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water balance of recommended flows serves as a first level check of the accuracy of the results. The investigator must also be concerned with the relationships between discharge, sediment yield, and channel structure. Channel dimensions are largely determined by the bankfull or dominant discharge. Changes in channel dimensions due to changes in the dominant discharge can be estimated through the use of hydraulic geometry equations. The shape and pattern of the channel are primarily determined by the amount and size of the sediment transported by the stream. Changes in shape and pattern are predictable when the sediment/ discharge ratio is altered, but actual quantification of a new channel structure requires the use of sediment transport models and considerable expertise. Discharge and channel structure combine to define the range of physical microhabitat conditions available to a species. Chapter 7 discusses the Physical Habitat Simulation System, with particular reference to microhabitat criteria, cover and substrate quantification, and the estimation of passage flows over natural barriers and through culverts. Several types of micro- habitat criteria can be used in the Physical Habitat Simulation System (PHABSIM): binary criteria, preference curves, multivariate suitability functions, and combinations of preference curves and suitability functions. All criteria are used to estimate a joint preference of a fish for a combina- tion of hydraulic and structural features at a specific location in a stream. The joint preference factor is found by multiplication of weighting factors for each variable when binary criteria or preference curves are used. The joint preference factor is computed directly when the multivariate suitability function is used. The advantages of the preference curve are that it can be constructed and modified by professional judgement and can represent very complex or discontinuous mathematical functions. The advantage of the multi- variate approach is the inclusion of interactions among variables in the joint preference factor. Cover and substrate are often represented by very complex, discontinuous functions. Therefore, preference curves are usually used to depict cover and substrate characteristics. The use of cover and substrate information in PHABSIM involves the development of a numerical code to depict various types and combinations of these characteristics, and a curve to describe the prefer- ences of an organism for each combination. Cover can also be treated as a discrete variable, with separate depth and velocity criteria associated with each cover type. Passage flows over natural barriers are evaluated by computing the width of stream meeting the clearance requirements of a species at each flow. Culverts present unique passage barriers and are evaluated by computing the time required for a fish to negotiate a culvert at different streamflows. Channel modification to enhance the physical structure of the stream is one way to increase or maintain the availability of habitat. This alternative is most feasible when channels have already been modified to increase water conveyance or when water supplies are so short that negotiation over instream flows will not succeed. Structural modifications to improve habitat include artificial cover devices, deflectors, weirs, and headgates. Nonstructural modifications include deepening pools, raising the elevation of riffles, ix