01Q070 - Dakota Creek @ Giles Rd.
Technical Notes: 2008 Water Year
Chuck Springer

The instantaneous stream gaging station on Dakota Creek at Giles Road was upgraded to 
continuous monitoring as part of the Drayton Harbor TMDL in November 2007.  The continuous 
stream gaging station operated throughout water year 2008.  During the water year, six discharge 
measurements were made and 22 discrete manual stage readings were taken at this station.

Rating Curve

This station began water year 2008 on Rating Table 6.  Table 6 reflects elevated stage heights due 
to beaver activity downstream of the gage, and covers a range of discharge from 1.4 to 974 cfs.  
Two of the eight discharge measurements used to develop this rating were taken during water 
year 2008.  The measured flows for this rating, ranging from 2.8 to 487 cfs, cover only 50% of 
the rating curve.  However, none of the discrete observations exceeded the measured range of 
flows while Table 6 was in effect during water year 2008.  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=2.5) calculated 
from the stage-discharge relationship.  The potential error for flows derived from this rating curve 
is ±13%.
 
During winter 2008, the beaver dam was broken up by high flows, shifting the stage-discharge 
relationship.  This shift is represented by Rating Table 7.  Table 7 covers a range of discharge 
from 2 to 974 cfs.  Four of the 20 discharge measurements used to develop this rating were taken 
during water year 2008.  The measured flows for this rating, ranging from 2.9 to 487 cfs, cover 
only 50% of the rating curve.  However, only one of the discrete observations exceeded the 
lowest measured flow while Table 7 was in effect during water year 2008.  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=2.5) 
calculated from the stage-discharge relationship.  The potential error for flows derived from this 
rating curve is ±15%.

Stage Record

The station logged continuously for most of water year 2008 with two major interruptions.  A 
large storm event flooded the station in early December 2007, resulting in a two month gap in the 
data set before repairs could be made.  Then in June 2008, a personal digital assistant (PDA) 
containing 6 weeks of downloaded data for this station was stolen during a vehicle prowl at a 
Bellingham hotel.  Both data gaps were filled using correlated data from station 01R090 - 
California Creek at Valley View Road.  

This station is tidally influenced; all but the lowest high tides result in a spike in the continuous 
stage record that typically lasts 1-3hours.  These spikes were removed from the data set by 
interpolating linearly between data points before and after each spike.  The resulting data values 
were qualified as estimates in each case.

The staff gage at this site is generally readable to within 0.02 ft during all flow conditions.  
Conditions surrounding the terminal pressure transducer are similar to those around the staff 
gage.  The stage height readings typically differed from manual staff gage readings by 0.01 to 
0.03 ft, and were as much as 0.06 ft.  Time-weighted corrective adjustments were made to the 
continuous stage record whenever the staff gage observations and datalogger readings differed.  
All adjustments are documented in the Hydstra Data Workbench.

Quality control measures were also taken to identify potentially erroneous staff gage 
observations.  A linear regression of staff gage observations versus tape down observations had 
an r2 of 0.997, with a standard deviation of 0.03 ft.  The regression did not identify any obvious 
outliers, indicating that the staff gage and tape down observations made during water year 2008 
are at least reasonably accurate.

The calculated potential error of the continuous stage data for this station is ±15%.  

Future Efforts

This station tends to see beaver activity every summer, followed by washouts of those dams 
during winter storm events.  Ongoing frequent discharge measurements and, when possible, point 
of zero flow measurements, will be necessary to monitor these shifts.