To measure water clarity, a Secchi Disk about
the size of a dinner plate is lowered into the water until it disappears.
Green Lake water clarity varies with the amount of suspended sediment
(clays and silts) and/or plankton (microscopic plants and animals also
known as algae) in the water.
A Secchi Disk about the size of a dinner plate is used to measure water
clarity. The disk is lowered into the water until it disappears. The distance
at which it disappears is a measure of water clarity. Shallow Secchi Disk
measurements indicate high levels of sediment (caused by surface runoff
following storms) or high levels of phytoplankton (caused by warm water
temperatures, sunlight, and nutrients). High levels of phytoplankton can
lead to unpleasant odors and scum.
On occasion, blue-green algae (a type of phytoplankton) can grow to great
density and may become a health risk. Blue-green algal blooms caused the
City Health Department to close Green Lake to swimming and boating in
1999, 2002, and 2003.
The Secchi depth of Green Lake varies throughout the year. In the past,
plankton blooms often reduced water clarity in the late summer to less
than 3 feet. In 2004, the City treated the lake with alum (aluminum sulphate)
to reduce the amount of phosphates in the water and hence reduce plankton
blooms. Alum combines with phosphate and settles to the bottom, removing
the phosphate from the water. The 2004 treatment provided excess alum
and the un-reacted alum settled to the bottom of the lake where it combines
with phosphate escaping from the bottom sediments. The goal of the alum
treatment is to maintain a Secchi depth at more than 2.5 meters, year
Click here* to see average
Green Lake water clarity over the years. 1995 and 1996 data show conditions
before the alum treatment in 2004, and 2005
to 2009 data show conditions after the treatment. The graph shows
that water clarity improved
following the alum treatment. The 2004 treatment continues to be effective
at reducing phytoplankton blooms. Water clarity in the late summer in
2008 was exceptionally high and we wait to see if the high clarity continues
West Green Lake Beach Bacteria Levels and Water Temperature Graphs and
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. FOGL also helps track the water level of the lake. A gauge on
the dock near the boat rentals was installed in 2004 and helps monitor
the lake's water level.
More about the alum treatment
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
the King County Lakes and Streams Monitoring Group (formerly Lake Stewarship
Program). Click here to see Green
Lake Water Quality Monitoring Results for Water Year 2012 Top:
Volunteer Billings Middle School students monitor and chart water
quality at dockside near the paddle boat rental (February 2010).
Click here to see more and larger images.
FOGL Monitoring Program
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
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 waterbirds on the lake. They take Secchi disk readings
for water transparency. Water samples and temperature are taken at one
meter below the surface and one meter above the bottom. Precipitation
measurements are taken daily year round by Dr. Fleming.
Milfoil Growth a Constant Problem and It's Increasing
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. The introduction
of non-reproducing grass carp (which eat the milfoil) failed to control
milfoil growth as much as was hoped. Milfoil removal projects have been
done by the Parks Department in the past, and another project is in the
works. A milfoil clean-up project is organized by FOGL once or twice in
the fall. Volunteers come with waders and gloves and are provided rakes
by Seattle Parks & Recreation. More volunteers are always welcome!