01F070 - S.F. Nooksack River at Potter Road
Technical Notes: 2007 Water Year
Chuck Springer

The telemetered stream gaging station on the South Fork Nooksack River at Potter Road 
operated throughout Water Year (WY) 2007.  During this time, ten discharge measurements were 
made and 31 discrete manual stage readings were taken at this station.  Cumulative potential 
error for this station for water year 2007 was ±14%.


Rating Curve

This station began WY 2007 on Rating Table 6.  Table 6 covers a range of discharge from 47 
cubic feet per second (cfs) to 26,700 cfs.  Five of the 16 discharge measurements used to develop 
this rating were taken during WY 2007.  The measured flows for this rating, ranging from 97.5 to 
11,700 cfs, cover only 44% of the rating curve.  However, flows only exceeded the measured 
range of flows 5% of the time during the five-and-a-half months Table 6 was in effect during 
WY 2007.  A total of 5% of flows exceeded the lowest measured flow and 1% exceeded the 
highest measured flow.  Flows greater than 11,700 cfs were modeled using a slope-conveyance 
discharge model, which had a confidence of ±14%.  The rating curve was interpolated between 
discharge measurements and extrapolated to half the lowest measured flow using Johnson’s 
method to temporarily straighten the rating curve using a log offset (e=5.6) calculated from the 
stage-discharge relationship.  The potential error for flows derived from this rating curve is 
±12%.

A large storm event in March 2007 caused moderate channel scour.  This shift is represented by 
Table 7.  Table 7 covers a range of discharge from 92.6 to 26,700 cfs.  Eight of the 20 discharge 
measurements used to develop this rating were taken during WY 2007.  The measured flows for 
this rating, ranging from 145 to 11,700 cfs, cover only 43% of the rating curve.  However, flows 
exceeded the measured range of flows only 12% of the time during the six-and-a-half months 
Table 7 was in effect during water year 2007.  A total of 11% of flows exceeded the lowest 
measured flow and 1% exceeded the highest measured flow.  Flows greater than 11,700 cfs were 
modeled using a slope-conveyance discharge model, which had a confidence of ±14%.  The 
rating curve was interpolated between discharge measurements and extrapolated to half the 
lowest measured flow using Johnson’s method to temporarily straighten the rating curve using a 
log offset (e=4.4) calculated from the stage-discharge relationship.  The potential error for flows 
derived from this rating curve is ±10%.

The weighted potential error for flows derived from the rating curves for this station for WY 
2007 is ±11%.

Stage Record

The station logged continuously throughout WY 2007 with only a few brief interruptions during 
February 2007 due to low battery voltage.  Data from the upstream USGS gage 12209000 - S.F. 
Nooksack River near Wickersham were used to fill these small gaps.  
The staff gage at this site is generally readable to within 0.02 ft during all flow conditions.  
Conditions surrounding the terminal end of the bubbler orifice are similar to those around the 
staff gage.  The stage height readings typically differed from manual staff gage readings by 
variable amounts, as much as 0.57 ft.  Drift at this station was variable and unpredictable, leaving 
very few periods of time that could be considered stable.  Time-weighted corrective adjustments 
were made to the continuous stage record whenever the staff gage and datalogger readings 
differed.  All adjustments are documented in the Hydstra Data Workbench.  Where drift 
conditions were considered unstable, and adjustments resulted in a 20% or greater change in 
discharge for any given day, the data for that day were qualified as estimates.  For WY 2007, 
there were no days that were qualified as estimates.

Quality control measures were taken to identify potentially erroneous staff gage observations.  A 
linear regression of staff gage observations versus tape down observations had an r2 of 0.993, 
with a standard deviation of 0.08 ft.  The regression did not identify any major outliers, although 
there were four minor outliers ranging from 0.14 to 0.30 ft, which could not be reconciled.  All 
four of these outliers were qualified as estimates in the Hydstra database.

The calculated potential error of the continuous stage data for this station is ±4%.  

Future Efforts

A more thorough high-flow modeling effort needs to occur for this site to solidify the upper end 
of the rating curve.  Also, point of zero flow (PZF) measurements can only be taken at this site 
during the lowest of flows.  PZF measurements should be taken whenever possible.