B.3.1 - Water Quality Status 



This section discusses the relevant stream and water quality data for Big Creek that characterize the 

 existing condition. 



Pfankuch Stream Channel Rating 



The Pfankuch stream channel rating (Pfankuch, 1978), was developed to "systemize measurements 

 and evaluations of the resistive capacity of mountain stream channels to the detachment of bed and 

 bank materials and to provide infomnation about the capacity of streams to adjust and recover from 

 potential changes in flow and/or increases in sediment production" (Pfankuch, 1975). This procedure 

 uses a qualitative measurement with associated mathematical values to reflect stream conditions. The 

 rating is based on 15 categories: six related to the bottom of the stream channel (the part of the 

 channel covered by water yearlong), five related to the lower banks (covered by water only during 

 spring runoff), and four related to the upper banks (covered by water only during flood stages). 

 Streams rated excellent (<38) or good (39-76) are less likely to erode during high flow than streams in 

 fair (77-114) or poor (115+) condition. Prime fish habitat usually occurs in streams with a good rating; 

 streams in exce//enf condition usually do not have adequate gravels for good spawning habitat. 



The rating is evaluated at a spot or reach of stream. Each rating represents one point in time; 

 therefore, a series of ratings must be made over several years to show the trend of stream stability; i.e., 

 whether the stream is headed towards or away from dynamic equilibrium. D. Sirucek reported a 

 statistically significant correlation between the change in Pfankuch stream channel ratings and several 

 measurements of stream health (e.g. McNeil core % fine sediment, water yield increase, nutrient levels 

 and chlorophyll A). 



In the late 1970s, stream channels at selected sites in the Big Creek drainage were rated as good using 

 the Pfankuch stream channel rating scale. Some of those same areas were rated as fair and poor in a 

 1992 survey. 



The good Pfankuch ratings of the late 1970s did not forecast that sediments that had been generated 

 from road construction associated with timber harvest of the 1 960s and 70s were working their way 

 downstream. Had surveys been made occasionally during the 1980s, they would have indicated that 

 the stream was becoming impaired. The fair and poor ratings of the 1992 survey are a result of 

 sediment moving downstream into areas that had previously been rated as good. These same sites 

 will rate good again, but it may not be for 1 0-20 years. 



After the Moose Fire during late October and early November Pfankuch ratings were done on the 

 triburaries and the mainstem of Big Creek within the fire boundary. These ratings would not reflect any 

 changes to the streams due to the fires, rather they were done to be able to measure any changes 

 following post-fire runoff events. The mainstem ratings were all fa/r; and the tributaries ranged from 

 good to poor, with the majority being fair. 



Riffle Stability Index 



The riffle stability index (RSI) is a quantitative methodology used to for assessing stream equilibrium 

 and channel stability (Kappesser, 1993). This technique looks at the relative mobility of streambed 

 matehal deposited on riffles during bankfull discharge. The largest size particles that are moved during 

 frequent (annual flood events) are measured at a deposition site such as a riffle point bar. That size 

 and all smaller particles are therefore considered mobile. The average size of the largest mobile 



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