05A105 - S. F. Stillaguamish River at Jordan Rd. Bridge 
Technical Notes:  2006 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 at roughly the 
same elevation as the roadway; and the slant-pipe drops abruptly 30 vertical feet over a 
steep embankment to the river.  The terminal end of the pipe drops into the deepest 
portion of the channel a few feet offshore.  The gage was telemetered and used a 
combination of Campbell Scientific 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.

Flow Characteristics during the Water Year

The Water Year (WY) 2006 annual mean daily discharge was 1200 cubic feet per second 
(cfs).   Data for 20 days are excluded from the calculation due to the presence of any 
instantaneous discharge value greater than two times the highest measured flow.  

The average of all 15-minute discharge values not exceeding two times the highest 
measured flow was 1280 cfs.

WY 2006 was generally quiet with no damaging high-flow events recorded.  The 
minimum flow for the water year was 128 cfs at a gage height of 4.76 feet on Sept 12 
and the highest recorded stage was 12.65 feet on January 30.

Rating Curve

The incised channel at this site remained stable throughout WY 2006.  Rating Table No. 1 
covered all of WY 2006 and continued 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 three discharge measurements were made 
between September, 2005 and October, 2006 ranging from 134 cfs at a gage height of 
4.80 ft. up to 1552 cfs at a gage height of 7.06 ft.   
 
During WY 2006, discharge was above Table No. 1’s highest measured flow 9% of the 
time and was above two times the highest measured flow 3% of the time.  Flows were 
below the lowest measured flow 2% of the time.  Flows remained above one-half of the 
lowest measured flow throughout the year.



Stage Record

The defective pressure transducer that caused errors in logged gage height values during 
the summer of 2005 was replaced in early January 2006.  Gage height data was not 
affected by this problem during WY 2006.  There were no data gaps longer than 45 
minutes during the water year.


Error Assessments

Calculated potential error due to rating uncertainty for Table No.1	±10.9%.
Calculated potential discharge error due to logger drift		± 1.0%
The calculated combined potential discharge error for WY 2006		±11.9%

Future Efforts

More frequent discharge measurements are needed, especially at the higher water levels.  

Accurate cross-sectional and longitudinal surveys are needed to improve high-flow 
modeling.