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" on physical and chemical vanables ha\;e been developed to descrioe and simulate the dynamics of the riverine habitat over t Imc and spacc. The physical prll\.:cssc~ of hydrology. hydraulics. and sedimentation. and their integration. pro- vide the theoretical basis for understanding the habitat con- sequences of river manipulations and management and can be used to formulate hypotheses of biological responses that should be the focus of future research (Trihey and Stalnaker 1985) . The primary purpose of habitat simulation is to describe the relationships between the streamflow and the usable quantities of the physical habitat in the stream. The function of physical habitat versus streamflow can be used not only to simulate habitat conditions over time but also as a sur- rogate for an economic production function in water man- agement decisions. The physical habitat represents the space in a river that can be used by a specific species and life stage of fish. The assumptions and calculation procedures used to determine one form of a physical microhabitat vs. flow function were described by Stalnaker (1979). The river sur- face area usable for certain recreational activities can also be simulated as a function of streamflow (Scott and Hyra 19771; Fritschen et al. 1984). . -~ Definitions Large vs. Small Rivers Stream classification has been a subject of several publi- cations where particular interest was focused on fluvial processes and sedimentation in river geomorphology (Leopold et al. 1964; Morisawa 1968; Dunne and Leopold 1978). This literature illustrates that the characterization of rivers as large or small is very arbitrary, since the geometry and hydraulic aspects of streams are often similar in small wadable streams and large deep rivers. We make no distinction here between large and small rivers insofar as the physical processes are concerned. Large rivers may have beds of gravel or sand; they may be cold or warm; and the gradient may be high or low. From a fishery biologist's viewpoint, large rivers are a matter of scale - and scale becomes important when one describes and measures important habitat variables. In practical terms, we consider any stream large if it has an average depth over 1 m and requires measurements to be taken from a boat with specialized gear. Another generality related to working on rivers is that the larger the river, the more specialized the gear requirements. Although there are many similarities between habitat s~mulation~ in ~l rivers, several characteristics of large flvers require different procedures from those used in small rivers: the vertical velocity distribution is usually more important in large rivers; much ofthe information on habitat tolerances and preferences of species are derived from small rivers, and may require transformation or extension before they are applicable to large ones; organisms in large rivers ~ Scott, J. W., and R. Hyra. 1977. Methods for determining mstream flow requirements for selected recreational activities in small and medium size streams. Paper presented to the 13th annual water resources association conference in Tucson, AZ. Available from ~quatic Systems Branch, National Ecology Center, 2627 Redwmg Road, Fort Collins, CO 80526-2899, USA. 14 often show a greater tendency tov.:ard haoitat zonation and isolation and the use of special1zed haoltat types, ~md thv emphasis on specialized hahitJh t-y these "re"oiL's ma\ reqUire more of a twc).dimenslOnal analYSIS of tcmfX'ralUrt: and water quality than is typically needed in small streams. Gravel vs. Sand-Bed Rivers The principal difference between a gta~el-bed river and a sand-bed river is the size of the bed material and the move- ment of sediment in the bed. In a sand-bed river, the bed is moving continually except during a few low flow periods. In contrast, a gravel-bed river is stable except during rela- tively high flows. Critical discharge levels capable of bed movement are consequently related to the size of the gravel in the stream. The ecological consideration of fines (particles < 1 mm diameter) differs between gravel and sand-bed rivers. In a sand-bed river the fines are continually cycled as deposits into the bed and then resuspended. In a gravel-bed river, the fines are absorbed into the bed gravels when the flow is low and are flushed from the gravel when flows exceed a certain discharge. This" flushing flow" is related to the size of the particles on the bed surface. Habitat Models The many stream habitat models developed during the past 30 years have been primarily responsive to particular threats to the stream fishery. In general. the models fall into one of four categories: physical microhabitat; water qual- ity; cover and channel structure; and the general .. health" of the stream substrate. These models focus on a few varia- bles that are assumed to be important to the success of river- ine fish and invertebrate species and that are used to assess specific potentially detrimental activities: streamflow alter- ation; pollution; channelization and realignment; and tim- bering, grazing, farming, and other sediment producing land-uses. The variables chosen for each class of models have been selected primarily because they are known to be significantly altered by the development activity and also are important from a biological perspective. The biological importance is usually supported by basic research of species responses to changes of the variables as reflected by changes in behavior, mortality, growth, reproductive success, or year-class strength. Important research continues to contribute to the habitat concepts of stream ecology in the form of multiple regres- sion analyses, in which fish standing crop is the principal dependent variable. Such regression models (Binns and Eiserman 1979) show that certain stream habitat variables are consistently important determinants of the condition or success of fish populations. These habitat models were recently reviewed by Fausch et al. (1988). Microhabitat, macrohabitat. and habitat - terms com- monly seen in literature on fluvial environments - are often used interchangeably. Gosse (1982)2 defined microhabitat 2Gosse, J. C. 1982. Microhabitat of rainbow and cutthroat trout in the Green River below Flaming Gorge Dam, Vol. I. Utah Divi- sion of Wildlife Resources, Contract #8/5049. 103 p. Available from Aqua-Tech Biological Consulting Firm, Logan, Utah 845321, USA.