01F070 – S.F. Nooksack R. @ Potter Rd.
Technical Notes:  2005 Water Year 
Chuck Springer

The telemetered streamgaging station on the S.F. Nooksack River at Potter Road operated 
throughout water year 2005.  A series of storm events in October and November caused 
fairly extensive flooding in the Nooksack basin, mainly in the agricultural lowlands.  At 
this station, these events caused noticeable changes to channel geometry, depositing large 
amounts of sediment in the measurement cross-section, and altering the stage to 
discharge relationship at lower flows.  A large rain-on-snow event in mid-January 
effectively marked the beginning of spring snowmelt.  However, abundant spring rainfall 
kept flows from dropping dramatically until late-July.  With insufficient snowpack to 
maintain summer flows, minimum summer flow at this station was 79 cfs in September 
2005, 34 percent lower than the average minimum flow for the previous two years of 
record.

Rating Curve
The rating curve for the station for water year 2005 covers a range of discharge from 75 
cfs to 26,700 cfs. The range of discharge encountered during the water year was from 
79.3 cfs in September 2005 to 25,600 cfs in November 2004.  The range of stage 
encountered during the water year was 9.85 feet.  The measured discharge, ranging from 
114 cfs to 11,700 cfs, covers only 44% of the magnitude of flows encountered during the 
water year.  However, flows were above the measured range of flows less than 2% of the 
time during the water year, and were below the measured range of flows only 8% of the 
time. Flows greater than 7,730 cfs had been modeled in late 2004  At that time, due to 
fairly poor interpolative strength, the extrapolative accuracy of the model was estimated 
to be ±40%.  However, a storm event discharge measurement taken in January 2005 
verified the model to within ±5% at 11,700 cfs.  Discharges greater than 11,700 cfs are 
considered accurate to within ±20%.    Sometime during the winter/spring of 2005, 
aggradation occurred in the stream channel at this site, causing a low-flow shift in the 
relationship between stage and discharge.  A comparison of cross-sectional profiles from 
several discharge measurements taken between fall 2004 and spring 2005 points to a 
storm event during the latter half of January 2005 as the likely cause of the shift.  A 
“phased” shift in the rating curve was applied over the duration of the falling limb of the 
storm event, whereby a gradual change in channel geometry over this time period is 
assumed.

Stage Record

The station logged continuously throughout the water year without interruption.  Channel 
conditions at the terminal end of the orifice pipe are fairly laminar.  However, the station 
seems to be prone to gradual instrument drift, which is measured as the difference 
between logged and measured stage values.  Drift at this site usually ranged from 0.01ft 
to 0.05ft in magnitude, but was as much as 0.48ft.  The drift was most extreme following 
the large storm events of the fall and winter.  Time-weighted corrective adjustments were 
made to the continuous stage record whenever the difference between logged and 
measured stage values resulted in a 5% or more difference in discharge.  All adjustments 
are documented in the Hydstra Data Workbench.

Future Efforts

To date, one storm peak discharge measurement has been taken at this site.  More high 
flow measurements at stages both above and below this measurement are needed.  A 
longitudinal elevation survey is also planned for Summer 2006.  The results of this 
survey will be used for Manning’s velocity modeling.  

cws