05A105 - S. F. Stillaguamish River at Jordan Road Bridge Technical Notes: 2008 Water Year Don Watt Introduction The gage is located on the left bank of the S.F. Stillaguamish River at the south end of the Jordan Road Bridge on the west side of the road. The gage house is at roughly the same elevation as the roadway; and the slant-pipe drops abruptly 30 vertical feet over a steep embankment to the river. The terminal end of the pipe drops into the deepest portion of the channel a few feet offshore. The gage was telemetered and used a combination of Campbell Scientific and Hydrological Services equipment for recording and transmitting data throughout the water year. The period of record for this station is from July 29, 2004 until the present. Discharge Statistics (cubic feet per second) Mean Annual Discharge: 1,610 cfs Median Annual Discharge: 1,170 cfs Maximum Daily Mean Discharge: 14,200 cfs Minimum Daily Mean Discharge: 223 cfs Maximum Instantaneous Discharge: 21,500 cfs Minimum Instantaneous Discharge: 214 cfs Discharge Equaled or Exceeded 10 % of Recorded Time: 3,130 cfs Discharge Equaled or Exceeded 90 % of Recorded Time: 477 cfs There were no days in which the discharge was higher than twice the highest measured flow or in which discharge was less than half the lowest measured flow. Events of Note Fall rains were already underway at the start of the water year. Two rain events in the first week of October produced flows above 9000 cfs. The highest water level of the year occurred during another warm rain event on December 3. Strong snowmelt runoff kept flows near 2000 cfs until early July. Rainy periods in late July and August held summer flows a little above normal. Low flow for the year occurred on September 20. Rating Curve The incised channel at this site remained stable throughout Water Year (WY) 2008. Rating Table No. 1 covered all of WY 2008 and continued in effect through 2010. This rating curve employs more than 30 measured flows ranging from 134 cfs to 21,400 cfs. The curve is generally defined by a Johnson’s Log Offset of 3.72 feet. Six discharge measurements were made in WY 2008. During WY 2008 the lowest measured flow was 211 cfs at a gage height of 5.27 ft on September 18. The highest measured flow was 2000 cfs at a gage height of 7.49 ft on May 12. During WY 2008 discharge was above Table No. 1’s highest measured flow less than 1% of the time. Stage Record Automated gaging equipment worked well through WY 2008. Only two significant data gaps occurred during the water year. The first lasted about 18 hours on November 26- 27. The second gap lasted for 40 hours on January 13-15. Both gaps were filled using reference trace data from Ecology’s Canyon Creek gaging station (05F080). The Canyon Creek data was scaled using the correlation equation Y=1.672*X-3.621 ft. The correlation coefficient for this relationship is 0.968. Error Assessments Calculated potential error due to rating uncertainty for Table No.1 ±11.3% Calculated potential discharge error due to logger drift ± 1.5% The calculated combined potential discharge error for WY 2008 ±12.8% Future Efforts Accurate cross-sectional and longitudinal surveys are needed to improve high-flow modeling.