The original subject area for the study was the Trinity River Basin as shown below, but it was decided that a subject area this large was unnecessary. Therefore, the HUC units 12030201 and 12030202 were chosen as the focus area based on the needs of the Append Aml and the desire to explore the linear address of two points of interest. The graphic below shows the river system of the Trinity Basin color coded by HUC unit. The NHD describes naturally occurring and constructed bodies of water and their paths. The organization of the NHD river system is broken up into two components, features and reaches, with each element having unique common identifier.
Features represent hydrographic elements such as paths through water flows and bodies of water.
They are each classified by a specific Feature Type, which has its own unique Feature Code. Features can be either points, lines, or areas. Groups of features then form reaches. Reaches are composed of hydrographic entities which have similar hydrologic characteristics. Each reach also has a unique identifying reach code.