05B090 - N. F. Stillaguamish River at Oso Technical Notes: 2005 Water Year Don Watt Introduction The gage is located on the right bank of the N.F. Stillaguamish River at the north end of the Whitman Road Bridge. The gage house is at the same elevation as the roadway; and the slant- pipe drops about 25 ft along the side of an old bridge abutment into the river. The terminal end of the pipe extends into a deep portion of the channel a few feet offshore. The gage is telemetered via GOES satellite and used a combination of Campbell Scientific, Inc. and Hydrological Services equipment for recording and transmitting data. The period of record for this station is from August 3, 2004 until the present. Significant Events Water Year (WY) 2005 Annual Mean Discharge 798 cubic feet per second (cfs) WY 2005 Annual Median Discharge 596 (cfs) The water year was highlighted by several brief high-flow events in the fall and winter, separated by unusually long periods of dry weather. The highest recorded stage of 10.88 ft occurred during a warm rain event on December 10 and 11, 2004. A cool and moist spring brought near normal flows and modest snowfalls in the highest elevations of the basin. Most of the light snowpack disappeared from the basin by late April. Summer started moist but ended dry and warm. The minimum flow for the water year was estimated at 122 cfs with a gage height of 0.32 ft on Sept 28, 2005. The water year ended with another warm rain event that brought much higher flows on the last two days of September. No major damage occurred at this station during WY 2005, however; some data was lost due to instrument failure. A "bubble chamber" was added to the terminal end of the station probe on August 5, 2005 to reduce data noise due to turbulence and sedimentation. Tapedown measurements from the Whitman Road Bridge were used as a secondary gage index throughout the year. Channel geometry at this location seemed stable throughout WY 2005. Rating Curve Rating Table No. 1 covers the beginning of record in 2004 until March 26, 2007. A total of 12 discharge measurements were used to develop curve No. 1 ranging from a low measured flow of 128 cfs at a gage height of 0.34 ft to a high measured flow of 4280 cfs at a gage height of 5.15 ft. Four of those discharge measurements were taken during WY 2005. Most flow measurements were taken from the downstream side of the Whitman Road Bridge, but three very low flows were measured at a wading cross section about 300 ft upstream of the gage. Discharge was above the highest measured flow 2% of the time and above two times the highest measured flow 1% of the time. Discharge was below the lowest measured flow 1% of the time and did not fall below half of the lowest measured flow. A logarithmic extrapolation of rating Table No. 1 is used to estimate flows between 4280 cfs and 8560 cfs or two times the highest measured flow. Discharge estimates beyond two times the highest measured flow are considered to be unreliable. Stage Record Significant gaps in the stage record for this station occurred during May and July of 2005. In all, about 16 days of data were lost due to pressure loss in the bubbler gage system. This gap was filled using reference data from the SF Stillaguamish River at Jordan Road (station 05A105) and the regression equation Y = 0.734 * X - 3.043. The correlation coefficient for the regression between 05B090 and 05A105 is 0.971. The standard error of estimate is 0.226. A defect in this station’s pressure transducer caused automated gage height readings to deviate from the actual water level on 16 days during August. The indicated stage varied by as much as ±0.08 ft. based on changes in air temperature. Data affected by this problem are considered to be estimated when the temperature-related stage variation caused an uncertainty known to be 10% or more of the indicated discharge. Error Assessments Calculated potential discharge error due to rating uncertainty for Table 1 ± 9.9% Calculated potential discharge error due to logger drift ± 1.8% The calculated combined potential discharge error for WY 2005 ± 11.7% Future Efforts The defective pressure transducer was replaced in early January of 2006. The new transducer is tracking well with the actual water level. More frequent discharge measurements are needed, especially at the higher end of the flow curve. New surveys are needed for both cross sections. A longitudinal survey should also be taken between the two cross sections.