05B090 - N. F. Stillaguamish River at Oso Technical Notes: 2008 Water Year Don Watt Introduction The gage is located on the right bank of the N.F. Stillaguamish River at the north end of Whitman Road Bridge. The gage house is at the same elevation as the roadway; and the slant- pipe drops about 25 feet 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 the GOES West 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. Flow Characteristics during the Water Year The water year started moist with the mean flow above 1000 cfs (cubic feet per second) in October. The winter was fairly uneventful with one high-flow event briefly topping a gage height of 9.00 ft. Flows remained light through the winter but increased with the snowmelt season in late spring and early summer. Showery periods through the summer months held flows above the usual low-flow levels until mid September. The low flow for the year was 180 cfs at a gage height of 0.74 ft on September 17. The annual mean discharge for Water Year (WY) 2008 was 1060 cfs. Data for two days were excluded from the mean discharge calculation due to the lack of reliable estimates for flows greater than two times the highest measured flow. Rating Curve Rating Table No. 2 covers all of WY 2008. There were eight discharge measurements taken during the year ranging from 226 cfs to 1140 cfs. Table No. 2 is used on a recurring basis as recently as WY 2010 and is supported by a total of 21 discharge measurements. Table No. 2 is identical to Table No. 1 at water levels above 5.15 ft and discharge above 4280 cfs. The December 2006 highest measured flow of 4280 cfs for Table No. 1 is also considered to be the highest measured flow for Table No. 2. In WY 2008, Table No. 2 described a fill event at water levels below gage heights of 3.5 ft. This filling may be a result of material continuing to move downstream from the large landslide that occurred two miles upstream from the station in January, 2006. During WY 2008 flows were above the 4280 cfs highest measured flow 2% of the time and above two times the highest measured flow 1% of the time. Flows were below the lowest measured flow 1% of the time and did not fall below half of the lowest measured flow during the water year. A logarithmic extrapolation of Table No. 2 is used to estimate flows between 4280 cfs and 8560 cfs or two-times the highest measured flow. Discharge estimates beyond 8560 cfs are considered to be unreliable. Stage Record Automated gaging equipment functioned well at the station through most of WY 2008. A temperature sensitivity problem with the dry pressure transducer affected stage data from early August through the end of the water year. Data in this period are subject to errors that may exceed ±10% of actual discharge and are coded as estimated data. There were no data gaps longer than 45 minutes during the year. Manual staff gage readings were taken about once each month. Time-weighted adjustments to the automated data record are documented as Data Shifts in the Hydstra Sites Database. Error Assessments Calculated potential discharge error due to rating uncertainty for Table 2 ± 10.2% Calculated potential discharge error due to logger drift ± 0.8% The calculated combined potential discharge error for WY 2008 ± 11.0%. Future Efforts New cross-section and longitudinal surveys are needed for this station.