lower Inflection point, the riffle bottom is being exposed at an accelerated 

 rate and the area available for food production greatly diminishes. 



The wetted perimeter-f low relationship may also provide an index of other 

 limiting factors that influence a stream's carrying capacity. One such factor 

 is cover. Cover, or shelter, has long been recognized as one of the basic and 

 essential components of fish habitat. Cover serves as a means for avoiding 

 predators and provides areas of moderate current speed used as resting and 

 holding areas by fish. It is fairly well documented that cover improvements 

 will normally increase the carrying capacity of streams, especially for larger 

 size fish. Cover can be significantly influenced by streamflow. 



Tn the headwater streams of Montana, overhanging and submerged bank vegetation 

 are important components of cover. The wetted perimeter-flow relationship for 

 a stream channel may bear some similarity to the relationship between bank 

 cover and flow. At the upper inflection point, the water begins to pull away 

 from the banks, bank cover diminishes and the stream's carrying capacity 

 declines. Flows exceeding the upper inflection point are considered to 

 provide near optimal bank cover. At flows below the lower inflection point, 

 the water 1s sufficiently removed from the bank cover to severely reduce its 

 value .is fish shelter. 



It has been demonstrated that riffles are also critical areas for spawning 

 sites of brown trout and shallow inshore areas are required for the rearing of 

 brown and rainbow trout fry (Sando, 1981). It is therefore assumed that, in 

 addition to maximizing bank cover and food production, the flows exceeding the 

 upper inflection point would also provide the most favorable spawning and 

 rearing conditions. 



Riffles are the area of a stream most affected by flow reductions (Bovee, 1974 

 and Nelson, 1977). Consequently, the flows that maintain suitable riffle 

 conditions will also maintain suitable conditions in pools and runs, areas 

 normally inhabited by adult fish. Because riffles are the habitat most 

 affected by flow reductions and are essential for the well-being of both 

 resident and migratory fish populations, they should receive the highest 

 priority for instream protection. 



The wetted perimeter/inflection point method provides a range of flows 

 (between the lower and tipper inflection points) from which a single instream 

 flow recommendation can be selected. Flows below the lower inflection point 

 are judged undesirable based on their probable impacts on food production, 

 hank cover and spawning and rearing habitat, while flows exceeding the upper 

 inflection point are considered to provide a near optimal habitat for fish. 

 The lower and upper inflection points are believed to bracket those flows 

 needed to maintain the low and high levels of aquatic habitat potential. 

 These flow levels are defined as follows: 



2-3 



