for others. For some situations, consideration of water quality may enter the decision 

 process at several points in the form of feedback loops (Figure 2). 



The term "water quality" encompasses a wide variety of chemical and physical 

 constituents of the water. In many cases, the potential limiting effects of water quality 

 may be determined by a simple screening procedure. Such procedures basically give a 

 "yes" or "no" answer to the question of the adequacy of water quality. Two important 

 aspects are addressed to answer the questions: (1) determination of constituent 

 concentrations, and (2) evaluation of the significance of those concentrations. 



Records may be used to determine both spatial and flow-related changes in water 

 quality. With no anticipated changes in source loading, such an empirical data base 

 may be used directly to determine concentrations of various constituents at different 

 streamflows. In many cases, such information is not available, and in others, changes 

 in streamflows may be accompanied by changes in source loadings. In these cases, 

 some type of water quality model will be required to evaluate constituent 

 concentrations. 



While the state-of-the-art in water quality concentration modeling has achieved a 

 high degree of sophistication, the same cannot be said regarding the development of 

 water quality biological evaluation criteria. ^^ po^ the most part, biological criteria 

 have been developed through the use of laboratory bioassays (See Mount and Gillett, 

 this monograph). This type of controlled testing may have little relevance beyond 

 defining threshold tolerances to animals in nature that are subjected to a variety of 

 simultaneous stresses. Should water quality constituent concentrations fall within 

 the criteria bounds this does not necessarily mean that no problem exists. Species 

 growth and behavioral responses are the least documented in terms of present 

 promulgated water quality standards. 



Channel structure. Channel structure refers to features of the channel which 

 provide resting and feeding areas for fish and fish-food organisms. These features 

 include channel morphology and alignment, substrate size and distribution and 

 cover characteristics. 



The size and shape of a channel is a function of the geology of the area through 

 which the stream flows, and of the flood flows carried by the stream. The alignment is 

 often a function of the watershed characteristics, but is frequently altered by man's 

 activity within the channel. Substrate size within the channel is dominated by the 

 sediment yield from the watershed. The distribution of various substrate sizes in the 

 stream is a function of both the yield and channel hydraulics. 



Channel structure may affect the biological community directly through changes 

 in sediment and cover characteristics. Indirect effects, primarily due to changes in 

 channel size, shape, or alignment, are caused by redistribution of depths and 

 velocities through the reach rendering the reach more or less usable by the organism 

 of interest. 



The contribution (yield) of water and sediment from the watershed to the stream 

 system, in addition to providing the energy source (coarse particulate organic matter) 

 and influencing the chemical quality, defines a dynamic equilibrium state with the 

 stream channel structure. Disturbances upon the watershed often upset this 

 equilibrium, resulting in a dramatic shift in channel form and sediment transport to 

 compensate and move toward a new equilibrium state. Frequently, such a disturb- 

 ance will cause changes in all three aspects of channel structure. However, it is 

 possible to retain its present shape, yet experience changes in sediment size. Converse- 

 ly, in many channel realignment (channelization) cases, the substrate size remains 

 approximately constant, but the shape and alignment of the stream is radically 

 altered. 



Modification of the flow regime, with or without a watershed disturbance, may 

 also upset the sediment-water equilibrium with similar results. A frequent mistake 

 made during instream flow studies is to recommend a flow regime which is 

 satisfactory from a microhabitat standpoint without checking to make sure the flow 



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