regime is sufficient to maintain the channel in its present form. Thus, maintaining a 

 stability of microhabitat by guaranteeing a flow regime may impose an instability in 

 the same microhabitat by modifying the channel shape and alignment. 



The equilibrium status of a channel may often be determined by a screening 

 process (as for water quality). Perhaps the easiest technique is to obtain United States 

 Geological Survey stage-discharge rating curves for 5 to 1 years for gauging stations 

 along the stream. An analysis of these rating curves can indicate equilibrium if the 

 same rating curve has been used for several years to predict discharge from stage 

 readings. Persistent changes due to aggradation or degradation of the bed are 

 apparent in the frequency of change in the rating curves. 



If the channel geometry and substrate are in equilibrium, they can be empirically 

 described and interface with streamflow in hydraulic models. Likewise, if the channel 

 shape is to be deliberately changed by channelization or by habitat improvement 

 techniques, the channel shape, substrate, and cover characteristics may be designed 

 and defined. 



Where channel structure is not in equilibrium, the analyst is not helpless in 

 assessing impacts of channel change. In those cases, several options exist: 



1. The system may be monitored over a period of years to determine time trends. 



2. Sediment routing on the macrohabitat level may also be determined empiri- 

 cally. This may be accomplished by sampling the suspended load and bedload 

 entering and leaving a reach of stream. A sediment-discharge rating curve is 

 thus constructed for each segment or reach boundary. From these empirically 

 based curves one can determine the flows at which coarse sediment (it is 

 necessary to segregate coarse load from wash load) is either scoured or 

 deposited within the reach. However, the source of scour and areas of 

 deposition can only be estimated. Simple mass balance equations can then be 

 used to approximate bed elevation changes within the reach. 



3. A state-of-the-art type sediment routing model may be applied to roughly 

 determine the amount of scour or deposition within a segment of stream. 

 Resultant streambed particle size may also be estimated. From this analysis 

 (meta-morphology) a new channel geometry assuming the same alignment 

 may be defined and passed on the Module 4 for microhabitat analysis. 



Microhabitat simulation To reiterate, watershed and water quality characteris- 

 tics are primarily longitudinal (macrohabitat) determinants offish distribution and 

 abundance. Channel structure and flow regime were discussed as they operate both 

 on the macrohabitat level and microhabitat level. The geographic distribution of a 

 species has been presented as a result of its interaction with its macrohabitat. 

 However, within a segment of the macrohabitat where the longitudinal habitat 

 characteristics are essentially homogeneous, fishes and invertebrates tend to select 

 those microhabitat conditions most favorable to a particular species and size class. 



Fishes have been shown to utilize instream habitat in a three-dimensional fashion 

 which is determined by the interaction of channel structure, depth, and velocity. 2" 

 For some fish species, "instream" or "overhead" cover further dictates distribution 

 within a reach. The distribution of the flow parameters, depth, and velocity within a 

 stream reach is very much a function of flow mechanics, sedimentology, and the 

 channel form. 



Therefore, the quantification of physical microhabitat must be addressed in a 

 manner similar to the process used in water quality. First, changes in the physical 

 environment must be identified and described. Second, the significance of those 

 microhabitat changes in terms of their usability to the target species must be 

 determined. It is this deterministic process which relates streamflow to the quantity 

 and quality of microhabitat in the stream that is central to the state-of-the-art 

 physical microhabitat analysis. 



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