01Q070 - Dakota Creek @ Giles Rd. Technical Notes: 2009 Water Year Chuck Springer The continuous stream gaging station on Dakota Creek at Giles Road, which was upgraded from an instantaneous monitoring site as part of the Drayton Harbor TMDL in November 2007, operated throughout water year 2009. During the water year, ten discharge measurements were made and 12 discrete manual stage readings were taken at this station. Rating Curve This station began water year 2009 in transition from Rating Table 7 (see WY 2008 technical notes for description) to Rating Table 8 amid beaver dam construction downstream of the gage. Table 7 reflects the elevated stage heights due to this beaver activity, and covers a range of discharge from 3.6 to 974 cfs. Three of the ten discharge measurements used to develop this rating were taken during water year 2009. The measured flows for this rating, ranging from 7.2 to 487 cfs, cover only 50% of the rating curve. Two of the discrete observations exceeded the lowest measured flow while Table 8 was in effect during water year 2009. 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=2.6) calculated from the stage-discharge relationship. The potential error for flows derived from this rating curve is ±13%. A large storm event during January 2009 washed away the beaver dam, shifting the stage- discharge relationship back to that represented by Rating Table 7. Six of the 20 discharge measurements used to develop this rating were taken during water year 2009. As mentioned above, Table 7 is described in the technical notes for water year 2008. During summer 2009, beaver activity again elevated stage heights at this site, shifting the stage- discharge relationship. This shift is represented by Rating Table 9, which covers a range of discharge from 0 to 974 cfs. Four of the 17 discharge measurements used to develop this rating were taken during water year 2009. The measured flows for this rating, ranging from 3.2 to 487 cfs, cover only 50% of the rating curve. However, no discrete observations were made while Table 9 was in effect during water year 2009. 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 ±14%. Stage Record The station logged continuously throughout water year 2009 without interruption. 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 0.02 to 0.08 ft, and were as much as 0.16 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 0.996, with a standard deviation of 0.05 ft. The regression identified one obvious outlier, which was identified as a misread tape down observation. However, given the moderately high standard deviation, minor outliers would be difficult to detect. The calculated potential error of the continuous stage data for this station is ±34%. 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.