05A105 - S. F. Stillaguamish River at Jordan Road Bridge Technical Notes: 2005 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 roughly the same elevation as the roadway; and the slant-pipe drops abruptly about 30 vertical feet over a steep embankment to the river. The terminal end of the pipe drops into the deepest portion of the channel which is a few feet offshore at this location. The gage is telemetered via the GOES West satellite and used a combination of Campbell Scientific, Inc. 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. Significant Events The annual mean daily discharge for Water Year (WY) 2005 was 927 cubic feet per second (cfs). Data for 13 days are excluded from the mean daily discharge calculation due to instantaneous discharge values exceeding two times the highest measured flow of 3430 cfs. Several brief high-flow events occurred in the fall and winter between unusually long periods of dry weather. The highest recorded stage of 15.72 feet occurred during a warm rain event on December 10. Spring was cool and moist and brought a mean flow above 1650 cfs during April. Modest spring snowfalls occurred in the highest elevations of the basin; but most of the snowpack disappeared by late April. Mean flows in May and June were below 900 cfs, well below monthly norms. Summer started moist but ended dry and warm. The lowest stage in the water year was 4.74 feet on Sept. 27 and was estimated at 125 cfs. The water year ended with another warm rain event that raised water levels above 8.00 ft on the last two days of September. The high-flow events of November, December, and January each brought water levels above 15.0 feet and caused major damage to the middle and upper staff gage segments. The staff gages were repaired and surveyed on February 1. Tapedown measurements from the bridge served throughout the year as backups to the staff gage readings. Rating Curve Rating Table No. 1 covered all of WY 2005 and continued in effect through 2010. This rating curve employs more than 30 measured flows ranging from 134 to 3430 cfs. The curve is generally defined by a Johnson’s Log Offset of 3.72 feet. A total of four discharge measurements were made between late September of 2004 and early February of 2006. Those measurements ranged from 134 to 1490 cfs. During WY 2005, discharge was above Table No. 1’s highest measured flows 5% of the time and was above two times the highest measured flow 2% of the time. Flows were below the lowest measured flow 3% of the time and remained above one-half of the lowest measured flow throughout the year. Stage Record The stage record for this station was continuous except for three gaps of 20 hours each from January 3-5. These gaps were the result of low battery voltage and cold overnight temperatures. The gaps occurred at a time of minimal stage variation and were filled using reference trace data from the Nooksack River above Middle Fork (Station No. 01A140). Manual staff gage readings were taken at monthly intervals. Time-weighted corrections were made to adjust the continuous record to match the manual observations. Each adjustment to the data record is logged in the Hydstra Data Workbench. A defect in the pressure transducer that generates continuous gage height records affected data throughout the year. The problem was most pronounced in warm weather and caused the indicated gage height readings to deviate from the actual water level by as much as 0.08 ft. Data points have been flagged as estimated when the stage deviation caused flow uncertainties of 10% or more of the discharge values. All of the flagged data occurred in August and amounted to 2% of the total data record for the year. These variations appear to have a negligible effect on the logger drift calculation shown below. Error Assessments Calculated potential error due to rating uncertainty for Table No.1 ±10.9%. Calculated potential discharge error due to logger drift ± 1.5% The calculated combined potential discharge error for WY 2005 ±12.4% Future Efforts The defective pressure transducer was replaced in early January of 2006. The new transducer tracks well with the actual water level. More frequent discharge measurements are needed at this site, especially focusing on flows at the higher end of the flow curve. More accurate cross-sectional and longitudinal surveys are needed to improve high-flow modeling.