05B090 - N. F. Stillaguamish River at Oso
Technical Notes: 2007 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 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

Water Year (WY) 2007 Annual Mean Discharge 	1030 cubic feet per second (cfs)

Streamflow remained between 150 cfs and 300 cfs through most of October, 2006 but rose 
abruptly with heavy rains in early November.  The peak stage for the water year was 11.21 ft on 
November 6 that produced major flooding in the Arlington area.  Water levels reached about 
10.00 ft during three other runoff events in January through March of 2007.  While July was also 
unusually damp, water levels dropped below 150 cfs by mid September, 2007.  September ended 
wet with flows again approaching 2000 cfs.  

Rating Curve

Rating curve No. 1 covers the period from the start of record for the station through the first half 
of WY 2007.  A total of 12 discharge measurements were used to develop curve No. 1, ranging 
from a low of 128 cfs at gage height 0.34 ft to the high measured flow of 4280 cfs at a gage 
height of 5.15 ft.  

Rating curve No.2 covers the remainder of WY 2007 and serves throughout WY 2008.  Six 
discharge measurements were taken in WY 2007, ranging from 197 cfs up to the 4280 cfs 
measurement.  Curve 2 describes a fill event for gage heights below about 3.5 ft.  This filling may 
be a result of material moving downstream from the large landslide that occurred two miles 
upstream from the station in January, 2006.

During WY 2007, flows were above the 4280 cfs highest measured flow 4% of the time and 
above two times the highest measured flow 1% of the time.  Flows did not fall below the lowest 
measured flows for the applicable rating tables (#1 or #2) during WY 2007.  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 8560 cfs are considered to be unreliable.  
Days in which the instantaneous flow exceeded two times the highest measured flow are not 
included in calculations for Annual Mean Discharge.

Stage Record

Automated gaging equipment functioned well at the station throughout WY 2007.  There were 
no data gaps longer than 45 minutes during the year.  Manual staff gage readings were taken 
once each month.  Time-weighted corrections were made to match the continuous record with 
the manual observations.  Each of these adjustments to the data record is logged in the Hydstra 
Data Workbench.  

The middle segment of the staff gage was destroyed during high water in early November, 2006.  
The staff segment was replaced in April of 2007.  Staff observations were impossible on three 
site visits during the interim.  Logger and staff readings were in excellent agreement on three 
other visits during this period.  A laser level pad and reference marks were installed in July, 2007 
to provide another backup gaging index for the site.

Error Assessments

Calculated potential discharge error due to rating uncertainty for Table 1	±  9.9%
Calculated potential discharge error due to rating uncertainty for Table 2	± 10.2%
Time-weighted potential discharge error for Tables 1 and 2 in WY 2007		±10.05%

Calculated potential discharge error due to logger drift			± 3.4%
The calculated combined potential discharge error for WY 2007			±13.45%

Future Efforts

WY 2007 brought the routine use of the Stream Pro ADCP for discharge measurements at this 
station.  The ADCP should allow more frequent discharge measurements at higher flows and 
should improve the accuracy of rating curves.  New cross-section and longitudinal surveys are 
needed for this station.