05A105 - S. F. Stillaguamish River at Jordan Road Bridge 
Technical Notes:  2009 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.

Discharge Statistics (cubic feet per second)
Mean Annual Discharge:                                1,370 cfs
Median Annual Discharge:                                864 cfs
Maximum Daily Mean Discharge:                        22,400 cfs
Minimum Daily Mean Discharge:                           162 cfs
Maximum Instantaneous Discharge:                     29,700 cfs
Minimum Instantaneous Discharge:                        160 cfs
Discharge Equaled or Exceeded 10 % of Recorded Time:  2,560 cfs
Discharge Equaled or Exceeded 90 % of Recorded Time:    204 cfs
 
There were no days in which the discharge was higher than twice the highest measured 
flow or in which discharge was less than half the lowest measured flow.

Events of Note

November produced two warm rain events that pushed discharge above 22,000 cfs.  
December was cold and snowy bringing good accumulations of lowland snow.  Warm 
rain returned in early January causing rapid snowmelt and an estimated high flow for the 
year of 29,700 cfs on January 8.  The minimum flow for the water year was 160 cfs on 
Sept 1. 

Rating Curve

The incised channel at this site remained stable throughout Water Year (WY) 2009.  
Rating Table No. 1 covered all of WY 2009 and continued through 2010.  This rating 
curve employs more than 30 measured flows ranging from 134 to 21,400 cfs.  The curve 
is generally defined by a Johnson’s Log Offset of 3.72 feet.  A total of nine discharge 
measurements were made during WY 2009 ranging from 190 cfs at a gage height of 5.16 
ft. up to 1930 cfs at a gage height of 7.42 ft.   
 



During WY 2009, discharge was above Table No. 1’s highest measured flow less than 1% 
of the time.  Flows remained above Table No. 1’s lowest measured flow throughout the 
year.

Stage Record

Automated gaging equipment worked well in WY 2009.  A cold-weather pressure leak 
caused a 10-hour data gap on January 26.  Flow at the time was in a slow decline and the 
gap was filled using linear interpolation.  A one-hour data gap occurred during a site visit 
in October with no significant loss of data.

Error Assessments

Calculated potential error due to rating uncertainty for Table No.1	±11.3%
Calculated potential discharge error due to logger drift		± 4.1%
The calculated combined potential discharge error for WY 2009		±15.4%

Future Efforts

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