The Lake:

Cottage Lake is located approximately four miles east of Woodinville. The lake covers 63 acres and the majority of the shoreline is surrounded by private homes. Cottage Lake Park is at the north end of the lake and provides public access with a large dock and a small beach.

The Big Issue:

Cottage Lake contains high phosphorus concentrations and is therefore on Washington State’s 303(d) list of lakes not meeting state water quality standards. High phosphorus levels lead to increased algae blooms that impact water quality and the long-term usability of a lake. These algae blooms, when severe, are potentially toxic to humans. Past studies show that phosphorus is coming primarily from incoming streams (Daniels Creek and Cottage Creek). Other suspected sources include harmful lawn-care products and ill-maintained septic systems.

Algae Blooms and Cyanobacteria (blue-green algae):

Due to the high phosphorus levels in the lake, Cottage Lake is home to a lot of algae (microscopic single-celled plants). During hot, sunny weather, the algae multiplies quickly and this is referred to as an "algae bloom"(algae forms large dense patches visible on the surface of the water). Most species of algae are harmless and the blooms are just a nuisance. Cyanobacteria (commonly referred to as "blue-green algae", but really a bacteria) is another organism living in the lake that multiplies during warm, sunny weather - again, cyanobacteria is generally harmless, but some can produce toxins which can be harmful to animals and humans. In particular, Microcystis (a type of cyanobacteria) has produced toxin blooms in other lakes in our area in the past (including Lake Sammamish). King County's website has an excellent article on the 1997 toxic algae bloom in Lake Sammamish which contains a lot of valuable information on blue-green algae (cyanobacteria). Another type of cyanobacteria has also been linked to swimmer's itch.

Cottage Lake Algae Watch:

• August 8th, 2005: A large bloom was seen on the lake and large clumps of blue-green algae were present on the shoreline (especially dense on the east side of the lake). A sample was sent to King County.
• August 17th, 2005: King County tested the sample we gave them and it contained no microcystins, which is a common toxin produced by blue-green algae. However, it is possible that algae in the lake could produce detectable toxins later in the season if there is a change in the varieties or a big increase in the amount of algae present.

Other Problems (updated!): 

In addition to the high phosphorus levels and algae, Cottage Lake is also home to at least three identified noxious weeds: Purple Loosestrife, Fragrant Water Lilies, and Eurasian Milfoil. Purple loosestrife can clog streams and crowd out beneficial native plants and is very difficult to remove once it is established. Fragrant water lilies grow in dense patches and crowd out native plants. The can even create stagnant areas with low oxygen levels underneath the leaves which impacts fish and other wildlife. Eurasian Milfoil was recently found (Fall 2007) and can also have a very serious impact on the ecosystem and recreation. See the new milfoil page for more information.

Frequently Asked Questions (FAQ):

See the Lake Issues FAQ for answers to frequently asked questions about the lake.

We are currently in the process of applying for grant money to monitor water quality and help determine the source of the phosphorus and other contaminants in the lake. See Funding and Projects for more information.

More Information:

To learn more about Washington State’s 303(d) list and phosphorus go to:
http://www.ecy.wa.gov/programs/wq/303d/

To read the Washington State Dept. of Ecology “TMDL” or “Total Maximum Daily Load” Analysis report
for Cottage Lake go to:
http://www.ecy.wa.gov/biblio/0310085.html

For more information on “TMDL” analysis please see:
http://www.ecy.wa.gov/programs/wq/tmdl/index.html