Incremental L ogic 



The Incremental Methodology is a structured procedure providing a series of 

 decision points and feedback loops that connect the major habitat components 

 elaborated by Gorman and Karr.^' The methodology itself is incremental because it is 

 possible to start with some set of initial conditions, vary conditions slightly in any 

 module, and determine the impact of the fluctuation. More specifically, the meth- 

 odology is designed for iterative use of a large number of initial conditions. 



For example, initial conditions might be: 



1. Watershed unaltered and stable. 



2. Water quality marginal but within bounds of criteria. 



3. Channel structure in equilibrium with primarily a cobble bed containing 

 extensive deposits of 10-25 mm gravel. 



Given this set of initial conditions, one might proceed directly to calculations 

 utilizing PHABSIM and determine habitat conditions for a median (l-in-2 year) flow 

 regime. By evaluating the input discharges, it might be possible to determine a flow 

 regime which requires less instream flow than the median flow regime, yet provides 

 sufficient instream habitat. 



For example, if by rerunning these habitat maintenance discharges through the 

 water quality module, it is determined that this flow regime will result in dissolved 

 oxygen concentrations during July and August which are too low when all additional 

 flows were diverted from the stream. Two solutions could be explored: 



1. Incrementally increase the flow until satisfactory levels of the dissolved oxygen 

 concentrations are attained. 



2. Incrementally increase the level of treatment to lower the biological oxygen 

 demand concentration in the river. Various combinations of streamflow and 

 levels of treatment should be investigated. 



Further investigation may also show that the flow levels providing good fish 

 habitat during February and June would allow deposition of sand-sized material 

 over the gravel bars during the two peak sediment production months. Sedimenta- 

 tion resulting in the replacement of gravel with sand substrate in PHABSIM shows 

 that while little effect on adult and juvenile fish habitat usability occurs, spawning 

 and insect production will be radically curtailed. Therefore, it is determined that 

 prevention of this sedimentation is desirable. 



The flow during these two months is then increased until a flow level is found which 

 is sufficient to prevent the deposition of sand on the gravel bars. However, on 

 running these flows through PHABSIM, it may be found that the flows required for 

 sediment transport are detrimental to newly hatched fry. This could be true since the 

 effect of increased flows beyond the threshold level for transport of sand floods out 

 shallow backwaters along the stream margin which provide the microhabitat 

 required by the young fish. By continued iteration, it is possible to determine the flow 

 regime which provides both fish habitat and sediment transport capability. 



Not all scenarios will work out this nicely since it is not always possible to identify 

 flows which will accommodate several uses at once. In such cases, some form of 

 trade-off decision, or alternative management plan, must be formulated. From the 

 previous example, suppose that flows required to move sediment were totally 

 incompatible with the flows required by young fish. One mangement alternative 

 might be to build sediment traps in tributaries to prevent the sand from reaching the 

 stream. This technique might trigger undesirable side effects, such as degrading the 

 bed, so it would have to be evaluated. Another alternative might be to wait until after 

 the passage of the sediment transporting flow, and then stock fingerlings of the 

 desired species. A third alternative might be to allow a high flow once every 3 or 4 

 years to remove accumulated sediment, with full knowledge that that year's recruits 

 would be sacrificed. Regardless of the alternative selected, the Instream Flow 

 Incremental Methodology provides a useful tool which can be employed to evaluate 

 effects on the system. 



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