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. managed. Sedimelllallon Illl'deb such as HEC-o (Hydrolog- ical Engineering Center 1976) h:wc heen developed to simu- laIc the change in channel form as a funcIlon of strellmflo'A over time, Predictions of channel response to changing flow regimes can be simulated fairly well in alluvial sand oed streams by using HEC-o. hut the HEC-o model cannot yet provide reasonable results in gravel-bed rivers. Improved transport functions for gravel bed streams need to be devel- oped. See Milhous et at. (1986) for a discussion of this problem. . Using phYSIcal process models, it will be possible to determine: . changes in stream bed elevations in sand bed streams due to alterations of sediment supply and/or flow regime. . flushing flows for removal of fines from the bed material in gravel bed streams . flow required to remove sprouting vegetation from bend and point bars. . flows required to transport sediment delivered to a river segment by increased sediment production from upstream sources such as tributaries. Flushing Flows In the normal course of events, in unregulated rivers, fines are deposited in and on the gravel substrate during low flows and are resuspended from the gravel substrate during higher flows. In many regulated rivers, this flushing of fines either does not occur or occurs infrequently. The purpose of a "flushing flow" management scheme, therefore, is to maintain the substrate in a healthy condition during biologi- cally critical times of the year. Most authors, when discuss- ing flow methods, have interchanged channel maintenance and flushing concepts. Hushing flows are of most concern in gravel-bed streams that transport fines in suspension. Many of the aquatic organisms in a gravel bed river require a clean gravel sub- strate for some of their life processes. For example,gravel is used for spawning and egg incubation by trout and salmon. As the interstitial spaces or voids fill with fines, many of these life processes cannot continue. Rieser et al. (1985) have recently reviewed flushing flow requirements for stream fishes, the stream flows required to maintain relatively silt free surface or interstitial spaces in gravel bed substrates. Typically, the necessity for regu- lated flushing flows occurs below a reservoir with sufficient capacity to significantly reduce peak flood flows, alter sedi- ment transport, or both. Milhous (1982b) and Milhous et al. (1986) postulated two general processes for deposition of fine material in gravel streams. The first assumes that fines are deposited predominantly on the surface and the second that they are deposited within the inteJ:stitial voids of the gravel particles and must be periodically resuspended. Flushing flows for both, involve a movement parameter, {J defined as follows: (23) {J =~~ RS (G. - I)Dso where R = the hydraulic radius, S = the energy slope, G. = the specific gravity of the bed material, and D50 = the median particle size of the bed surface material (armor layer) Field observations suggest that the bed surface needs only to be slightly disturbed to move fines from the surface. This movement has heen found to occur at a (J value of 0.021 fMilhous et at. ILJR6) In contrast. the armor particle on the hed surface would have to rw moved significantly to remove fines from the voids withlll the gravel. Milhous et a1. (1986) recommended a f3 value of at least 0,030. and preferably 0.035. for interstitial flushing in streams which tend to armour. The relation between the movement parameter f3 and discharge, in the Williams Fort River, Colorado, is given in Fig. 10. Using f3 values of 0.021 and 0.035, respec- tively. Fig. 10 indicates that surface flushing occurs at about 7.1 m3.s-1 and interstitial flushing at about 18.3 m3.s-l. Unless the watershed upstream is disturbed, the Williams Fork River should require only periodic surface flushing, because the suspended sediment load is small and the sus- pended particles are relatively large, judging by watershed geology. It must be emphasized, however, that a flushing flow has a different purpose than a channel maintenance flow. A discharge of 7. I m3. s - I may keep the streambed clean, but may not be sufficient to scour out pools, prevent vegetation encroachment, or avoid disequilibrium. It is more likely that a higher flow would be needed to maintain the current channel configuration at current rates of sedi- ment input. If sediment were totally interrupted, then a flow of 7.1 m3.s-1 would be more appropriate, although the stream dimensions would probably shift to fit this new domi- nant discharge. In this case, a decision must be made as to which is more important to preserve: channel dimensions or substrate composition. Under this scenario, it is unlikely that both can be achieved. O.OB -. ~ c: 13 RS w 0,06 I- (G. - 1>050 W ::E c( c: c( 0.04 D. I- Z w ::E Depth w 0.02 Flushing > 0 ::E Surface Flushing 0,00 1.0 5 10 50 DISCHARGE <m3'S-1) FIG, 10. Movement parameter ((3) versus discharge in the Wil- liams Fork River, Colorado (from Milhous et al. 1986). Conclusions Habitat analyses in large and small rivers have many com- mon elements, which may give the appearance that the same procedures are followed, regardless of stream size. Certain aspects of these studies, such as hydraulic simulation, may actually be easier in large rivers because the hydraulic characteristics are often less variable over time and space. Closer examination reveals that there are numerous critical distinctions between large and small river habitat analyses. One of the most important size-related differences is the simulation of nose velocities, rather than mean column velocities for demersal species. If the subject species 27 ! ~. ~'~l