01A140 – Nooksack River above the Middle Fork Technical Notes: 2008 Water Year Chuck Springer The telemetered stream gaging station on the Nooksack River above the Middle Fork Nooksack River operated throughout water year 2008 with only one interruption. During the water year, ten discharge measurements were made and 21 discrete manual stage readings were taken at this station. Rating Curve This station began water year 2008 on Rating Table 5. Table 5 covers a range of discharge from 215 cubic feet per second (cfs) to 17,800 cfs. Two of the 11 discharge measurements used to develop this rating were taken during water year 2008. 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 5 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=4.3) calculated from the stage-discharge relationship. The potential error for flows derived from this rating curve is ±17%. A large storm event in December 2007 caused minor channel scour, shifting back to the stage- discharge relationship reflected by Table 2. Table 2 covers a range of discharge from 215 to 17,800 cfs. Four of the 14 discharge measurements used to develop this rating were taken during water year 2008. The measured flows for this rating, ranging from 505 to 5,260 cfs, cover only 27% of the rating curve; however, flows only exceeded the highest measured flows 3% of the time while Table 2 was in effect during water year 2008. One percent of flows exceeded the lowest measured flows, and 2% of flows exceeded the 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=-16.1) calculated from the stage-discharge relationship. The potential error for flows derived from this rating curve is ±20%. A moderate storm event in May 2008 caused a substantial amount of channel scour, shifting the stage-discharge relationship back to that reflected by Rating Table 3. Table 3 covers a range of discharge from 186 to 17,800 cfs. Two of the 14 discharge measurements used to develop this rating were taken during water year 2008. The measured flows for this rating, ranging from 373 to 5,260 cfs, cover only 28% of the rating curve; however, flows exceeded the highest measured flows only 3% of the time while Table 3 was in effect during water year 2008. 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 series of moderate storm events in August and September 2008 caused a moderate amount of channel fill, shifting the stage-discharge relationship back to that reflected by Table 1. Table 1 covers a range of discharge from 350 to 17,800 cfs. One of the 14 discharge measurements used to develop this rating was taken during water year 2008. The measured flows for this rating, ranging from 693 to 5,260 cfs, cover only 26% of the rating curve. Flows exceeded the measured range of flows 20% of the time while Table 1 was in effect during water year 2008. Eighteen percentof flows were below the lowest measured flow, and 2% of flows were above 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=4.2) calculated from the stage- discharge relationship. The potential error for flows derived from this rating curve is ±16%. Stage Record The station logged continuously throughout water year 2007 with one interruption from September 18-25 due to an internal failure in the datalogger. A continuous stage record for this time period was modeled by regressing stage at this site against stage at USGS station 12205000 - N.F. Nooksack River below Cascade Creek, located 22.3 river miles upstream (Y=0.745X+3.558, r2=0.90). The wire weight gage at this site is generally readable to within 0.05 ft during low-flow conditions, and the readability deteriorates 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 highly variable amounts, as much as 1.8 ft. Many of the large differences were attributed to a failing pressure transducer. Time-weighted corrective adjustments were made to the continuous stage record whenever the wire weight gage readings and datalogger readings differed. All adjustments are documented in the Hydstra Data Workbench. Quality control measures were also taken to identify potentially erroneous wire weight gage observations. A linear regression of wire weight observations versus tape down observations had an r2 of 0.999, with a standard deviation of 0.02 ft. The low standard deviation is surprising given the difficulty inherent in reading both the wire weight gage and tape down at this site. However, recent changes in channel geometry have made both wire weight and tape down observations easier 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 2008. 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 ±73%. This high potential error is largely due to the readings taken during the time period (October 2007 to January 2008) when the pressure transducer in the datalogger at this site was failing. Future Efforts This station primarily oscillates between three primary stage-discharge relationships, and does so rather frequently. Continued frequent discharge measurements will be vital to ensuring an accurate discharge record.