01F070 - S.F. Nooksack River at Potter Road
Technical Notes: 2008 Water Year
Chuck Springer

The telemetered stream gaging station on the South Fork Nooksack River at Potter Road 
operated throughout Water Year (WY) 2008.  During the water year, seven discharge 
measurements were made and 23 discrete manual stage readings were taken at this station.  
Cumulative potential error for this station for water year 2008 was ±13%.

Rating Curve

The station operated throughout WY 2008 on Rating Table 7.  Table 7 covers a range of 
discharge from 92.6 to 26,700 cfs.  Seven of the 20 discharge measurements used to develop this 
rating were taken during WY 2008.  The measured flows for this rating, ranging from 145 to 
11,700 cfs, cover only 43% of the rating curve; however, flows exceeded the highest measured 
flows only 1% of the time during the water year.  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%.

Stage Record

The station logged continuously throughout WY 2008 without interruption.  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 0.04 to 0.06 ft., 
and by as much as 0.17 ft.  Drift at this station was very constant through much of the water 
year.  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 are considered unstable, and adjustments result in a 
20% or greater change in discharge for any given day, the data for that day are qualified as 
estimates.  For WY 2008, 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 versus tape down observations had an r2 of 0.998, with a standard 
deviation of 0.06 ft.  The regression identified three outliers, ranging from 0.12 to 0.16 ft.  Two 
of these outliers were identified as mis-corrected tape down readings, and the third was 
identified as an erroneous staff gage reading.  All three readings were qualified as estimate in the 
Hydstra database.

The calculated potential error of the continuous stage data for this station is ±3%.  

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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.