01A140 – Nooksack River above the Middle Fork Technical Notes: 2007 Water Year Chuck Springer The telemetered stream gaging station on the Nooksack River above the Middle Fork Nooksack River operated throughout water year 2007 without interruption. During this time, eight discharge measurements were made and 34 discrete manual stage readings were taken at this station. Rating Curve This station began water year 2007 on Rating Table 3. Table 3 covers a range of discharge from 186 cubic feet per second (cfs) to 17,800 cfs. One of the 14 discharge measurements used to develop this rating were taken during water year 2007. The measured flows for this rating, ranging from 373 to 5,260 cfs, cover only 28% of the rating curve. Flows exceeded the measured range of flows 46% of the time while Table 3 was in effect during water year 2007. Forty percent of flows were less than the lowest measured flow, and 6% of flows were greater than the highest measured flow. Flows greater than 5,260 cfs were modeled using a slope- conveyance model developed for this site. 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=2.7) calculated from the stage- discharge relationship. The potential error for flows derived from this rating curve is ±16%. A large storm event in November 2006 caused a substantial amount of channel fill, shifting the stage-discharge relationship back to that reflected by Rating Table 2. Table 2 covers a range of discharge from 215 to 17,800 cfs. Five of the 14 discharge measurements used to develop this rating were taken during water year 2007. The measured flows for this rating, ranging from 431 to 5,260 cfs, cover only 27% of the rating curve; however, flows exceeded the highest measured flows only 4% of the time while Table 2 was in effect during water year 2007. Flows greater than 5,260 cfs were modeled using a slope-conveyance model developed for this site. 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=-16.1) calculated from the stage-discharge relationship. The potential error for flows derived from this rating curve is ±20%. A series of moderate storm events in March 2007 caused a moderate amount of scour on a prominent shelf on the left side of the channel, shifting the stage-discharge relationship for mid- range flows. This shift is reflected by Rating Table 4. Since only mid-range flows were affected by this shift, the statistics for Table 2 still apply. Flows exceeded the highest measured flow 5% of the time while Table 4 was in effect during water year 2007. The potential error for flows derived from this rating curve is ±14%. During spring snowmelt the channel filled back in slightly more than the level reflected in Table 2. This shift is reflected in Table 5. This relationship remained in effect for the remainder of the water year. Table 5 covers a range of discharge from 215 to 17,800 cfs. Five of the 11 discharge measurements used to develop this rating were taken during water year 2007. The measured flows for this rating, ranging from 431 to 5,260 cfs, cover only 27% of the rating curve. However, flows exceeded the measured range of flows only 2% of the time while Table 5 was in effect during water year 2007. One percent of flows exceeded each the lowest and highest measured flows. Flows greater than 5,260 cfs were modeled using a slope-conveyance model developed for this site. 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=4.3) calculated from the stage- discharge relationship. The potential error for flows derived from this rating curve is ±17%. As part of the rating curve review process for water year 2007 for this station, historic rating curves for water years 2003-2006 were recalculated using current methods, resulting in some changes to the previously reported discharge record for those years. Stage Record The station logged continuously throughout water year 2007 without interruption. The wire weight gage at this site is generally readable within 0.05 ft during low-flow conditions, and the readability deteriorates to as much as ±0.10 ft during high flows. Conditions surrounding the terminal end of the bubbler orifice are similar to those around the wire weight gage. The stage height readings typically differed from manual wire weight gage readings by about 0.2 ft, and were as high as 0.7 ft. Time-weighted corrective adjustments were made to the continuous stage record whenever the wire weight gage readings and datalogger readings were different. All adjustments are documented in the Hydstra Data Workbench. Quality control measures were taken to identify potentially erroneous wire weight gage observations. A linear regression of wire weight observations versus tape down observations had an r2 of 0.996, with a standard deviation of 0.06 ft. The relatively low standard deviation is surprising given the difficulty inherent in reading both the wire weight gage and the tape down at this site. The regression identified one outlier, which was attributed to an uncorrected tape down reading in the database. There were no obviously erroneous wire weight gage observations during water year 2007. Turbulent conditions surrounding the terminal end of the bubbler orifice, particularly at high flows, resulted in a great deal of noise in the continuous stage data at times. This noise gives the appearance of highly variable flows at this site, when in fact the variability is due to turbulence. Time periods of extreme noise were “smoothed” using a 5-point moving mean to eliminate the effects of the turbulence and reduce the appearance of highly variable flow. The calculated potential error of the continuous stage data for this station is ±16%. Future Efforts This station primarily oscillates between three primary stage-discharge relationships, and does so frequently. Continued frequent discharge measurements will be vital to ensuring an accurate discharge record.