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degrel'-day simulation and used as input for the effective spawllIng and incubation analysis Then. thl' rh~ ..i..:al microhahitat simulations and the effective spawnmg and incubation programs are used as discussed above to deter- mine the composite areas suitable for spawning and incuba- tion during the time period. Such composite simulations subsequently illustrate the general success for alternative water management schemes for each year across a given baseline hydrologic time series. Channel Dynamics As a subject of riverine habitat studies, channel morphol- ogy represents a variety of possible definitions. The physi- cal configuration of a channel, both in cross section and plan view, is governed by several factors: the slope of the chan- nel, the materials (including vegetation) through which the channel is cut, the size and volume of sediments delivered to the channel, and the stream flows that occur in the stream. A change in any of these variables has the potential to effect a change in the shape, pattern, or other physical characteris- tics of the stream. In order to address the issue of channel morphology, one needs first to evaluate which of the aforementioned varia- bles might change, and then, which avenues of analysis are applicable. In some studies, only the flow regime will be changed, and the investigator can address the issue by deter- mining what flows would be needed to maintain the channel in its current configuration, the duration of such flows, and how often they need to be repeated. In other studies, only the volume of sediment input to the channel will be changed. In such cases, the channel mayor may not shift to a different configuration, but the size of the substrate materials may be altered. This may dictate a focus upon altering discharge to cleanse the substrate rather than upon maintenance of chan- nel morphology, per se. In other cases, several variables may be altered simultaneously, resulting in changes in chan- nel dimensions, as well as substrate composition. Such cases cannot be addressed by a "channel maintenance" philoso- phy, and require an analysis of what the future channel may look like. These are typically much more complex problems than those concerned only with maintaining the status quo. Channel Morphology A variety of empirical relationships have been derived to address aspects of channel morphology with respect to individual variables. These relationships are sometimes referred to as regime equations. The most familiar of these are the hydraulic geometry relationships described by Leopold et al. (1964): (5) v = k Q"' (6) d = c Qf (7) w = a Qh where Q is a representative discharge for the stream, v is the average velocity at the discharge Q, d is the mean depth at the discharge Q, and w is the stream top width at the dis- charge Q. The terms k, m, c, f, a, and b are regression coefficients. When channel morphology has been consid- ered in a riverine habitat study, the principal goal was usually to define a "channel maintenance" flow. The most common assumption made in determining this flow when a reservoir or diversion is proposed, is that the channel struc- 24 ture should be similar 10 that under pre-construction condi- lions v.hile stili m<lllltaining the caracit~ of the strcam to transpon the annual scdiment YIeld. The width and depth of a stream channel are often presented as power functions of a representative channel-forming discharge, The represen- tative discharge (Q) most often used in these equations is the mean annual streamflow. the mean of a time series of annual peak flows, or the bankfull discharge. In many situa- tions, the bankfull discharge is assumed to be the same as the annual peak flow with a return period of 1.5 yr. Nearly all of the existing methods for determining channel main- tenance flows are based on equation 7. The Tennant method (Tennant 1976) indicates that a high flow of twice the mean annual flow. assumed to approximate bankfull discharge, is required to retain the same channel geometry. An analytical procedure, following the same philosophy but not the same mathematical approach is described by the U.S. Forest Service (USFS 1986). This procedure was developed to estimate flow regimes that would be needed to preserve channel equilibrium if the volume of sediment entering the stream were unchanged. The use of the annual peak flow with a return period of 1.5 yr is based on stream morphology studies in unregulated streams (Leopold et al. 1964). This approach is used in riverine habitat studies to maintain the state of dynamic equilibrium currently existing. In some cases, a change in this condition produces better habitat conditions for some species, whereas such changes in other systems are deterimental. Some forms of disequilibrium, such as aggra- dation, tend to be detrimental to many aquatic species. The principal advantage of the channel maintenance approach is that it attempts to keep a channel in its current configuration, so that the impacts of disequilibrium need not be addressed. Unfortunately, there are many instances where such channel modifications are unpreventable. When it becomes necessary to consider changes in the channel morphology over time, we can reference the chan- nel morphology equations to the existing conditions and rewrite them as follows: (8) v = v Q"' o Qo (9) d = d Qf o Qo (10) w = w Qh o Qo where the subscript 0 refers to the existing conditions. If the assumption is made that the channel-forming dis- charge is the 1.5 yr or the mean annual peak flow, there will be no change in velocity. depth, and width - in other words no change in the channel morphology - when there is no change in the 1.5 year or mean annual peak flow. Values of the power coefficients for equations 8, 9. and 10 are shown for three streams in Table 3. Calculated changes in average velocity, depth, and width as a function of an assumed reduction of the peak annual flow are provided in Table 4. Many regime equations are derived through univariate regression analysis. This means that, in general, one could expect about the same regression slopes provided that there is some degree of similarity among the streams measured. The agreement among the coefficients in Table 3 may seem f~' 1; " ' "..""i"" .;J ,.....-;:;;~ -~~ .'~:,;i~ - - ---.~<N.... ...:2 ~-:;~ .c.:~ qs "~~f# ,~~