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illustrates a river reach with initial transect placement overthe <br />hydraulic controls. The placement of these transects ensures better <br />hydraulic predictability. If such placement can be made to correspond to <br />a major type of habitat in the stream, so much the better. <br />Having placed the control transects, the next step is to place <br />transects which sample major habitat types in the study area which were <br />missed in the initial placement. At this stage the field crew must resist <br />the temptation of proliferating the area with cross sections- As a <br />general rule, never use two cross sections where one would suffice. <br />Figure 18 shows the hypothetical study area with the-addition of transects <br />delineating major habitat. types. <br />Depending on the level of detail desired, additional transects may be <br />added to define the transition zones from one type of habitat to another. <br />During this phase of study area setup, the tendency to dot the landscape <br />with headstakes becomes very strong and must be resisted with an iron <br />will. Remember that the greater the detail with which a reach is modeled, <br />the more dissimilar from adjacent reaches it becomes. In the hypothetical <br />reach we have used as an example, two additional transects could be added <br />to delineate the transitions around the pool, as shown in Figure 19. <br />Additional cross sections could have been added, but would not have <br />contributed much to the description of the reach. <br />The final suggestion concerning the placement of transects deals <br />with location of transitional transects at the heads of pools. It is <br />recommended that these transects be placed well into the transition zone, <br />toward the pool. At high flows this transition zone will be a <br />distinctively pool-type habitat. At low flows, the transition zone will <br />be more riffle-like. Thus, the "head" of the pool will migrate up and <br />downstream in accordance to the discharge. Additionally, these zones are <br />especially prone to the development of eddies. An eddy represents an area <br />of negative velocity (i.e. the direction of flow is "upstream"). While <br />these areas are of great biological importance in many streams, they are <br />extremely difficult to represent adequately in a simulation model. <br />SITE IMPOSED LIMITATIONS TO HYDRAULIC SIMULATION <br />The discussion of the theory of hydraulic simulation mentioned <br />several general limitations to various approaches. This section <br />addresses limitations to the capabilities of hydraulic simulation <br />approaches which are site-imposed. Initially, limitations have been <br />classified as either "problems of complexity" or "problems of stability." <br />Conml ex Channels <br />For the purposes of this paper, a complex channel.is defined as one <br />of the following: <br /> <br />sf