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3 . o cr: CJl c."") SECTION 13(a) ASSESSMENTS ORIENTATION TO RIVER SYSTEMS Section 13(a) water availability assessments are addressed to river sys- tems. River systems can be divided into natural components (i.e., flowing reaches, reservoi rs, deltas, etc). Section 13( a) assessments also are intended to quantitatively describe the potential impacts of such developments within different river basins -- without gathering site-specific data. For planning purposes, the U.S, Water Resources Council has deline- ated 18 Water Resources Regions based on the major drainage basins of the 48 states. Each of these regions has been further divided (for purposes of the proposed methodology) into Riverine Analysis Areas superimposed on the National Water Data Network Accounting Units of the Water Resources Council, The number of RAA's was minimized by conforming to subregion and accounting unit boundaries where feasible. A Technical Advisory Panel (TAP), drawn from a number of organiza- tions with expertise and interest in stream resources, contributed to the development of the Riverine Analysis Areas and to the methodology in general. FOCUS . The section 13(a) assessments are of a reconnaissance nature conducted for the purpose of evaluating the relative impacts among different geo- graphical regions given the same proposed development. The methodol- ogy discussed here considers only physical changes in the stream sys- tem and the projected effects of these changes upon the more important fish species and river-oriented recreational activities. The more subtle changes in chemical water quality associated with specific processing plants requires a much more detailed analysis and is consequently high- ly data-intensive. Such analyses are nonetheless very important and must be addressed in section 13(b) and (c) assessments. Figure 1 pre- sents a typical section 13( a) data projection. CONCEPTS Representative Reaches The selection and conceptual simulation of representative stream reaches is paramount to the successful application of the methodology. Because any RAA is likely to contain many streams of differing sizes and morph- ologies, certain procedures and criteria must be followed to ensure that the streams simulated to represent an RAA truly are representative of conditions in that area. These procedures are outlined below and de- scribed in more detail on pages 6 to , .