"yes" or "no," depending on the land use activities of the watershed. If the answer is 

 "yes," the user may proceed directly to the second module. If the answer is "no," the 

 investigator is faced with the problem of determining the direction and magnitude of 

 the change. Several avenues of analytical approach are available: 



1. Monitoring watershed and associated habitat changes over a period of years to 

 determine time trends. 



2. Using simple watershed models and equations to estimate water, sediment, and 

 chemical yields. 



3. Investing in independent modeling expertise to build a sophisticated watershed 

 model, or obtain the output of such models from other agencies. 



4. Measuring the stream characteristics in a similarly affected watershed, and 

 scaling those measurements up or down to fit the stream and watershed of 

 interest. 



Occasionally, an analyst may suspect watershed disequilibrium but is not able to 

 authoritatively say so. If the analyst subsequently proceeds to the next module, it 

 may have been falsely assumed even without saying so that the watershed is in 

 equilibrium. 



Warningsigns indicating disequilibrium include: (1) more rapid runoff resulting in 

 drastic high and low water levels of streams as compared with historical flow records; 

 (2) large amounts of nutrients and sediments lost from the terrestrial to the aquatic 

 component, often over short time periods; (3) increased fluctuations in stream 

 temperature; (4) increased streambank erosion as the stream attempts to reestablish 

 its equilibrium by channel cutting; and (5) decreased diversity and stability in the 

 biotic component. . .as a resuU of the less stable environment.^' 



Water quality. Water quality is a dominant macrohabitat feature which, on a 

 macro-scale, determines the longitudinal distribution of fishes and invertebrates in a 

 river system. This is consistent with the river continuum theory of aquatic ecology. 

 Theoretically, the distribution of water quality characteristics should be graded 

 through a river system. That is, the headwaters should start out with the lowest 

 temperature, lowest dissolved solids, and highest dissolved oxygen. As the river 

 descends through the watershed, the temperature should systematically increase, as 

 should the total dissolved organic and inorganic solids. If all systems operated this 

 way, and all dissolved solids were nondecaying, a simple dilution model would suffice 

 to relate flow regime to water quality. 



Unfortunately, the real world is not so simple. Whereas longitudinal inversions are 

 common in watersheds, they are the rule rather than the exception where water 

 quality is concerned. Temperature and inorganic dissolved solids are among the few 

 water quality parameters which often follow normal longitudinal gradations. 

 However, even temperature is subject to longitudinal inversions wherever an 

 abnormal heat source is present. 



Concentrations of nonconservative pollutants seem to be functions of many 

 system characteristics. Their initial concentrations are determined in the watershed, 

 and are subject to augmentation (point sources) and dilution as they move 

 downstream. However, as they move, they react with each other and with oxygen in 

 the water. The reaction rates are functions of temperature, oxygen concentration, 

 and initial concentration of the pollutant. These in turn are affected by travel time, 

 mix rates, and dilution which are functions of channel geometry and flow regime. 

 Therefore, when addressing water quality, it is virtually impossible to ignore 

 watershed and hydraulic features of the stream. 



As in the case of an assumed steady state for the watershed, it is frequently 

 "convenient" to ignore water quality or to assume that maintenance of adequate 

 water quality is guaranteed if sufficient flow is proved for fish habitat. For many 

 streams, this assumption is valid. However, for many others, water quality is either a 

 constraint on production, or will be under an altered flow regime. While some water 

 allocation studies can legitimately ignore water quality, it should be the starting point 



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