Methods 



The procedure utilized by the MDFWP to quantify instream flow 

 recommendations is the wetted perimeter method. Wetted perimeter is 

 the distance along the bottom and sides of a stream channel cross section 

 that is in contact with water. Wetted perimeter is a direct function of 

 discharge but the rate at which it increases or decreases is not constant 

 throughout the discharge range and is modified by channel configuration. 

 Wetted perimeter increases rapidly with small increases in flow up to the 

 point where the stream nears its maximum width. The rate of increase then 

 slows as the water moves up the stream banks with increasing discharge. 

 In small streams, there are usually two points, termed the lower and upper 

 inflection points, which can be depicted on a plot of wetted perimeter 

 versus discharge and which represent significant changes in the rate of 

 increase or decrease of wetted perimeter with flow. 



Studies have shown that the wetted perimeter-discharge relationships 

 for selected channel cross sections can be used to quantify instream flow 

 needs for the maintenance of trout habitat (Nelson, 1980). As the wetted 

 perimeter decreases and pulls away from stream banks, accompanying losses 

 of riffle habitat for the production of benthic food organisms, spawning 

 sites, inshore rearing areas and streambank cover can occur. In this 

 manner, wetted perimeter can be related to various levels of aquatic habitat 

 potential . 



The methods used to derive wetted perimeter-discharge relationships 

 on Mount Haggin streams are given in Nelson (1980b). A study section or 

 subreach was established on each stream that was examined at Mount Haggin. 

 Each subreach was selected to typify the stream habitat and generally 

 contained riffle and run habitat. Pool habitat occurred in few of the 

 subreaches due to high gradients of these headwater streams. Five cross 

 sections were selected for study within each subreach. 



Data collection usually consisted of three sets of stage (water surface 

 elevation) measurements for each cross section at three different known 

 discharges. The stage-discharge data were collected at high (as runoff 

 is receding), intermediate (near the end of runoff) and low (late summer) 

 flows. A fourth stage-discharge measurement was collected at a second 

 intermediate flow on some of the streams. The final set of data collected 

 in the field consisted of a measurement of the stream channel profile 

 through each cross section. The profile measurements were made during 

 the low flow period. 



The stage-discharge data and cross sectional profile measurements 

 were entered into the wetted perimeter predicative computer program (WETP) 

 (Nelson, 1980b) for analysis. The WETP program uses stage-discharge data 

 to derive a stage-discharge rating curve for each cross section by least 

 squares fit. The rating curve, when coupled with the cross-sectional profile, 

 was used to predict wetted perimeter at selected flows of interest for each 

 stream. 



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