Microhabitat methods. These methods differ from the threshold methods above 

 in that the species criteria are often weighted and a stream reach is described in terms 

 of the spatial distribution of the hydraulic parameters of depth and velocity over 

 suitable substrate. The areal extent of suitable habitat vs. discharge is easily 

 determined for several different discharges. Maximum area of suitable habitat vs. 

 discharge is also easily determined from these analyses, but any selection of minimum 

 levels of flow is quite subjective. 



Fishery scientists in the Pacific Northwest developed an approach which uses a 

 series of overlay maps, delineating areas where the depth, velocity, and substrate are 

 within the preferred ranges for spawning salmon. The ranges for these criteria are 

 termed "binary" criteria, where the utility of a variable takes on the integer value of 

 zero, or one, depending on whether or not it falls within the preferred range of the 

 animal. Areas of intersection of all three preferred ranges are identified and meas- 

 ured by planimeter. This procedure is repeated for a range of discharges, and a plot of 

 discharge vs. preferred spawning is developed.'^ The U.S. Geological Survey, 

 Tacoma, Washington, has developed a computer program (DVA TRAPES! ARRAY) 

 used in producing these plots for the Washington Department of Fisheries and 

 Game. Although time consuming, the method is straightforward and fairly simple in 

 design (necessary attributes for presentation to water administrators). Since 

 calculations are based upon empirical field data and are graphical in nature, the 

 results of the method are easily understood. 



A refinement in the early 1970s was an outgrowth of work initiated by the 

 California Department of Fish and Game. The basic concept is the same except that 

 they substituted for binary criteria, weighting factors which ranged from zero to one, 

 to represent the relative habitat values of the three stream attributes to obtain an 

 equivalent "optimum quality streambed area."'* These weighting factors could be 

 varied as a function of species, life stage, or principal food organisms. They could be 

 estimated for many species from information available in the literature and from 

 professional judgment; but for some species, this information could be obtained only 

 from new research. 



The principal drawback to the physical habitat methodologies was the intensive 

 labor needed to acquire hydraulic information from the stream. During 1976, several 

 researchers reported upon the use of hydraulic modeling techniques to simulate 

 hydraulic conditions at unobserved discharges and minimize time in the field. 19,20,21,22,23 

 However, at that time the use of hydraulic simulation modeling for habitat analysis 

 was in its infancy and simply a "spinoff of flood routing models. The hydraulic 

 models available could best be described as macrohabitat models, giving output in 

 terms of depth, mean velocity, and wetted perimeter at a cross section. As such, the 

 models were not precise enough for the in-depth microhabitat quantification 

 practiced in the Pacific Northwest and California and most were used for threshold 

 analysis only. 



THE INSTREAM FLOW INCREMENTAL METHODOLOGY 



During 1977 and 1978, the Cooperative Instream Flow Service Group (IFG) in 

 Fort Collins, Colorado, took on a major role of synthesis, documentation, and 

 refinement of training relating to physical habitat analyses. This effort was set first in 

 a hierarchical framework of macro- and micro-stream habitat considerations. 

 Secondly, it utilized a modular approach upon which to focus and set the boundaries 

 of the problem studied. Finally, the progression from initial planning to system 

 management and operation was used to identify the level of precision which, along 

 with the level of measured or simulated detail, determined the degree of sophistica- 

 tion of analysis along levels compatible to the level A, B and C studies described by 

 the Water Resources Council. *''2'' Since this methodology is generally accepted as the 

 state-of-the-art site specific, or Level C approach, it is discussed in some detail below. 



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