natural or manmade features (canyon rims, roads and ridge tops, etc.) and legally identifiable property lines.

Interagency Wild & Scenic Rivers Council