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, .' ., occupies a mid-column position or is pelagic. there may he little difference hetween the mean column and nose veloci- ties, However. simulation of mean column vehlCIlies for a demersal species can have two detrimental results, First. there will be an overall tendency for the amount of available habitat to be underestimated for the higher flows. because the mean column velocity will consistently be too fast for the fish. Second, when usable microhabitat is plotted against discharge, the resulting curve will peak at a relatively low discharge, and will decrease rapidly at higher flows. Use of nose velocities typically results in a more robust habitat vs. discharge relationship (i.e., a broader curve) which may peak at a higher flow than one based on mean column veloci- ties. We have avoided a definition of what differentiates a large river from a small one. Many investigators would probably make this distinction on the basis of channel width. mean annual discharge, drainage area, or some other size-related characteristic. Based on the foregoing discussion, the best distinguishing characteristic may be channel depth, as this will determine the vertical location of the mean column velocity measurement. When measuring stream velocities, as a rule of thumb, hydrologists change from a single mea- surement at 0.6 of the depth to an average of two measure- ments at 0.2 and 0.8 of the depth, when the depth exceeds about 1 m. Therefore, it is suggested that whenever the depth exceeds one meter, nose velocities be used instead of mean column velocities, and that this is one criterion differentiating small from large rivers. Based on this stan- dard, there are relatively few small streams in North America. However, there is one other characteristic that distin- guishes truly large rivers - the relative importance of specialized or isolated habitat types associated with the river margin. In small and medium-sized streams, the same spe- cies is found at suitable locations throughout the stream. There may be much more habitat partitioning in larger rivers, with certain species associated only with edge habitats, and others found almost exclusively in mid-stream. It should be noted that this tendency is related more to the combined effects of physical and hydraulic structure than to the size of the channel. If a stream that would otherwise be classified as large has the same structural diversity of a small stream, the same species will probably be scattered through- out the channel. However, many large rivers tend to have a definite zonation, consisting of a hydraulically efficient (and often biologically devoid) main channel, and the bio- logically rich zones associated with the edges. In contrast, the biologically rich zones tend to overlap in small rivers. In many large rivers, the role of streamflow in the main channel is little more than providing water to the zones occupied by the fish. The determination and simulation of flow-induced habitat fluctuations in these specialized habitat areas provides the challenge in large-river investigations, because of their tendency to be hydraulically controlled by a variable backwater. The hydraulic complexities of these areas can prove to be difficult to simulate, but are rarely insurmountable. The more perplexing problem is that many instream flow investigators do not (or cannot) recognize variable backwaters when they see them. This emphasizes the advantage of including expertise in hydraulics, hydrol- ogy, and biology on any riverine habitat analysis team. The determination of instream flow requirements to pro- 28 vide suitable habitat for lishes can be assisted by use of physical habitat versus srreamllow functions and the analy- sis of instream benefits produced by alternative flow regimes. These can be compared to benetits from diversion- ar~ uses in order to obtain a reasonable mix of uses as the !!~al of the allocation of the water resource. ~ If a habitat versus streamflow relationship is used as a sur- rogate production function. a water management project can be formulated to include instream flows as an equal among various project purposes in the overall project production function (MiIhous 1983). In contrast, if the function is used only for the evaluation of impacts, the resulting instream flow requirements are treated as constraints on the alloca- tion of water to "useful" purposes (Henrikson 1980; Hoff- man 1980; Christiano5). The use of the habitat-streamflow function in a reservoir analysis was given by Milhous (1982a), Sale et al. (1982), and Olive and Lamb (1984). I t i ~ I t i f 1 : References ANNEAR, T. C., AND A. L. CONDER. 1984. Relative bias of several fisheries instream flow methods. N. Am. J. Fish. Manage. 4: 531-539. AMERICAN SOCIETY OF CIVIL ENGINEERS. 1962. Nomenclature of hydraulics. ASCE manuals and reports on Engineering Prac- tice No. 43. New York, NY. 501 p. BAlDRIGE, J. E., AND D. AMOS. 1981. A technique for determin- ing fish habitat suitability criteria: a comparison between hab- itat utilization and availability, p.251-258. In N. B. Armentrout [ed.] Acquisition and util ization of aquatic habitat inventory information. Am. Fish. Soc., Bethesda, MD. BINNS, N. A., AND F. M. EISERMAN. 1979. Quantification of flu- vial trout habitat in Wyoming. Trans. Am. Fish. Soc. 108: 215-228. BOVEE, K. D. 1982. A guide to stream habitat analysis using the instream flow incremental methodology. Instream Flow Information Paper No. 12. U.S. Fish Wild\. Servo FWS/0BS-82/26. 248 p. 1985. Evaluation of effects of hydropeaking on aquatic macro invertebrates using PHABSIM, p. 236-241. In F. W. Olson, R. G. White. and R. H. Hamre [ed.] Proc. Symp. on small hydropower and fisheries. Am. Fish. Soc., Bethesda, MD. 1986. Development and evaluation of habitat suitability criteria for use in the instream flow incremental methodology . Instream Flow Information Paper No. 21. U.S. Fish Wildl. Serv. BioI. Rep. 86(7): 235 p. BRAY, D. I. 1982. Regime equations for gravel-bed rivers, p. 517-542. In R. D. Hay, J. C. Bathurst, and C. R. Thorne [ed.] Gravel-bed rivers. John Wiley and Sons, London. BRUNGS, W. A., AND B. R. JONES. 1977. Temperature criteria for freshwater fish: protocol and procedures. U.S. Envi- ronmental Protection Agency, Ecol. Res. Series. EPA-600/3-77-061. BURNS, C. V. 1971. Kansas streamflow characteristics, Part 8, In-channel hydraulic geometry of streams in Kansas. Kansas Water Resour. Bd., Tech. Rep. 8. CAIRNS, J., JR. 1%8. Suspended solids standards for the protec- tion of aquatic organisms. Purdue Univ. Eng. Bull. 129(1): 16-27. 5 CHRISTIANO, D. J. 1981. Negotiating for instream water. Presented at the 1981 Conference of the American Water Works Association, St. Louis, MO, USA.