05B090 - N. F. Stillaguamish River at Oso Technical Notes: 2006 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 period of record for this station is from August 3, 2004 until the present. Flow Characteristics during the Water Year Water Year (WY) 2006 Annual Mean Discharge 1080 cfs. After a cool and dry autumn, the winter turned very wet from late December thru early February. While January was one of the wettest on record in much of western Washington, there were no extreme high-flow events at the station. Saturated soils resulted in a major landslide that blocked the N.F. Stillaguamish River two miles upstream from the station on January 25. The channel blockage lasted for six hours before being re-opened by erosion and human efforts. The spring of 2006 was less eventful, and summer was dry. The minimum flow for the water year was 157 cfs at a gage height of 0.48 feet on Sept 17 and the highest recorded stage was 8.38 feet on January 10. 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. 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. Four discharge measurements were taken during WY 2006. Discharge was above the highest measured flow 3% of the time and above two times the highest measured flow 1% of the time. Discharge did not fall below the lowest measured flow for Table 1 during the water year. A logarithmic extrapolation of Table 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 The defective pressure transducer that caused data errors in WY 2005 did not cause any significant errors in WY 2006. The defective part was replaced in early January of 2006. The new transducer tracks well with the actual water level. The slant pipe was damaged and became unsecured during one of the moderate high-flow events in January. Records during a period of 34 days are considered to be estimated data until the pipe was re-secured on February 7. A data gap of three days occurred at the end of June due to loss of pressure in the bubbler system. The gap was filled using reference trace 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. Error Assessments Calculated potential discharge error due to rating uncertainty for Table 1 ± 9.9% Calculated potential discharge error due to logger drift ± 1.1% The calculated combined potential discharge error for WY 2005 ± 11.0% Future Efforts More frequent discharge measurements are needed, especially at the higher flows. New cross- section and longitudinal surveys are needed for this station.