Name________________________________________________Per.______
Analyzing the Spring Flow of the Stillaguamish River
The following charts show data from the Stillaguamish River at three different locations: Arlington,
Granite Falls, and Stanwood, WA. Gage height (depth of the river) is given for all three rivers, and
the discharge is also given for the Stillaguamish in Arlington. The steep drop in flow on March 22nd
in Arlington was due to the landslide upriver in Oso, WA.
Arlington (North Fork of the Stillaguamish):
1. What was the discharge of the river at its highest flow on March 19th? _________________
What is the median discharge for the Stillaguamish on this day? _____________________
What would cause the flow to be so much higher than the median?
2. Calculate the velocity of the river at various times:
Discharge = width x depth x velocity
Rearranged: velocity =
Discharge
width x depth
Depth and discharge are given to you. To estimate the width of the river, open Google Earth and go
to the location of the water sampling station: 48 15’42”N, 122 02’47”W. Use the ruler tool to
measure the width of the river at this location. Make sure to use feet as your units. Show your
calculations and label your units.
River Width: __________
2a. Calculate the velocity at the highest flow on March 19th:
High Flow Velocity =
2b. Calculate the velocity for the median flow for March 19th:
Median Flow Velocity:
3a. What is the difference in water velocity between the highest flow and the median flow? __________
3b. What assumption did you make when you calculated the velocities during different times at this
location?
4. On March 25th, the discharge of the Stillaguamish was about the median. Assume that the gage height
on this day is about the median gage height for this time of year.
What is the difference in water depth between the highest flow and the median flow? ___________
5. People often raft down rivers at various flow stages. Describe a possible danger of rafting the river
when the discharge is very low, and a danger of rafting the river when the discharge is very high.
6a. By how much did the discharge drop following the landslide in Oso, WA? Note that the discharge
scale is not even. Use the minimum discharge in your estimate:
6b. How long did it take for the river to return to normal flow after the landslide upriver in Oso?
6c. People were very concerned that the landslide in Oso might also cause catastrophic flooding
downstream if the river was dammed for too long by the landslide. Calculate how much water was
held back over the period you recorded in 6b and using the difference in discharge from 6a. There
are 86,400 seconds in one day.
_______days x 86,400 sec x ____________ft3 =
1 day
1 sec
ft3
6d. If the Stillaguamish River had taken 8 days to return to normal flow, how many cubic feet of water
would have been dammed by the landslide?
7. Go to http://waterdata.usgs.gov/wa/nwis/uv/?site_no=12167000&PARAmeter_cd=00060,00065.
This is the site that contains the river information for the Stillaguamish in Arlington.
7a. What is considered flood stage for this river? ___________
7b. Define flood stage (click on the flood stage link on this page for a good definition).
7c. March, 2014 was a record-breaking month for rain in the Puget Sound
region. Seattle Rainfall is given for Jan-March in the figure to the
right, with total rain for the year-to-date given at the top.
How many more inches of rain did the Seattle area receive in March
in 2014 compared to the average?
How many more feet would be needed for the Stillaguamish to hit
flood stage on March 19th?
7d. Despite the record rain, this is nowhere close to flood stage. What
other factors do you think might play a role in whether a river hits
flood stage in the Puget Sound region?
Seattle Rainfall, 2014
Year-to-Date: 19.25”, Avg: 12.79”
8. Granite Falls (South Fork of the Stillaguamish): Fly to 48 06’14”N, 121 56’40”W. Granite Falls is
on another fork of the Stillaguamish River and was not affected by the Oso landslide. The Arlington
profile would look like this one if the Oso slide had not happened.
8a. Measure the width of the Stillaguamish at the Granite Falls station. Compare/contrast the width and
depth (gage height) of the Stillaguamish at Granite Falls and Arlington.
8b. If there was discharge data for the Stillaguamish River at Granite Falls, do you think it would be
higher or lower than at Arlington? Why?
9. Stanwood: Fly to 48 12’41”N, 122 20’10”W. This is the gage height station for the Stillaguamish
River in Stanwood, WA.
9a. Measure the river width at this station: ________________
How does it compare to the other two stations? Do you think the discharge at this station is higher or
lower than the river at the Arlington station? Why?
9b. Why do you think the river here is so much deeper than at either Arlington or Granite Falls?
9c. What is causing the variable pattern you see in the river depth at Stanwood?
9d. Follow the river west to where it flows into Puget Sound. What is the feature at this boundary
between the river and the Sound called?
9e. Notice that there is farmland on either side of the river before it flows into Puget Sound. Describe
why it is a good idea to have farmland in this area, and probably not a good idea to have houses or
other buildings here (Note that there are some buildings—probably farms).