The water in Green Lake was clear when this
picture was taken on January 20, 2005 (10 months after the alum treatment).
Transparency, or the clarity of the water, is measured using a black and
white disk, called a Secchi disk. The disk is lowered into the water using
a rope. The depth at which the disk is no longer visible is referred to
as the Secchi depth. The Secchi depth of Green Lake varies throughout
the year. The algae blooms of recent years' late summer months have reduced
the Secchi depth to less than 1 meter. The goal of the alum treatment
is to maintain a Secchi depth at more than 2.5 meters, year round.
Alum treatment monitoring
Take a look at this graph of our 2005-2007 water clarity measurements
to see the seasonal changes during the years. November
1, 2007 Secchi depth chart (PDF)*
Measuring Lake Temperature
The seasonal changes of the lake's temperature are shown in these measurements.
See the November 1, 2007
Lake Temperature data (PDF)*
Measuring Lake Water
Level
A new gage on the dock near the boat rentals will help all interested
parties with monitoring the lake's water level. This gauge was installed
in December 2004; past measurements by FOGL were adjusted to reflect the
levels measured on this gauge.
September 10, 2006
Survey of Milfoil Growth
The September 10, 2006 milfoil survey was designed to determine if the
milfoil was present in the same areas as in August 2005, and if it is
spreading or receding. Proliferation of milfoil weed in Green Lake paradoxically
gets worse when lake quality improves. The alum treatment actually helps
the milfoil grow and spread as sunlight penetrates deeper in the water.
Recent introduction of non-reproducing grass carp (which eat the milfoil)
is expected to help control milfoil growth at least to some degree. The
survey will also measure the impact of two milfoil removal projects done
by the Parks Department early in the summer.
* This linked document is a PDF file, and you'll need
Acrobat Reader to display it. If you don't have that program, you may
download it free from Adobe.
Above: Friends of Green Lake members
Gayle Garman and Richard Fleming take water samples and collect information
on water conditions from two kayaks positioned side-by-side as part of
King County's Lake Stewarship Program. Top: Volunteer high school
student Alex Gray at various stages of dockside monitoring in 2007. Photos
by Gail Barker
FOGL Monitoring Program
Level I
During the spring of 2003, while trying to gather information about the
condition of the lake, FOGL learned that no agency was regularly monitoring
the lake's water clarity. Friends of Green Lake decided to make the standard
Secchi measurements to record water clarity changes through the summer,
and in the fall we also began measuring water temperature and the lake
level. Gail Barker collected weather observations, secchi disk (water
transparency), surface water temperature, and water surface elevation.
Richard Fleming collected rainfall as part of that program. Billings Middle
School students currently collect Level I data dockside. Their data is
incorporated in the Secchi depth, water level, and lake temperature charts.
FOGL Monitoring Program
Level II
Beginning in 2005, Green Lake was included in the King County Lake Stewardship
Program, and FOGL members collected Level II information from kayaks.
Samples are taken eveny two weeks from mid May to mid October. FOGL volunteers
Richard Fleming and Gayle Garman note general weather conditions and the
number of boats and geese on the lake. They take Secchi disk readings
for water transparency. Water samples and temperature are taken at one
meter below the surface. Twice over the summer, water temperature and
samples are taken one meter above the bottom. Precipitation measurements
are taken daily year round by Dr. Fleming.
West Green Lake Beach
Bacteria Levels and Water Temperature Graphs and Data
The King
County Lake Swimming Beach Monitoring Program page shows temperature
and bacterial count charts and data
for West Green Lake Beach only. FOGL has requested monitoring of both
East and West Green Lake Beach - the only public warm-water beaches in
the city. Green Lake park is the most intensely used park in the state.
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©2008 Friends of Green Lake, Seattle, Washington
Updated
May 8, 2008
Webmaster - Ellen Hewitt