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,i PURPOSE AND SCOPE The purpose of this paper is to familiarize the field investigator with the theoretical aspects of hydraulic simulations and limitations to various approaches. Discussions of the data requirements, site limi- tations, and level of reliability of different approaches will aid the investigator in the selection of the appropriate technique to,meet the level of accuracy and reliability required for individual situations. The approach utiTized should have a level of reliability in output which is consistent with the overall approach to the determination of the value of insteam flows. Descriptions of various approaches are designed for the technical nonengineer. Therefore, descriptions and discussions of the theoretical aspects of hydraulic simulation have been generalized and simplified somewhat. Conversely, the discussions of site suitabilities and limitations to methods imposed by site selection are very practical matters for the field investigator, and are discussed in considerable detail. Finally, data requirements and specifications for various approaches are given to aid the investigator in establishing a work plan and to ensure maximum preservation of reliability for any given approach. Theoretical considerations for hydraulic simulation are discussed as two separate processes: (I) the determination of the stage-discharge relationship, and (2) the determination of the- velocity distribution- discharge relationship.. The discussion of the theoretical approaches of determining these relationships is followed by a discussion of the potential limitations and relative errors associated with the different approaches. Methodological concepts and limitations to approaches associated with site selection are introduced in a subsequent discussion. A final section discusses the data requirements and accuracy specifications for different approaches. Data collection procedures and equipment requirements are contained-in several appendices, designed to aid the field practitioner unfamiliar with the data collection techniques involved with the various approaches. More detailed expositions of these topics may be found by review of the references included with this paper. INTRODUCTION The purpose of this paper is to describe the. theory and data requirements for hydraulic simulation models. However, before the "how" of modeling is discussed, it is appropriate to discuss modeling and its use. Whether we choose to admit it or not, we all model at one time or another. A biologist standing on the bank of a river may visually assess the fishery habitat of a river at a particular floc and form an opinion about the fish's response to some alternate stream discharge. Having never been a fish, the biologist translates the things he can see (depth r